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The Facts On File DICTIONARY of

The Facts On File DICTIONARY of BOTANY

Edited by Jill Bailey The Facts On File Dictionary of Botany

Copyright © 2003 by Market House Books Ltd

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Library of Congress Cataloging-in-Publication Data

The Facts on File dictionary of botany / edited by Jill Bailey. p. cm. Includes bibliographical references (p. ). ISBN 0-8160-4910-6 (hc)—ISBN 0-8160-4911-4 (pbk.) 1. Botany—Dictionaries. I. Title: Dictionary of botany. II. Bailey, Jill. III. Facts on File, Inc.

QK9.F33 2002 580'.3—dc2l 2002035202

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This book is printed on acid-free paper PREFACE

This dictionary is one of a covering the terminology and concepts used in important of . The Facts On File Dictionary of Botany is planned as an additional source of information for students taking Ad- vanced Placement (AP) Science courses in high schools, but will also be help- ful to older students taking introductory college courses.

This volume covers the whole area of pure and applied science includ- ing and , plant , , bi- ology, , , and . It also covers the and classification of , with entries for the higher-ranking taxa. The defini- tions are intended to be clear and informative and, where possible, we have provided helpful diagrams and examples. The book also has a selection of short biographical entries for people who have made important contribu- tions to the field. The appendices include lists of webpages and an informa- tive bibliography.

The book will be a helpful additional source of information for anyone studying AP , notably the sections on and , and of Plants and , and and Cells. However, we have not restricted the content to this syllabus. Modern plant science is a subject of considerable importance and we hope that this book will be useful to anyone interested in the subject.

ACKNOWLEDGMENTS

Consultant B.Sc., Pd.D.

Contributors Eve Daintith B.Sc. Elizabeth Tootill B.Sc. CONTENTS

Entries A to Z 1

Appendixes I. SI Units 248 II. Webpages 249

Bibliography 250

A

ABA See . The organized loss of part of a plant, usually a , , or unfertilized abaxial In such as a . An abscission zone occurs at the and , the side facing away from the base of the . Here a separation layer main axis, i.e. the lower surface. In lateral (abscission layer) is formed by breakdown organs such as leaves, abaxial is synony- or separation of cells and final severance mous with the underside. Compare adax- occurs when the vascular bundles are bro- ial. ken mechanically, e.g. by wind or rain. The abscission layer is activated by increasing abiotic environment The nonliving levels of and a decreasing concen- factors of the environment that influence tration of ecological systems. Abiotic factors include , chemical pollution, geographical absolute humidity See humidity. features, etc. absolute frequency (APF) See pollen analysis.

absorption 1. The uptake of liquid by cells and organs. In plants, water and min- eral salts are absorbed mainly by the hairs, just behind the root tips. 2. The capture of radiant by plant pigments. About 80% of the visible light falling on a leaf is absorbed, and about Abscisic acid 10% of the infrared radiation. abscisic acid (ABA)A , absorption spectrum A plot of the ab- that functions chiefly as an inhibitor of sorbance by a substance of radiation at dif- growth and cell elongation. Abscisic acid ferent wavelengths, usually of , has a variety of effects related to dor- visible, or infrared radiation. It can give in- mancy and stress responses: it regulates formation about the identity or quantity of expression in seed development a substance. , for example, leading to dormancy and is one of the hor- have absorption peaks in the red and blue mones involved in dormancy; it regu- (and therefore reflect green light). Com- lates stress responses by, for example, pare action spectrum. closing stomata in times of water shortage and increasing the ability of to carry accessory cell See subsidiary cell. water. It can also promote root growth and inhibit growth. Despite its name it accessory chromosome See B chromo- does not directly promote abscission, but some. only indirectly through increasing ethylene production. Formerly it was known as ab- accessory pigment See photosynthetic scisin II or dormin. pigments.

1 acellular acellular Denoting relatively large tis- achene A dry indehiscent fruit formed sues or organisms that are not composed of from an with a single carpel contain- discrete cells and are, in effect, unicellular. ing a single seed, e.g. (Quercus). Dif- In flowering plants, for instance, the early ferent types of achenes include the stages of development are often , CYPSELA, , and SAMARA. acellular. Other examples include aseptate fungal hyphae and certain green , acicular Needle-shaped. such as Acetabularia. The term is used in preference to unicellular to distinguish acid A substance that gives rise to hy- + such structures (which are often multinu- drogen ions (or H3O ) when dissolved in cleate) from conventional cells and show water. An acid in aqueous solution will their equivalence to multicellular struc- have a pH below 7. Lowry–Brønsted tures. See also coenocyte. theory defines an acid as a substance that exhibits a tendency to release a proton, and Aceraceae A of temperate and a base as a substance that tends to accept a tropical and that includes the proton. Strong acids (e.g. HNO3) react +. maples and sycamores. completely with water to give H3O Weak acids (e.g. CH3COOH) are only partly dis- + acetaldehyde (ethanal) An aldehyde, sociated because H3O is a stronger acid than the free acids. CH3CHO, that is an intermediate in the conversion of pyruvic acid to ethanol dur- See ing the final stage of glycolysis during acidic stain staining. anaerobic respiration in plants. It is in- acid rain The deposition of acids by nat- volved in the synthesis and breakdown of ural precipitation, mainly by rain but also the threonine. by snow and fog. Acids are formed by re- action of gaseous waste products, particu- acetic acid (ethanoic acid) A carboxylic larly dioxide, and also acid, CH COOH, obtained by the oxida- 3 oxides, with moisture in the air to form sul- tion of ethyl alcohol. Acetic acid is a com- furic and nitric acids. The subsequent pre- ponent of vinegar (which is obtained by cipitation has led to raised acidity in forests bacterial oxidation of wine waste). It is and lakes, especially in Scandinavia and used as an alternative carbon source by cer- some other parts of northern Europe, dam- tain . When combined with aging the environment. Acid rain is often COENZYME A to form ACETYL COA, it plays a used more loosely for any atmospheric pol- key role in AEROBIC RESPIRATION. lutant that dissolves in precipitation caus- ing environmental damage, such as carbon acetocarmine A stain used to color monoxide or ozone, and these may interact chromosomes deep reddish-black for view- with the acids. ing with a light microscope. Tissues are The most serious pollutant is sulfur fixed in acetic acid before applying aceto- dioxide, which comes mainly from burning carmine. See fixation; staining. coal and is, consequently, less serious now than in the twentieth century. Sulfuric acid acetyl CoA (acetyl coenzyme A) A com- in soils may lead to the formation of am- pound made up of acetyl and coenzyme A monium sulfate, which causes the release linked by a sulfur bridge. Acetyl CoA plays of toxic aluminum and heavy metal ions a key role in , being a precursor that inhibit metabolic activity. In water- of the KREBS CYCLE and GLYOXYLATE CYCLE, ways these can damage the gills of fish. Un- and the starting point for synthesis of fatty polluted rain is normally slightly acidic acids, terpenes, and some amino acids. The with a pH of 5.0–5.6. synthesis of acetyl CoA is a high-energy process, requiring energy from ATP, which acid soil A soil with a pH less than 6.0. is converted to AMP. Such soils usually form in areas of heavy

2 activation energy rainfall, which causes leaching of lime form are rodlike structures. The from the surface layers, or over acid sub- Actinobacteria are distinguished by pro- strata such as granite or sand. In the acidic ducing actinospores: entire cells encysted conditions, decomposition of organic ma- in thick walls to form resistant . terial in the soil is slow. Acid soils often contain substantial concentrations of iron actinodromous (palmate; digitate) De- and aluminum hydroxides. See brown scribing a form of leaf VENATION in which earth; podsol. three or more primary veins radiate from the base of the lamina toward the margin, Acrasiomycota (cellular slime molds)A as in sycamore (Acer pseudoplatanus), re- of protoctists in the Rhi- sulting in a leaf that has several large lobes zopoda. Formerly considered to be fungi, or several leaflets all originating at the they are made up of independently living same point. amebas that feed on bacteria, which they ingest by phagocytosis. Under conditions actinomorphy See radial symmetry. of food shortage or other stresses, they come together to form a slug-like aggrega- actinomycetes See Actinobacteria. tion of amebas called a pseudoplasmo- dium, which acts like an individual actinostele A of protostele in . which the is star shaped and the lies between the points of the acrocarpous Describing in star. Protosteles are found in the primary which the reproductive organs are borne at roots of higher plants, and in some spe- the top of the main axis, which is usually cies of Lycopodium and Psilotum. See erect, so that subsequent growth is sympo- stele. dial. action spectrum A graph showing the acrocentric See centromere. effect of different wavelengths of radiation, usually light, on a given process. It is often acropetal Describing a process (such as similar to the absorption spectrum of the growth or development) that progresses substance that absorbs the radiation and from the base or point of attachment, so can therefore be helpful in identifying that that the oldest parts are at the base and the substance. For example, the action spec- youngest are at the tip. Compare basipetal. trum of is similar to the ab- sorption spectrum of . It shows actin A globular contractile protein (G- the net assimilation plotted against wave- actin) that makes up 10–15% of the total length. See absorption spectrum. See illus- cell protein in eukaryotic cells. It is a major tration overleaf. component of the CYTOSKELETON. It also controls motility, cellular movements, activation energy The minimum extra and cyclosis. G-actin can polymerize into energy that must be put into a system to en- helical strands that coil together to form able a reaction to occur. It increases the en- microfilaments called fibrous actin (F- ergy levels of participating molecules, and actin). hence raises their reactivity, often by rais- ing electrons to an excited state. Actinobacteria (ray fungi; actinomycetes) enhance reaction rates by lowering the ac- A phylum of Eubacteria that contains the tivation energy. This is often achieved by true actinobacteria and the coryneform the attraction of the reactants to the en- bacteria. The true actinobacteria form zyme’s active site, thus coming into closer branching filaments that resemble small proximity, or by the causing con- fungal mycelia, with hyphae usually less formational or electrostatic changes in one than 1.5 m m in diameter; the coryne- or more of the reactants.

3 activator

Action spectrum of photosynthesis

Absorption spectrum of chlorophylls

activator 1. A metal ion that acts to- the efficiency of the enzyme activity. See al- gether with an enzyme or its substrate to losteric site. bring about a reaction. 2. A protein that positively regulates the The transport of mol- transcription of a . ecules or ions across a against a concentration gradient, with the active site The particular part of an en- expenditure of energy, usually in the form zyme that combines with and acts of ATP. It is probably an attribute of all on the substrate. The active site consists of cells. Anything that interferes with the pro- amino acids arranged in a configuration vision of energy will interfere with active specific to a particular substrate or type of transport. The mechanism typically in- substrate. Binding of an inhibiting com- volves a carrier protein that spans the cell pound elsewhere on the enzyme molecule membrane and transfers substances in or may change this configuration and hence out of the cell by changing shape.

4 ADP acuminate Describing a structure that Hedyotis on the Hawaiian Islands, but the gradually narrows to a point, such as cer- term can apply to the range of in tain leaves. any or family. acylglycerol (glyceride) An of adaxial In structures such as a leaves glycerol and one or more fatty acids. They and petals, the side facing toward the main may be mono-, di-, or triacylglycerols axis, i.e. the upper surface. Compare abax- according to the number of –OH groups ial. esterified. The fat stores of the body consist mainly of triacylglycerols (triglycerides). adder’s tongue See Ophioglos- These can form a source of energy when sales. levels are low, being broken down by lipases into fatty acids, which A nitrogenous base (6-amino- can enter metabolic pathways. Glyco- ) found in DNA and RNA. It is also (glycosyldiacylglycerols) are diacyl- a constituent of certain coenzymes, e.g. glycerols with a attached to the unes- NAD and FAD, and when combined with terified hydroxyl group. See also . the sugar ribose it forms the nucleoside ADENOSINE found in AMP, ADP, and ATP. The extent to which an or- Adenine has a purine ring structure. ganism, or a physiological or structural characteristic of an organism, is suited to a particular environment. Specialist organ- adenosine (adenine nucleoside) A nu- isms that have become highly adapted to cleoside formed from adenine linked to D- one environment are then often not so ribose with a b-glycosidic bond. Adenosine adaptable as less specialized organisms triphosphate (ATP) is a nucleotide derived and are at a disadvantage in a changing en- from adenosine. vironment (adaptation versus adaptabil- ity). adenosine diphosphate See ADP. adaptive enzyme See inducible enzyme. adenosine monophosphate See AMP. adaptive radiation The formation See ATP. through evolution of a number of different varieties or species from a common ances- Adiantaceae (maidenhair ferns) A fam- tor. It is often seen most clearly on isolated ily of ferns (Filicinophyta) found through- oceanic islands, e.g. the many forms of out the world, especially in the moist trees, shrubs, , and of the genus American tropics.

Active site

5 adventitious

Aerobic respiration

ADP (adenosine diphosphate) A nu- aerobe An organism that can live and cleotide consisting of adenine and ribose grow only in the presence of free , with two phosphate groups attached. The i.e. it respires aerobically (see aerobic res- second phosphate is attached to the first by piration). All plants and most bacteria and a high-energy bond. fungi are aerobes. Compare anaerobe. adventitious Describing plant organs aerobic respiration Respiration in that arise in unexpected places, for exam- which free oxygen is used to oxidize or- ple the development of adventitious roots ganic substrates to and from stems, and adventitious from water, with a high yield of energy. The re- leaves. action overall is: C6H12O6 + 6O2 = 6CO2 + 6H2O + aerenchyma A plant containing energy It occurs in a number of stages, the first of large intercellular air spaces, usually which (GLYCOLYSIS) also occurs in anaero- formed as a consequence of the death of bic respiration in the cell cytoplasm. With cells. Aerenchyma is typical of as the substrate, a sequence of re- the stems of many aquatic plants whose actions results in the formation of pyru- leaves float at the surface of ponds and vate. The remaining stages, which do not lakes, giving added buoyancy to the tis- occur in anaerobic respiration, take place sues. in the mitochondria. Pyruvate is converted to ACETYL COA, which enters a cyclic series aerial root A root that arises above soil of reactions, the KREBS CYCLE, with the pro- level. Examples include the roots of epi- duction of carbon dioxide and hydrogen phytes and climbers, which hang down in . These and other hydrogen atoms the air or stick to a or . The produced at earlier stages are now passed roots of many orchids and other epiphytes to the ELECTRON-TRANSPORT CHAIN (involv- developed a sheath of dead cells, the vela- ing CYTOCHROMES and FLAVOPROTEINS). men, which helps to absorb water from the Here they combine with atoms of free oxy- atmosphere. gen to form water. Energy released at each

6 air bladder stage of the chain is used to form ATP dur- gels are extensively used for growing ing a coupling process (see oxidative phos- and tissue cultures. phorylation). There is a net production of 38 ATPs (2930 kJ) per molecule of glucose Agaricales (agarics) An of basid- during aerobic respiration, a yield of about iomycetes that contains the mushrooms 19 times that of anaerobic respiration (300 and toadstools. The cap of the fleshy fruit- kJ), and the mechanism of the majority of ing body – the mushroom or toadstool – organisms. Compare anaerobic respira- bears a series of parallel gills on its lower tion. surface that greatly increase its surface area. These are covered in the - aerotaxis (aerotactic movement)A bearing layer (hymenium). Agarics are TAXIS in response to an oxygen concentra- common in leaf litter and rotting , tion gradient. For instance, motile aerobic where they live as saprobes. bacteria are positively aerotactic, whereas motile obligate anaerobic bacteria are neg- aggregate fruit (compound fruit) A fruit- atively aerotactic. Some photosynthetic eu- like structure that is made up of several in- karyotic unicells are also aerotactic. dividual derived from the carpels of a single flower, e.g. (Fragaria), aerotropism A TROPISM in which the (Rubus fruticosus). These may orientating is oxygen. be achenes, follicles, , or drupelets. An aggregate fruit is sometimes called an afforestation The establishment of for- etaerio, but this term may also be restricted est on land not previously forested, either to an aggregate of drupelets. A fruit that by natural succession or by planting. develops from a group of is termed a multiple fruit, e.g. (Ananas co- aflatoxin One of a group of carcino- mosus) and hop (Humulus lupulus). genic toxins produced by fungi of the genus Aspergillus, especially A. flavus. This Agrobacterium A genus of soil bacte- species is a common contaminant of , ria, the species A. tumefaciens being the such as peanuts. Contaminated feed causes causative agent of CROWN GALL, a type of serious outbreaks of disease among live- tumor in plants. A segment of DNA (trans- stock. ferred DNA, T-DNA) from a PLASMID in the bacterium is transferred into the after-ripening The collective name for DNA and induces tumor formation. Since processes that are necessary before germi- the plasmid is capable of independent nation can take place in certain , even replication in host cells of many dicotyle- though external conditions may be suit- donous plants, it has been used as a able. For example, a period of dormancy vector in . may be imposed on the seed, preventing premature before an unfavor- agroforestry A system of cultivation able season such as winter. After-ripening common in many parts of the tropics, es- is common in areas with marked seasonal- pecially in rainforest regions, in which ity. and arable farming are mixed. This reduces soil degradation, as freshly agamospermy See . tilled soils are shaded and protected from heavy rains and run-off by the trees. The agar A gelling agent prepared from sea- trees help to draw nutrients up from the weed (normally a red alga, Gelidium), used deeper soil layers into the zone occupied by to set liquid nutrients. It can withstand the roots of the plants. sterilization at high temperatures and is re- sistant to attack by most bacteria. Agar A branch of deal-

7 air plant ing with soil management and crop pro- alcohol dehydrogenase An enzyme duction. that converts ethanol into acetaldehyde (ethanal). It is important in a plant’s ability air bladder 1. A structure on the pollen to withstand waterlogging. Ethanol is a grains of some , such as Pinus, that product of anaerobic respiration in plants, aids the wind dispersal of the pollen. It and waterlogged roots suffer from a short- consists of an air-filled protuberance on age of oxygen. Accumulation of ethanol is each side of the pollen grain developed by harmful to a plant. the separation of the exine layers, giving the pollen a characteristic winged appear- alcoholic fermentation A form of ance. ANAEROBIC RESPIRATION in which glucose is 2. An air-filled chamber in the thallus of broken down to form ethanol and carbon many Phaeophyta (brown algae) that in- dioxide. It is carried out by yeasts and creases the buoyancy of the thallus, en- some other fungi and certain bacteria. abling it to float toward the surface when Alcoholic fermentation is catalyzed by underwater to increase light absorption enzymes of the zymase complex, which are and gaseous exchange for photosynthesis. secreted by the cells or released after the Air bladders are highly developed in cer- cells die. The process is self-limiting, since tain wracks, such as the bladder wrack the organisms usually die once the alcohol (Fucus vesiculosus). concentration in the medium exceeds 15%. Alcoholic fermentation is the basis of the air plant See epiphyte. preparation of alcoholic beverages, and of bread making (in which the carbon dioxide alanine A simple AMINO ACID, one of the early products of photosynthesis. Alanine is formed by transamination when an amino group is donated by gutamine to pyruvic acid. It may be deaminated back to pyruvate for use in the KREBS CYCLE. albumin One of a group of simple, low- molecular-weight found in plants, for example in the endosperm of and wheat seeds. Albumins are water- soluble and coagulate when heated. albuminous cell 1. A vertically elon- gated parenchyma cell, found in groups in the rays of the secondary phloem in gym- nosperms, where they are associated with sieve cells. Unlike the companion cells of angiosperms, they are not derived from the same mother cell as the sieve cell. 2. An albumin-containing cell found in cer- tain seeds. alcohol A type of organic compound of the general formula ROH, where R is a hy- drocarbon group. Examples of simple alco- hols are methanol (CH3OH) and ethanol (C2H5OH). Ethanol is a product of anaer- obic metabolism. Alcoholic fermentation

8 released causes the dough to rise). During carbon atoms) and glucose (with six car- alcoholic fermentation acetaldehyde acts bon atoms). as a hydrogen acceptor instead of oxygen. The pyruvic acid formed by GLYCOLYSIS is grain (aleurone body) A mod- broken down into acetaldehyde and car- ified found in the and en- bon dioxide. The acetaldehyde is further dosperm of seeds and containing mostly reduced by NADH to form ethanal, releas- reserve proteins, but also phytic acid and ing only about a tenth of the energy that various enzymes associated with mobiliza- would be released by aerobic respiration. tion (digestion) of these reserves. The pro- teins and phytic acid are present in aldehyde A type of organic compound crystalline form in the dormant seed. with the general formula RCHO, where the –CHO group (the aldehyde group) con- aleurone layer The outermost protein- sists of a carbonyl group attached to a hy- rich layer of the endosperm of grass fruits drogen . Simple examples of (e.g. grains). At germination, the em- bryo produces , which stimu- lates the aleurone layer to synthesize enzymes, especially amylase. The latter causes hydrolysis of the in the en- dosperm. The enzymes are synthesized from the amino acids supplied by break- down of ALEURONE GRAINs.

algae (sing. alga)A large mixed group of photosynthesizing eukaryotic organisms, now usually placed in the Protoc- tista. They often resemble plants and are found mainly in marine or fresh-water , although some algae are terres- trial. Algae differ from plants in lacking any real differentiation of leaves, stems, and roots, in having no layer of sterile cells around the reproductive organs,and in not having an embryo stage in their cycle. Algae can be unicellular (e.g. Chlamy- domonas), colonial (e.g. Volvox), filamen- tous (e.g. Spirogyra), or thalloid (e.g. Fucus). All algae contain chlorophyll but this may be masked by various accessory pigments, these being one of the major characteristics used to divide the algae into their various phyla. Other characters used to classify the algae are the Aldose of storage products, the type of , the form and number of undulipodia aldehydes are methanal (formaldehyde, (flagella), ultrastructural cell details, type HCHO) and ethanal (acetaldehyde, of food reserves, and reproductive pro- CH3CHO). cesses. aldose A SUGAR containing an aldehyde alkaline soil See calcareous soil. (CHO) or potential aldehyde group. Ex- amples include the ribose (with five alkaloid One of a group of organic com-

9 allele pounds found in plants, which are poiso- DIOECIOUS and MONOECIOUS. Most plants nous insoluble crystalline compounds. are , but have mechan- They contain nitrogen and usually occur as isms such as self-INCOMPATIBILITY that salts of acids such as citric, malic, and suc- promote allogamy. Compare autogamy. cinic acids. Their function in plants re- See also heterostyly; chasmogamy; di- mains obscure, but it is suggested that they chogamy. may be nitrogenous end-products of me- tabolism, or they may have a protective allopatric species See species. function against , since they taste bitter. Important examples in human allopolyploidy A type of in- use are quinine, , atropine, , volving the combination of chromosomes , codeine, and strychnine. They from two or more different species. Al- occur mainly in the poppy family, the but- lopolyploids usually arise from the dou- tercup family, and the nightshade family of bling of chromosomes of a between plants. species, the doubling often making the hy- brid fertile. The properties of the hybrid, allele (allelomorph) One of the possible such as greater vigor and adaptability, are forms of a given gene. The alleles of a par- retained in the allopolyploid in subsequent ticular gene occupy the same positions generations and such organisms are often (loci) on homologous chromosomes. A highly successful. Many plant species have gene is said to be homozygous if the two been derived originally from allopoly- loci have identical alleles and heterozygous , e.g. cultivated wheat. Compare when the alleles are different. When two autopolyploidy. different alleles are present, one (the domi- nant allele) usually masks the effect of the allosteric enzyme An enzyme whose other (the recessive allele). The allele deter- catalytic activity can be modified by the mining the normal form of the gene is usu- noncovalent binding of a particular ally dominant while mutant alleles are metabolite (a modulator) at a site (the AL- usually recessive. Thus most LOSTERIC SITE) other than the active site. It show in the only when they are homozygous. In some cases one allele is not may either inhibit or enhance the enzyme completely dominant or recessive to an- activity. Allosteric enzymes may have more than one modulator. other allele (INCOMPLETE DOMINANCE). Thus an intermediate phenotype will be produced in the heterozygote. See also in- allosteric site A part of an enzyme to complete dominance; multiple allelism. which a specific effector or modulator can be attached. This attachment is reversible. allelomorph See allele. Allosteric enzymes possess an allosteric site in addition to their ACTIVE SITE. allelopathy Inhibition of the germina- tion, growth, or of an organ- allotetraploid (amphidiploid) An allo- ism effected by a chemical substance polyploid whose chromosomes are derived released from another organism. It is prob- from two different species and which ably an anticompetition mechanism in therefore has four times the haploid num- plants; for example, barley secretes an al- ber of chromosomes, e.g. Spartina anglica, kaloid substance from its roots to inhibit derived from the diploids S. alterniflora competing weeds, but it is not known how and S. maritima. See allopolyploidy. important it is generally. alluvial soil A type of soil formed on allogamy Cross-fertilization in plants. river floodplains and deltas, where new This promotes genetic variation in the sediment is deposited on the land during especially in plants that are floods.

10 amino acids alpha helix A highly stable structure in alternation of generations The occur- which peptide chains are coiled to form a rence of two, or occasionally more, gener- spiral. Each turn of the spiral contains ap- ations during the life cycle of an organism. proximately 3.6 amino acid residues. The In all plants and some algae there is an al- R group of these amino acids extends out- ternation between sexual haploid and ward from the helix. Hydrogen bonding asexual diploid stages. They usually differ between successive coils holds the helix to- markedly in morphology. The haploid gether. If the alpha helix is stretched the plant produces mitotically and is hydrogen bonds are broken but reform on thus termed the while the relaxation. The alpha helix is found in diploid plant produces spores meiotically muscle protein and keratin. and is called the . The gametes fuse to form a , which develops into alpha-naphthol test (Molisch’s test)A the sporophyte, and the spores germinate test for detecting the presence of carbohy- and produce the gametophyte, so forming drates in solution. The test solution is a cycle. In (mosses, liverworts placed in a test tube, and a small amount of and ) the haploid gametophyte is alpha-naphthol added; concentrated sulfu- the dominant phase of the life cycle and the ric acid is then trickled slowly down the sporophyte is represented only by the cap- side of the tube. If carbohydrate is present, sule, , and foot. In vascular plants the a violet ring will form at the junction of the diploid sporophyte is the dominant phase liquids. and in the ferns, for example, the gameto- phyte is a small prothallus. The concept of alpine Describing a (regional an alternation of generations can be ex- ) of plants above the treeline tended to the flowering plants, in which the and below the snowline on high moun- embryo sac and pollen represent the much tains. The lower limit of the alpine zone reduced female and male gametophyte gen- varies in different mountain regions, ac- erations respectively. cording to the rainfall and other climatic and topographic factors, from 100 meters ameboid Describing an organism that above sea level in parts of Scotland to 3700 resembles an ameba in shape and move- meters in the western Himalayas. There are ment. often considerable differences between the nature and distribution of on amensalism An association between north- and south-facing slopes and be- two different species, at either the level of tween windward and leeward slopes. the individual organism or the population Alpine vegetation is often similar to TUN- level, in which one is harmed and the other DRA vegetation, being adapted to harsh cli- is unaffected. Compare commensalism; mate conditions, including high wind . speeds. amino acids Compounds containing alternate Describing a leaf arrangement both carboxylic acid and amino groups in in which there is only one leaf at each node, their molecules. The amino acids have the as in hazel. This is the commonest form of general formula RCHNH2–COOH. Here, leaf arrangement. See . the group R ranges from a simple hydrogen atom to complex ring structures. All amino alternate host Any host other than acids are white, crystalline, soluble in the main (most common) one. For exam- water, and with the sole exception of the ple, many rusts overwinter on alternate simplest member, glycine, all are optically hosts. Some parasites and pests are able to active. live on more than one host, and some Amino acids are the basic components need a second host to complete their life of proteins, which consist of chains of cycle. amino acids. Plants also contain some 200

11 amino acid sequencing

Amino acids amino acids that do not occur in proteins, Amino acids contain both acids (car- some of which have protective or storage boxylic acids) and bases (amino group), so roles, while others are intermediates in the they react with both acids and bases. Thus synthesis of commoner amino acids. the charge on the amino acid depends on

12 amitosis

Amino acids the pH. This feature is used to separate amino acids for analysis by chromatogra- phy and other methods. Each amino acid has its own characteristic isoelectric point, where the net charge on the molecule is zero. See illustration overleaf.

13 AMP amino acid sequencing The determina- the starch molecule, while a-amylase can tion of the amino acid sequence of a pro- attack bonds within the starch molecule. tein or peptide. This sequence is the primary Both a- and b-amylases occur in plants, the structure of a protein; it influences sec- latter particularly in malt (being used in the ondary, tertiary, and quaternary structure. brewing industry), but only b-amylase is found in animals, having an important role amino sugar A monosaccharaide sugar in digestion. During seed germination a- in which a hydroxyl group (OH) has been amylase breaks down the starch of the replaced by an amino group (NH2). Glu- endosperm, the transcription of the a- cosamine (from glucose) is one of the amylase gene being greatly enhanced by commonest amino sugars. Galactosamine produced by the embryo. (from galactose) is a major component of a-amylase is also involved in the release of glycolipids. Amino sugars are important stored starch from . components of bacterial cell walls. amylopectin The water-insoluble frac- amitosis Nuclear characterized tion of STARCH. It is a branching of by the absence of a nuclear spindle and glucose units. leading to the production of daughter nu- clei with unequal sets of chromosomes. A that synthesizes The ordered process of division, duplica- and stores starch grains. are tion of chromosomes, dissolution of nu- common in storage organs, e.g. the potato clear membrane, and production of a . They have a physiological role in the spindle as in is apparently absent. root cap and elsewhere, where the starch Cells produced amitotically inherit vari- grains act as statoliths. able numbers of chromosomes. The chances of a daughter cell lacking essential amylose The water-soluble fraction of are less than may be expected since STARCH. It is an unbranched polymer of many cells that characteristically divide glucose units. amitotically are polyploid, e.g. the en- dosperm nucleus in angiosperms. Compare anabolism Metabolic reactions in endomitosis; mitosis. which molecules are linked together to form more complex compounds. Thus, an- AMP (adenosine monophosphate)A abolic reactions are concerned with build- nucleotide consisting of adenine, ribose, ing up structures, storage compounds, and and phosphate. AMP has an important complex metabolites in the cell. Starch, role in the regulation of glycolysis, pro- fats, and proteins are all products of ana- moting the production of bisphos- bolic pathways. Anabolic reactions gener- phate from fructose 6-phosphate. See ATP; ally require an input of energy, usually cyclic AMP. provided by ATP produced by catabolism. An example is the . See also amphidiploid See allotetraploid. metabolism. Compare catabolism. amphimixis True anaerobe An organism that can live and by fusion of gametes. Compare apomixis. grow in the absence of free oxygen, i.e. it respires anaerobically (see anaerobic respi- amylase An enzyme, found widely in ration). Anaerobes can be facultative, in plants, animals, and microorganisms, that that they usually respire aerobically but hydrolyzes starch or glycogen to the sugars can switch to anaerobic respiration when maltose, glucose, or dextrin. Amylase hy- free oxygen is in short supply, as are most drolyzes the a(1-4)glycosidic bonds in yeasts; or obligate, in that they never starch. It occurs in two forms, a and b. b- respire aerobically and may even be poi- amylase attacks the nonreducing ends of soned by free oxygen, as are denitrifying

14 aniline stains bacteria and lactic acid bacteria. Compare hermaphrodite flowers (or occasionally aerobe. bearing male and female flowers, i.e. MO- NOECIOUS). Compare gynodioecious. anaerobic respiration Respiration in which oxygen is not involved, found in al- androecium The collective name in most all organisms. The organic substrate higher plants for the male parts of a plant, is not completely oxidized and the energy i.e. the . It is denoted in the floral yield is low. The glycolytic pathway, formula by a letter A. whereby glucose is degraded to pyruvate, with the production of a small amount of andromonoecious Describing species energy, is the same as in aerobic respira- in which both male and hermaphrodite tion, but after that, in plants and fungi, flowers are borne on the same plant. Com- ethanol and carbon dioxide are produced pare gynomonoecious. as the end products of the process, nor- mally known as FERMENTATION. In animals anemophily by wind. Plants lactic acid is produced and normally reme- pollinated in this manner (e.g. grasses) usu- tabolized with oxygen at a later stage. Only ally have insignificant unscented flowers two molecules of ATP are produced by this with large, often feathery stigmas. process. See anaerobe; glycolysis. Compare aerobic respiration. aneuploidy The condition, resulting from nondisjunction of homologous chro- analogous Describing structures that mosomes at , in which one or more are apparently similar (structurally or chromosomes are missing from or added to functionally) but have a different evolu- the normal somatic chromosome number. tionary origin, and thus a different embry- If both of a pair of homologous chromo- ological origin and structure. Phyllodes are somes are missing, nullisomy results. Mono- analogous to leaf blades, but are derived somy and trisomy are the conditions in from petioles (leaf stalks). See also ho- which one or three homologs occur respec- mologous. tively, instead of the normal two. Poly- somy, which includes trisomy, is the anaphase The stage in mitosis or meio- condition in which one or more chromo- sis when chromatids are pulled toward op- somes are represented more than twice in posite poles of the nuclear spindle. In the cell. See nondisjunction. mitosis each of the chromatids moving to- ward the poles represents a single complete aneuspory The production of an un- chromosome. During anaphase I of meiosis usual number of spores during meiosis of a pairs of chromatids still connected at their spore mother cell. In dandelion (Tarax- centromere move to the spindle poles. Dur- acum), for example, the chromosomes of ing anaphase II the centromeres divide and the mother cell remain in one single chromatids are drawn toward the cell after the first meiotic division, forming poles, thus sister chromatids are separated. a restitution nucleus. The second meiotic division thus gives rise to two cells with the anatomy The organization of the parts diploid number of chromosomes, one of of the body and the structural relationships which divides parthogenetically to form an between them, including the arrangement embryo. This is a form of APOMIXIS. Since of tissues and their component cells. crossing over can occur during the first meiotic division, it does allow for some ge- anatropous See . netic variation in the offspring. See par- thenogenesis. androdioecious Describing species in which there are two forms, one bearing angiosperms The flowering plants con- only male flowers, the other bearing only stituting by far the largest phylum (Antho-

15 anion phyta or Angiospermophyta) of vascular annual ring (growth ring) The annual seed plants. They differ from conifers and increase in girth of the stems or roots of other by having the ovule woody plants, as a result of cambial activ- enclosed within an ovary, which after fer- ity. The annual rings of plants growing in tilization develops into a fruit. The female temperate can be seen in cross- gametophyte is represented by the embryo as two consecutive rings of light- sac, the archegonia being absent. The con- and dark-colored xylem tissue. These are tents of the pollen grain represent the male formed from a zone containing larger ves- gametophyte. The pollen grain germinates sel elements produced by the cambium in on a special extension of the carpel, the the spring (lighter layer), followed by a , and results in a zone containing smaller vessel elements diploid zygote and triploid (usually) en- (darker layer) produced during the late dosperm nucleus. Angiosperms are divided summer. This process is repeated annually into two major classes, the monocoty- so that the number of light or dark rings in- ledons (MONOCOTYLEDONAE) and dicoty- dicate the age of that part of the plant. ledons (DICOTYLEDONAE), with a small basal group known as primitive dicotyle- annular thickening Rings of thickening dons. laid down on the inner wall of protoxylem vessels and . Such thickening al- angstrom Symbol: Å A former unit of lows extension of the XYLEM between the length equal to 10–10 meter (one thou- rings so that it is not ruptured as the sur- sandth of a micrometer, one tenth of a rounding tissues grow. nanometer). It is still used occasionally for measurements of wavelength or inter- annulus 1. The ring of tissue surround- atomic distance. ing the stalk of the mature fruiting body (mushroom or toadstool) of the basid- aniline stains A group of stains with iomycete fungi. The annulus is all that re- various properties that are derived from mains of the veil, which joined the rim of aniline. For example, aniline hydro- the pileus (cap) to the stalk in the immature choloride and aniline sulfate will stain mushroom or toadstool. It is sometimes yellow. See staining. termed the velum. 2. A special arc or ring of cells in the spo- anion A negatively charged ion, formed rangia of ferns that constitutes the mecha- by addition of electrons to atoms or mol- nism for spore dispersal. The cells of the ecules. In electrolysis anions are attracted annulus are thickened except on the outer to the positive electrode (the anode). Com- wall, so that they contract on drying out. pare cation. This causes the stomium to rupture, expos- ing the spores within the . The cap- anisogamy () The sexual fu- sule wall gradually bends back as the water sion of nonidentical gametes. Anisogamy in the annulus cells continues to evaporate grades from situations in which the ga- until a point when the remaining water in metes differ only in size to the extreme of the cells suddenly turns to vapor. This oogamy, in which one is a large results in the wall springing back to its nonmotile ovum and the other a small original position, the sudden movement motile sperm. Compare isogamy. See dispersing any remaining spores. oogamy. 3. A ring of large cells separating the epi- dermis from the in certain bryo- annual A plant that completes its life phytes (e.g. Funaria). cycle within a year. Examples are the com- mon field poppy (Papaver rhoeas) and the anterior Designating the part of a sunflower (Helianthus). Compare biennial; flower or facing away from ephemeral; perennial. the axis or stem, respectively.

16 antiport anther The part of the that pro- One of a group of water- duces the pollen. The anther is usually soluble pigments found dissolved in the joined to the tip of the filament (stalk) and of higher . is made up of two lobes. Each lobe con- are red, purple, and blue and are widely tains two pollen sacs that produce very distributed, particularly in flowers and large quantities of small pollen grains. The fruits where they are important in attract- pollen is released when the lobes split open ing insects, birds, etc. They also occur in longitudinally. buds and sometimes contribute to the The anther is made up of an outer epi- autumn colors of leaves. They are natural dermis, a middle fibrous layer, and an pH indicators, often changing from red to inner nutritive layer, the . The hap- blue as pH increases, i.e. acidity decreases. loid pollen cells develop in the tapetal zone Color may also be modified by traces of from spore mother cells. iron and other metal salts and organic sub- stances, for example cyanin is red in roses anther culture (pollen culture) The gen- but blue in the cornflower. See flavonoid. eration of haploid plants from immature pollen grains or intact excised anthers. The Anthophyta See angiosperms. resultant plants are generally smaller than their diploid counterparts. antibiotic One of a group of organic compounds, varying in structure, that are antheridial cell The cell from which the produced by microorganisms and can kill antheridium develops. In seed plants it is or inhibit the activities of other micro- the generative cell in the pollen grain, organisms. One of the best-known exam- which divides in the to produce ples is penicillin, which is produced by the two sperm cells. mold Penicillium notatum. Another exam- ple is streptomycin, from the actinobac- antheridium The male sex organ of the terium Streptomyces griseus. Antibiotics algae, mosses, liverworts, hornworts, are also commonly used in medicine to ferns, horsetails, clubmosses, and fungi. It combat bacterial infections. Antibiotics are may be made up of one cell, or one or widely used in research to inhibit protein many layers of cells. It produces gametes synthesis. Some antibiotics selectively in- that are usually motile. Compare archego- hibit protein synthesis by the ribosomes of nium. prokaryotes, but not by those of eukary- otes (excepting the ribosomes of chloro- antherozoid (spermatozoid) The male plasts and mitochondria), lending support gamete of algae, some gymnosperms, and to the theory that these evolved the non-seed-bearing plants. It is mo- from symbiotic microorganisms. Antibi- tile and is produced in an antheridium, otic-resistant genes are used as markers in except in certain gymnosperms (e.g. Cycas, genetic-engineering experiments. ) in which antherozoids develop from the generative cells of the pollen anticlinal Describing something that is tube. at right angles to a surface. An anticlinal division results in the formation of anticli- Anthocerophyta (hornworts) The small nal walls between daughter cells, allowing phylum of nonvascular plants. Like thal- a tissue to increase its circumference. The lose liverworts, the hornworts have an anticlinal wall of a cell is perpendicular to alternation of generations in which the the surface of the plant body. In cylindrical gametophyte is a dorsiventral flattened organs, such as stems and roots, the term green thallus and the sporophyte is an up- radial is often used instead of anticlinal. right structure that has a foot embedded in Compare periclinal. the gametophyte, from which it draws nu- trition. anticodon A nucleotide triplet on trans-

17 antisense DNA fer RNA that is complementary to and with many chambers, e.g. Myrmecodia, or bonds with the corresponding codon simple hollows in internodes, leaf veins (triplet) of messenger RNA in the ribo- (Coffea arabica), petioles (Tococa guya- somes during protein synthesis, thus ensur- nensis) or stipular spines, e.g. whistling ing the correct sequence of amino acids is thorns (Acacia sphaerocephala). bonded together in the final peptide. See transfer RNA. APF See pollen analysis. antioxidant A substance that slows or aphid (greenfly; blackfly; plant louse)A inhibits oxidation reactions, especially in bug of the family Aphididae of the insect biological materials or within cells, thereby order Hemiptera. Aphids feed by sucking reducing spoilage or preventing damage. plant juices. Many occur in such numbers Natural antioxidants include vitamin E that they are serious pests of crops and and b-carotene. plants. Others carry plant viruses. antipodal cells The three haploid cells found in the embryo sac of seed-bearing aphotic zone The lower part of a body plants that migrate to the chalazal end of of water where light intensity is insufficient the sac farthest from the micropyle. These to support photosynthesis. Compare nuclei arise as a result of the three meiotic photic zone. divisions that produce the egg cell, syn- ergid cells, and polar nuclei, but they do Aphyllophorales See bracket fungi. not themselves take part in the fertilization process. They are eventually absorbed by aphyllous Without leaves. the developing embryo, and their function is uncertain. They do not occur in all flow- Apiaceae (umbellifers; carrot family)A ering plants. family of distinguished by its , which are . Most antiport A membrane transport protein species are herbaceous, but a few are in which the energy liberated by the passive shrubs and trees. The plants have a charac- movement of H+ ions across the membrane teristic odor and some species, such as is coupled to the active transport of an- parsley, fennel, and dill, are used as herbs; other solute, e.g. Na+, in the opposite di- the seeds of many others are popular rection but against its own electrochemical spices, e.g. aniseed, , coriander, potential gradient. cumin. Many species develop swollen tap- roots, and the family includes several im- antisense DNA The DNA strand that is portant root , e.g. carrot and not transcribed. In transcription the DNA parsnip; the leaf stalks of celery are also double helix unwinds and only one of the eaten. strands acts as the template for messenger RNA synthesis. The phenomenon in which the presence of a growing apical bud A chemical that re- on a plant inhibits the growth of lateral duces . Antitranspirants may buds. It is controlled by the interactions of work by depositing a waterproof film over plant hormones, particularly AUXIN (pro- the stomata or by inducing stomatal clo- duced by the shoot tip) and ABSCISIC ACID. sure. apical The actively dividing plant A plant that is adapted to pro- cells constituting the growing point at the vide a home for . Such plants usually tip of the root or stem in vascular plants. have one or more domatia, cavities in New cells are cut off on the lower side to stems, leaves or roots that house ants. form new stem tissue at the stem apex, and These may be elaborate systems of cavities on both sides in the root apex to form root

18 tissue and a protective root cap. The apical Apomixis is comparable to the conditions in the lower plants consist of of apogamy and apospory, which are seen one cell only, as in the ferns, but become in many ferns. It includes the process more complex and consist of groups of whereby a diploid cell of the nucellus de- cells in the higher plants. See histogen velops into an embryo giving a diploid seed theory; –corpus theory; meristem. with a genetic constitution identical to the parent. Another form of apomixis in which aplanospore A nonmotile spore, char- seeds develop from unfertilized gametes acteristic of the pin molds and certain can also be termed . Seeds green algae of the phyla Clorophyta and produced in this way may be either haploid Chrysomonada. It is an asexual spore or diploid depending on whether or not the formed in a and is usually megaspore mother cell undergoes meiosis. thick walled. Often, in the process termed , entry of the male gamete is required to apocarpous See carpel. stimulate the development of the female gamete, even though nuclear fusion does apocarpy An ovary made up of unfused not occur. Such cases of apomixis are diffi- carpels, as in the buttercup (Ranunuculus). cult to distinguish from true sexual repro- Compare syncarpy. duction. Compare amphimixis. See also apogamy; apospory; parthenogenesis; sex- apoenzyme An ENZYME whose cofactor ual reproduction. or prosthetic group has been removed (e.g. via dialysis) rendering it catalytically inac- apoplast The system of cell walls and in- tive. It is the protein part of a conjugate en- tercellular material extending through a zyme. When combined with its prosthetic plant body and along which water contain- group (coenzyme) it forms a complete en- ing mineral salts, etc. can move passively. It zyme (holoenzyme). Examples include the is an important pathway for movement of cytochromes, which require ferrous or fer- these substances outside the xylem, for ex- ric ions for activity. ample across the root . Compare . apogamy In certain ferns, algae, and fungi, the development of the sporophyte apospory The development of the ga- directly from a cell of the gametophyte, so metophyte directly from the cell of a sporo- fusion of gametes is bypassed. It frequently phyte, thus bypassing meiosis and spore occurs in that have been production. Gametophytes produced in produced aposporously and are thus this manner are thus diploid instead of diploid. The term also describes the devel- haploid. If such gametophytes produce fer- opment of an unfertilized female gamete tile gametes, the resulting sporophyte is into the sporophyte, a phenomenon de- then tetraploid, and large polyploid series scribed as PARTHENOGENESIS. See apospory; may subsequently be developed. Apospory apomixis. is found in some mosses and ferns. In an- giosperms, a diploid embryo may develop apomixis Any form of asexual repro- from a cell of the nucellus or ; this duction. The term is sometimes used in a is also apospory. See also apogamy; restricted sense to describe a modified form apomixis. of reproduction by plants in which seeds are formed without fusion of gametes, a apothecium See ascoma. process also termed agamospermy. A plant that reproduces in this way is called an apposition The deposition of successive apomict, e.g. many species of Rubus. layers of on the inner surface of a Apomicts are often polyploids that would plant cell wall after elongation is com- have difficulty reproducing sexually. pleted, resulting in an increase in thickness

19 archaebacteria of the wall. These extra layers are called archaebacteria See Archaea. the . Compare intussus- ception. Archean (Archeozoic) See Precambrian. appressorium (pl. appressoria) In many The female sex organ of parasitic fungi, a flattened hyphal structure the mosses, ferns, clubmosses, horsetails, that presses closely against the host epider- most gymnosperms, and some red and mis. From its undersurface a narrow infec- brown algae. It is a multicellular flask- tion hypha or penetration tube pushes shaped structure made up of a narrow neck through the cell wall or intercellular spaces and a swollen base (venter) that contains and develops into a series of hyphae or a the female gamete. Compare antheridium. . archesporium The single cell or group aquaporin (water pore) A protein chan- of plant cells in the sporophyte from which nel in the plasma membrane that facilitates spore mother cells may eventually develop water movement across the membrane. in a sporangium. Aquaporins can open or close to increase or decrease water flow. Arecaceae (Palmae; palms)A family of monocotyledonous plants found mainly in Archaea (archaebacteria) In the Five tropical regions. The typical growth form Kingdoms classification scheme, a sub- is an with unbranched trunk kingdom of the BACTERIA, but in most mod- and a crown of large spirally arranged ern classifications, a in its own leaves at the apex. The trunk is often right. It contains bacteria-like organisms sheathed in old leaf bases. Some palms, with distinctive ribosomal RNA whose nu- e.g. the doum palms (Hyphaene), branch cleotide sequences differ significantly from dichotomously. Many palms grow in the those of other prokaryotes. Because RNA understorey of rainforests in Asia and the is considered to evolve very slowly, these Americas. differences are considered important in as- signing the Archaea the status of domain. arginine An AMINO ACID found espe- The shape and structure of their ribosomes cially in the histone proteins that surround are more similar to those of DNA in chromosomes. than to bacterial ribosomes. Whether they are more closely related to the eukaryotes aril A brightly colored fleshy or hairy than to the bacteria is hotly debated. They outgrowth from the funicle at the base of appear to have a mixture of prokaryotic the ovule that may partly or completely and eukaryotic genes. Like eukaryotes, but cover the seed. The mace-yielding out- unlike bacteria, they have introns in their growth around the fruit of the is an transfer RNA, but like bacteria they have example, as is the red fleshy cup surround- polycistronic operons (gene regulators). ing a yew seed (Taxus baccata). See also Archaean cell walls lack murein, a com- caruncle. mon component of bacterial cell walls, and their lipids have ether links rather than artificial parthenogenesis See partheno- ester links. genesis. The Archaea contain many organisms of extreme environments, such as hy- ascocarp See ascoma. drothermal vents, hot springs, saline sedi- ments, volcanic craters and boiling muds. ascogonium (pl. ascogonia) The female They include methane-generating organ- gametangium of certain fungi of the phy- isms (methanogens), sulfate reducers, and lum (e.g. Erysiphe, Eu- extremophiles. rotium).

20 ATP ascoma (ascocarp) The fruiting body of cial reproductive structures. It occurs in most fungi of the phylum Ascomycota, in many plants, usually by vegetative propa- which the asci are borne. gation or spore formation; in unicellular organisms usually by fission or budding; ascomycete See Ascomycota. and in some algae by fragmentation. Such ‘offspring’ are genetically identical clones Ascomycota (sac fungi; ascomycetes)A of their parents. See apomixis. large phylum of fungi characterized by their distinctive reproductive structure, the asparagine An AMINO ACID formed by ASCUS. In some species the asci are cylindri- the addition of ammonia to aspartic acid. cal, and the ascospores are discharged vio- This is a way of removing ammonia, which lently as a result of hydrostatic pressure is highly toxic, from living cells. As- building up inside the ascus. In others, the paragine can later be broken down and the asci are globular, and the ascospores are re- nitrogen used in amino acid synthesis. leased passively. The tip of the ascus may open through a simple pore, or it may have aspartic acid An AMINO ACID formed by a cap. Most ascomycotes form branching transamination, in which the amino group septate hyphae, but some are yeastlike. of glutamic acid is transferred to ox- Ascomycotes are found in a wide range aloacetic acid. Aspartic acid can be broken of habitats. Some are saprobes, such as down in the KREBS CYCLE. It is an important blue and green molds (e.g. Neurospora), precursor of several amino acids, and also morels, and truffles, which are important acts as a source of the amino group in var- decomposers. There are also parasites, ious metabolic reactions. such as ergot of rye (Claviceps purpurea), peach leaf curl (Taphrina), the powdery association A climax mildews (Erysiphales), and Fusarium wilt. named according to the dominant type of A few are symbionts in and mycor- species. Examples are heath associations rhizas. The yeasts are economically impor- and coniferous forest associations. See also tant in fermentation processes such as consociation. brewing (Saccharomyces cerevisiae), wine making, and bread making. Some yeasts assortative mating Sexual reproduc- are parasites of humans, e.g. Candida albi- tion in which the pairing of male and cans; others, such as Penicillium, are female is not random. If similar pheno- sources of antibiotics. types (e.g. plants with similar flower shapes and color that attract the same in- ascorbic acid See vitamins. sects) breed together, this a positive assor- tative mating; it may lead to INBREEDING. ascospore One of (usually) eight hap- This is likely in insect-pollinated plants, loid spores produced inside an ASCUS by where the insect looks for similar colors members of the phylum Ascomycota. and guide markings. Negative assortative mating occurs when dissimilar ascus (pl. asci) The spore-producing cell breed together, leading to OUTBREEDING. of fungi of the phylum ASCOMYCOTA. It is a saclike structure that is formed either aster See centriole. singly or in large numbers in ascomata. After meiosis the ascus contains four or (Compositae) A large family eight haploid ASCOSPORES that are liberated of flowering plants, with some 21 000 through a pore at the end of the sac. species found worldwide and including al- most every kind of growth form, but The formation mainly herbs. The leaves, which may be of new individuals from a single parent simple or compound, are usually alternate, without the production of gametes or spe- occasionally opposite or whorled, and

21 ATPase often in basal rosettes. Latex is present in tions of an individual organism or a single some genera, e.g. lettuce (Lactuca). Tap- species with the living and nonliving com- roots are common. Cultivated composites ponents of its environment. Compare syn- include sunflower (Helianthus annuus), a ecology. source of and seeds; Jerusalem arti- choke (H. tuberosus), grown for food; autodiploid 1. A diploid plant produced globe artichoke (Cynara scolymus), whose by doubling the chromosomes of a haploid young flower heads are eaten; salsify plant. Such a plant is homozygous for (Tragopogon porrifolius), whose roots are every allele. eaten; chicory (Cichorium intybus), used as 2. A gamete containing a diploid number a substitute for coffee; lettuce (Lactuca of chromosomes, rather than the usual sativa); and Chrysanthemum cinerari- haploid number. Such a gamete may be ifolium and C. coccineum, which are produced by tetraploid tissues, or it may be sources of the insecticide pyrethrum. the result of faulty meiosis. Two diploid gametes may fuse to give a tetraploid zy- astrosclereid An irregularly branched gote, or gamete may develop directly by SCLEREID found in the mesophyll of leaves parthenogenesis. of certain dicotyledons. autoecious Denoting rust fungi that re- ATP (adenosine triphosphate) The uni- quire only one host species to complete the versal energy carrier of living cells. Energy various stages of their life cycle. Examples from respiration or, in photosynthesis, are Puccinium antirrhini found on antir- from sunlight is used to make ATP from rhinum and the mint rust (Puccinia men- ADP. It is then reconverted to ADP in var- thae). Compare heteroecious. ious parts of the cell, the energy released being used to drive cell reactions. ATP is a autogamy Self-fertilization in plants. nucleotide consisting of adenine and ribose This restricts genetic variation but allows with three phosphate groups attached. Hy- isolated individuals to reproduce. It is drolysis of the terminal phosphate bond re- found particularly in pioneer weed species leases energy (30.6 kJ mol–1) and is and in such as the , coupled to an energy-requiring process. where insect vectors are rare. Very few Further hydrolysis of ADP to AMP some- plants rely exclusively on autogamy. Com- times occurs, releasing more energy. The pare allogamy. pool of ATP is small, but the faster it is used, the faster it is replenished. autolysis The self-destruction of cells by digestive enzyme activity. It is the final ATPase (adenosine triphosphatase ) An stage of cell senescence resulting in com- enzyme that catalyzes the hydrolysis of plete digestion of all cell components. See ATP to ADP and inorganic phosphate. lysosome. Found in every cell, it is responsible for the release of energy to power metabolic reac- autonomic movements (autogenic move- tions and active transport. In oxidative ments; spontaneous movements) Move- phosphorylation, it forms part of the en- ments of plants in response to internal zyme complex ATP synthetase, catalyzing rather than external stimuli. Examples the synthesis of ATP from ADP and inor- are cytoplasmic streaming, chromosome ganic phosphate using energy from the movement during nuclear division, and proton gradient generated by the electron- growth itself. Compare paratonic move- transport chain. ments. atropous (orthotropous) See ovule. autophagy The process by which worn- out cell organelles are enclosed in vacuoles, autecology The study of the interac- then broken down by hydrolytic enzymes

22 Azotobacter secreted into vacuole, the breakdown auxanometer An instrument designed products being reabsorbed afterward. See to measure the increase in length of a plant also lysosome. Compare autolysis. part. A growing plant is attached by thread to the end of a lever that magnifies any autopolyploidy A type of polyploidy growth movement. The opposite end of the involving the multiplication of chromo- lever is used to record a trace on a slowly some sets from only one species. Autopoly- rotating drum. ploids may arise from the fusion of diploid gametes that have resulted from the auxin Any of a group of plant hor- NONDISJUNCTION of chromosomes at meio- mones, the most common naturally occur- sis. Alternatively, like allopolyploids, they ring one being indole acetic acid, IAA. may arise by the nondisjunction of chro- are made continually in growing matids during the mitotic division of a zy- shoot and root tips. They are actively gote. Autopolyploids often have larger moved away from the tip, having various cells than the parent plants but are mor- effects along their route before being inac- phologically very similar. They may, how- tivated. They regulate the rate of extension ever, be at least partially isolated from each of cells in the growing region behind the other reproductively and occupy different shoot tip and are involved in phototropic ecological niches. Compare allopolyploidy. and gravitropic curvature responses of shoot tips (but probably not root tips) autoradiography A technique whereby moving laterally away from light and to- a thin slice of tissue containing a radioac- ward gravity. tive isotope is placed in contact with a pho- Auxins stimulate cell enlargement, tographic plate. The image obtained on probably by stimulating excretion of pro- development shows the distribution of the tons leading to acid-induced wall loosening isotope in the tissue. and thus wall extension. They help main- tain apical dominance by inhibiting lateral autosomes Paired somatic chromo- bud development. Root initiation may be somes that play no part in sex determina- stimulated by auxin from the shoot, and tion. Compare sex chromosomes. auxins have been shown to move toward the root tips. Pollen tube growth is stimu- autotetraploid An autopolyploid that lated by auxin and its production by devel- has four times the haploid number of chro- oping seeds stimulates fruit set and mosomes. See autopolyploidy. pericarp growth in fleshy fruits. It interacts synergistically with and cy- autotrophism A type of in tokinins in stimulating and which the principal source of carbon is differentiation in the cambium. A high inorganic (carbon dioxide or carbonate). auxin: ratio stimulates root Organic materials are synthesized from in- growth but inhibits regeneration of buds in organic starting materials. The process tobacco pith . It is antagonistic to ab- may occur by use of light energy (photo- scisic acid in abscission. autotrophism) or chemical energy (chemo- autotrophism). Autotrophic organisms In grass flowers (), a stiff (autotrophs) are important ecologically as bristlelike structure formed from a contin- primary producers, their activities ulti- uation of the central nerves of the LEMMAS mately supplying the carbon requirements or GLUMES. of all heterotrophic organisms. Compare heterotrophism. See chemotrophism; pho- axenic culture A culture that consists of totrophism. only one type of organism or cell, i.e. it is

23 B

Bacillariophyta See Diatoms. cluding the soil. Some derive their carbon from inorganic sources, and their hydrogen bacillus (pl. bacilli) Any rod-shaped bac- from hydrogen sulfide or other inorganic terium. Bacilli may occur singly (e.g. compounds, while other bacteria may be Pseudomonas), in pairs, or in chains (e.g. photosynthetic or saprobic. Lactobacillus). Some are motile. Bacterial cells are simpler than those of animals and plants. They lack well-defined backcross A cross between an individ- membrane-bound nuclei and histone- ual and an individual of the original parent coated chromosomes, and do not contain generation. If a homozygous dominant AA complex organelles such as is crossed with a homozygous recessive aa, and mitochondria. Their DNA is usually the F2 generation obtained by selfing the F1 located in a specific part of the cell, called would be 25% AA, 50% Aa, and 25% aa. the nucleoid. Bacteria may divide every 20 To distinguish between the phenotypically minutes and can thus reproduce very identical AA and Aa, these can be back- rapidly. They also form resistant spores. crossed to the homozygous recessive par- The Bacteria are divided into two main ent aa (a test cross). The offspring from AA groups or subkingdoms: the ARCHAEA, or ´ aa will all be identical (Aa) whereas in the archaebacteria, which often occur in ex- cross Aa ´ aa, 50% will have the dominant treme conditions, such as hot springs; and phenotype (Aa) and 50% will show the re- the EUBACTERIA, which include the vast ma- cessive character (aa). jority of bacteria. Some taxonomists con- sider these groups to be full kingdoms in Bacteria A kingdom containing all the See prokaryotic organisms, formerly called their own right. Actinobacteria; Monera or Prokaryotae. This is a large and ; myxobacteria; sulfur bac- diverse group of organisms, which, in teria. See also prokaryote. terms of numbers and variety of habitats, includes the most successful life forms. In bacterial chromosome The naked nature, bacteria are important in the nitro- DNA of bacteria and Archaea. Bacteria do gen and carbon cycles, and some are useful not have true CHROMOSOMES – linear pieces to humans in various industrial processes, of DNA complexed with histone proteins. especially in the food industry, and in tech- Their DNA is circular and is not com- niques of genetic engineering (see also plexed with proteins. It lacks the cen- ). However, there are also tromere, telomere, and other structures of many harmful parasitic bacteria that cause the chromosome. diseases such as botulism and tetanus. Oth- ers are sources of antibiotics such as strep- bacterial transformation See transfor- tomycin and erythromycin. Some bacteria mation. require oxygen for respiration, while oth- ers can survive without oxygen. There may bactericidal Used to describe a com- be at least ten times more bacteria living in pound that has a lethal effect on bacteria. cracks in the rocks deep inside the earth A bactericidal compound may act by inter- than in the surface layers of the planet, in- fering with a vital biochemical pathway, or

24 base by destroying the molecular structure of South Pacific in 1768 with Captain James the cell. Cook on the Endeavour. This voyage lasted for three years and resulted in the Any of several discovery of the unique and fauna of types of chlorophyll found in photosyn- Australia. On his return with his enormous thetic bacteria, such as the . collection of specimens, he received an in- There are seven forms, designated bacteri- vitation to meet King George III, who had ochlorophylls a–g. All are structurally sim- expressed a great interest in the expedition. ilar to of plants. The His continued friendship with the king in- absorb light at longer creased his influence in scientific circles wavelengths than chlorophyll a enabling and he became president of the Royal Soci- far-red and infrared light to be used in ety for life in 1778. Following his appoint- photosynthesis. See photosynthetic pig- ment to the post of honorary director of ments. Kew , he helped to establish a major collection of living plant species and bacteriophage (phage) A virus that in- to make Kew famous as a center of excel- fects bacteria. Phages usually have com- lence for plant cultivation. He is also fa- plex capsids (protein coats) composed of a mous for introducing the tea plant to India polyhedral head, containing the nucleic (from China) and breadfruit to the West acid (DNA or RNA), and a helical tail, Indies (from Tahiti). His extensive library through which is injected into and are housed in the British the host. Phages are usually specific to par- . ticular species or even strains of bacteria. Infection by a phage may or may not lead bark The outermost tissue of the stem to the death of the bacterium, depending and roots in woody plants, on the outside on the phage and on the conditions. All of the phellogen ( cambium). If the RNA phages cause the death of the host same phellogen functions from year to cell. After reproduction of the viral nucleic year, as in beech and oak, then the bark is acid the host cell usually undergoes lysis smooth and consists only of tissue external (LYSOGENY). Some DNA phages, called to the phloem, mainly cork. If new cork TEMPERATE PHAGES, may become integrated cambia arise every few years, as in most into the host DNA and are replicated when species of woody plants, then the bark in- it replicates. Some of these phages are cludes dead phloem and cortex as well as capable of transferring genes between bac- cork, and is termed rhytidome. As bark in- teria in a process called TRANSDUCTION. In creases in thickness, the outer layers may GENETIC ENGINEERING, nonviral DNA can split in distinctive patterns. The texture be inserted into a phage, which is then used and patterning of the bark is often charac- as a cloning vector. teristic of the species. The bark is punctu- ated by lenticels, small patches of loose bacteriostatic Used to describe a com- cells, which allow air to penetrate the living pound that prevents reproduction of bacte- tissues below. ria, but does not kill them. basal body See kinetosome. bacteroid A modified bacterial cell in a , typically a cell of the bac- base 1. ()A compound that terium Rhizobium in the root nodule of a reacts with an acid to produce water plus a leguminous plant (family ). salt. In solution, it forms ions that can react with hydrogen ions. Banks, Sir Joseph (1743–1820) British 2. (Biochemistry)A nitrogenous molecule, botanist. Banks used his considerable for- either a PYRIMIDINE or a PURINE, that com- tune to finance several important expedi- bines with a pentose sugar and phosphoric tions, including his most famous one to the acid to form a nucleotide, the fundamental

25 base pairing unit of nucleic acids. The most abundant Basidiomycota (basidiomycetes) A phy- bases are cytosine, thymine, and uracil lum of the Fungi that includes the mush- (pyrimidines) and adenine and guanine rooms and toadstools. Basidiomycotes are (). characterized by their spore-bearing struc- tures (see basidium), each of which usu- base pairing The bonding relationship ally bears four sexually produced spores between purine and pyrimidine bases in the (basidiospores) on its outer surface. In nucleotides of DNA and RNA. The pairing most basidiomycotes the basidia are borne of these bases, sustained by hydrogen on a special fertile layer, the hymenium, on bonding, is highly specific: adenine (A) on the surface of a fruiting body, the BASIDI- one strand of the nucleic acid pairs only OMA. with thymine (T) in the complementary strand of DNA, or with uracil (U) in RNA; basidiospore See Basidiomycota; basid- guanine (G) pairs only with cytosine (C). ium. The large numbers of hydrogen bonds that form between the base pairs along a DNA basidium (pl. basidia) The cell of the BA- or RNA molecule stabilize the double SIDIOMYCOTA that produces the sexual helix. The specificity of the pairing ensures spores (basidiospores). In most basidiomy- cotes the basidia develop in the BASIDIOMA that during REPLICATION and TRANSCRIP- from hyphal tips in the hymenium and may TION accurate copies are made of the ge- netic material. The number of base pairs is be club-shaped or cylindrical. a measure of the length of the DNA or basifixed Stamens in which the anther RNA molecule, which is often expressed in lobes are attached at their base to the fila- units of 100 bases (kilobases), meaning ment, and are not capable of independent 100 base pairs. movement. Compare dorsifixed; versatile. base ratio The ratio of the molar quan- basipetal Developing from the apex tities of two bases in DNA, or of one base downward so that the youngest structures pair with the other . The base are furthest from the apex. Basipetal differ- ratio of 1:1 for the bases adenine and entiation is seen in the formation of proto- thymine and the bases cytosine and gua- and metaxylem in the stem. The term may nine was the first clue to the specificity of also be applied to the movement of sub- BASE PAIRING in the nucleic acids. The base stances toward the base, for example the pair ratio, AT:GC, is constant within a movement of auxin in shoot tissues. Com- species but differs between species. pare acropetal. See also centrifugal; cen- tripetal. basic stain See staining. B chromosome (accessory chromosome) basidioma (pl. basidiomata) The fruit- An extra chromosome found in addition to ing body of the Basidiomycota, excepting the normal number of chromosomes char- the rusts and smuts, formerly called the ba- acteristic of a species (the A chromosomes). sidiocarp. It may be of a fleshy, corky, or B chromosomes are known from more spongy nature. The basidioma is composed than 20 families of plants and are also mainly of sterile tightly packed hyphae; in found in some fungi. most species there is a distinct fertile layer, the hymenium, on the surface of the basid- beet sugar See . ioma that bears the basidial hyphae which, after meiosis, give rise to the basidiospores Benedict’s test A test for the presence of (see basidium). a dissolved reducing sugar. Benedict’s solu- tion is an alkaline solution of copper(II) basidiomycete See Basidiomycota. sulfate, which is reduced to insoluble cop-

26 per(I) oxide by reducing sugars, giving a chain and the C=O and NH groups on the red precipitate. other chain. The structure is folded in reg- ular ‘pleats’. having this type of Bentham, George (1800–84) British structure are usually composed of amino botanist. Having developed an interest in acids with short side chains. The chains from the age of 17, Ben- may run in the same direction (parallel) or tham abandoned (1833) his career in law opposite directions (antiparallel). See con- to devote himself entirely to his botanical formation; secondary structure. collection and library. In 1854 he pre- sented his collection to the Royal Botanic bicollateral bundle The arrangement Gardens at Kew, where he worked for the of tissues in the vascular bundles in which rest of his life. His famous Handbook of the phloem is situated on both sides of the British Flora was published in 1858. xylem. This is thought to aid transport of Between 1862 and 1883 he collaborated nutrients in plants with long trailing stems, with Joseph HOOKER to publish Genera such as the marrow. Compare concentric Plantarum, a world flora describing 7569 bundle; collateral bundle. genera and 97 000 species, which is con- sidered his greatest achievement. biennial A plant that completes its life cycle within two years. In the first year it benthic Of, relating to, or living in the produces foliage only and photosynthe- zone at the bottom of a sea, lake, or other sizes. The food is stored during the winter See Compare body of water. benthos. in a swollen underground root or stem. In pelagic. the second year, the stored food is used to produce flowers, fruits, and seeds. Many benthos The organisms that live on the important crops, such as carrot and bottom sediments of freshwater and ma- parsnip, are biennials. Some biennials can rine ecosystems. They include plants and be induced to act as annuals and flower in algae (phytobenthos) that grow below low the first year by appropriate cold or hor- tide mark. mone treatments. Compare annual; A succulent fruit, usually contain- ephemeral; perennial. ing more than one seed, that does not burst open when ripe. The ovary wall remains bilateral symmetry The arrangement fleshy after fertilization of the ovule except of parts in an organism in such a way that for the development of a thin outer skin. A the structure can only be divided into sim- berry with a hard rind, such as a , ilar halves (mirror images) along one is called a pepo.A berry with a leathery plane. Bilateral symmetry is characteristic rind in which the seeds are separated by of most free-moving animals, where one segments, as in the citrus fruits, is called a end constantly leads during movement. In hesperidium. The epicarp and mesocarp plants, bilateral symmetry is seen particu- may be brightly colored thus attracting larly in flowers (e.g. snapdragon), the con- animals, which disperse the fruit, as in the dition being termed zygomorphy. See also . Compare . See also pericarp. radial symmetry. beta-oxidation See oxidation. binomial distribution (normal distribu- tion; Gaussian distribution) The ideal- beta-pleated sheet (b -pleated sheet)A ized distribution of a population of type of protein secondary structure in statistical values centered around a mean, which polypeptide chains run close to each when departures from the mean are due the other and are held together by hydrogen random occurrence of a large number of bonds at right angles to the main chain be- small independent effects. It represents the tween the NH and C=O groups on one probability of each value occurring and is

27 bioassay represented on a graph by a bell-shaped nonbiodegradable compounds, such as curve or histogram. most plastics. See also decomposer; pollu- tion. binomial nomenclature A system of classification introduced by Linnaeus, the The number and variety of Swedish botanist, in which each species is organisms in a given locality, community, given two names. The first is the generic or . High biodiversity is typical name, written with a capital letter, which of complex and highly productive ecosys- designates the genus to which the species tems, such as tropical rainforests, where a belongs. The second is the specific name, small area can contain many different indicating the species. The generic and spe- species of animals, plants, and other or- cific names are in Latin and are printed in ganisms. Biodiversity is often used as an in- italic type. For example, the sunflower be- dicator of the health of such ecosystems. longs to the species Helianthus annuus. See also Convention on Biological Diver- Helianthus is the generic name and annuus sity. is the specific name. See International Code of . bioenergetics The study of energy transfer in living organisms. bioassay An experimental technique for measuring quantitatively the strength of a biogenesis The theory that living things biologically active chemical by its effect on originate only from other living things as a living organism. For example, the vita- opposed to nonliving matter. The theory min activity of certain substances can be became accepted as a result of the work of measured using bacterial cultures. The in- Redi and Pasteur, who showed that dirt, crease in bacterial numbers is compared for example, does not itself produce bacte- against that achieved with known stan- ria or maggots, but that bacteria and mag- dards for vitamins. Plant growth hormones gots come only from spores or eggs already can be estimated by their effect in causing existing in the dirt. This theory satisfacto- curvature of oat . rily explains the occurrence of existing or- ganisms, but not the origins of the first Biochemical Oxygen Demand (BOD) organisms. The standard measurement for determin- ing the level of organic pollution in a sam- biogenic Describing structures pro- ple of water. It is the amount of oxygen duced as a result of the activity of living or- used by microorganisms feeding on the or- ganisms, in particular rocks (e.g. chalk, ganic material over a given period of time. limestone, diatomaceous earth) and fossils. Sewage effluent must be diluted to comply with the statutory BOD before it can be biogeochemical cycle The movement disposed of into clean rivers. of chemical elements from the physical en- vironment into organisms and back to the biochronology The measurement of geo- environment in a more or less circular logical time using biological events. Partic- pathway. If the elements concerned are es- ular strata of rocks can be approximately sential to life, this cycle is called a nutrient dated according to the presence or absence cycle, e.g. the NITROGEN CYCLE. Some of of certain fossils. The first and last appear- these cycles are regulated mainly by physi- ances of particular fossil species represent cal factors. For example, the main con- specific moments in geological time. straint on the cycle is the slow release of phosphates from rocks by weath- biodegradable Describing organic com- ering. Others, such as the nitrogen cycle, pounds that can be decomposed by bacte- involve biological feedback mechanisms, ria and other microorganisms, such as the as nitrates are incorporated into proteins constituents of sewage, as compared with and released by decomposition. In the CAR-

28 biosynthesis

BON CYCLE, photosynthetic organisms are abiotic factors that influence these. This thought to be important in balancing the may involve human management of a habi- cycle by ; carbon com- tat. Modern biological conservation may pounds are slowly released as dead organ- also include sustainable use of biological isms decompose. resources, which sometimes conflicts with the conservation of maximum diversity. biogeographical barrier A barrier to See World Conservation Union. the migration of plants and animals that prevents different populations mixing. biological control The use of natural Such barriers may be physical, such as predators or parasites, instead of chemi- mountain ranges, large rivers, lakes, or cals, to control pests. The most famous seas; or they may be climatic. successful example was the introduction of the gray moth, Cactoblastis cactorum, into biogeographical region A subdivision Australia to control the prickly pear, Op- of the Earth’s surface based on the particu- untia inermis, which was overrunning vast lar species of plants and animals present. tracts of land. The moth’s caterpillars eat There are several different schemes of bio- the of the plant. geographical regions, but the most widely recognized are: Boreal (northern Eurasia biology The study of living organisms, and North America), Paleotropical (Africa including their structure, function, evolu- and southeast Asia as far as New Guinea), tion, interrelationships, behavior, and dis- Neotropical (South America, except the far tribution. south), Australasian, Cape (South Africa), and Antarctic (New Zealand and southern The weight or volume of living South America). These biogeographical re- organisms of one particular species per gions are subdivided into biogeographical unit area (species biomass), or of all the provinces. species in the community (community bio- mass). See also pyramid of biomass. The study of the distrib- ution of plants and animals, both past and biome A major regional community of present, and its interpretation in terms of plants and animals with similar life forms ecology, in particular the distribution of and environmental conditions. It is the world vegetation types and the interactions largest geographical biotic unit, and is between humans and their environment. named after the dominant type of vegeta- tion, such as tropical rain forest, grassland, The creation and main- or coral reef. tenance of databases of biological informa- tion. With the rapid expansion of biometry The application of to biological knowledge and research, sophis- the analysis of biological phenomena. ticated computerized information-retrieval systems are necessary to speed up access to biorhythm A periodic physiological or biological information. behavioral change that is controlled by a biological clock. Circadian rhythms are an biological clock The internal mecha- example. nism of an organism that regulates CIRCA- DIAN RHYTHMS and various other periodic The part of the earth and its cycles. atmosphere that is inhabited by living or- ganisms. The Earth is made up of three biological conservation The preserva- concentric zones: the lithosphere (solid tion of the world’s natural biological diver- earth), the hydrosphere (water layer), and sity and natural habitats and the the atmosphere. The biosphere includes maintenance, as far as is possible, of the parts of all three of these zones. It is some-

29 biosystematics times considered to represent a single yolk, kidney, liver, and yeast being good ecosystem, the ecosphere. sources. Biotin is required as a coenzyme for carboxylation reactions in cellular me- biosynthesis Chemical reactions in tabolism. which a living cell builds up its necessary molecules from other molecules present. biotrophic Obtaining nutrients from See anabolism. the living tissues of another organism. Compare autotrophism. biosystematics The area of in which experimental taxonomic tech- biotype 1. A naturally occurring group niques are applied to investigate the rela- of individuals all with the same genetic tionships between taxa. Such techniques composition, i.e. a clone of a pure line. include serological methods, biochemical Compare ecotype. analysis, breeding experiments, compara- 2. A physiological or form within a tive morphology, cytological examination, species that is morphologically identical and RNA analysis, in addition to the more with it, but differs in genetic, physiological, established procedures of comparative biochemical, or pathogenic characteristics. anatomy. Evidence from ecological studies may also be brought to bear. See also mol- bipinnate Describing a pinnate leaf ecular systematics. whose leaflets are pinnately subdivided, e.g. sensitive plant (). biota The plants, animals, and other liv- ing organisms that live together in a par- bisexual See hermaphrodite. ticular place. Biuret test A test for the presence of biotechnology The application of tech- proteins, in which sodium hydroxide is nology to biological processes for indus- added to the test solution, followed by trial, agricultural, and medical purposes. drops of 1% copper(II) sulfate solution. A For example, bacteria such as Penicillium violet color indicates the presence of pro- and Streptomycin are used to produce anti- teins or peptides (excluding dipeptides). biotics and fermenting yeasts produce alco- Amino acids have no effect. hol in beer and wine manufacture. Recent developments in genetic engineering have bivalent Describing any pair of homolo- enabled the large-scale production of hor- gous chromosomes when they pair up mones, blood serum proteins, and other during MEIOSIS. Pairing of homologous medically important products. Genetic chromosomes (synapsis) commences at one modification of farm crops, and even live- or several points on the chromosome and is stock, offers the prospect of improved pro- clearly seen during PACHYTENE of meiosis I. tection against pests, or products with novel characteristics, such as new flavors black earth See chernozem. or extended storage properties. See also genetic engineering. bladder 1. A modified leaf, found on the stems of members of the bladderwort fam- biotic environment The biological fac- ily, that develops into a distended structure tors acting on an organism, which arise for trapping small invertebrates. The blad- from the activities of other living organ- der is thin-walled and closed by a one-way isms, as distinct from physical factors. valve. 2. An air-filled sac found in large numbers biotin A water-soluble vitamin generally in the thalli of certain seaweeds (e.g. blad- found, together with VITAMINS in the B derwrack). group, in the vitamin B complex. It is widely distributed in natural foods, egg blending inheritance Inheritance in

30 bracteole which the characters of the parents appear iomycote fungi having fleshy mushroom- to blend into an intermediate form in the shaped fruiting bodies in which the hyme- offspring. It is seen in characters controlled nium (spore-bearing surface) lines the by a single gene whose alleles are codomi- inner surface of vertical tubes (pores) on nant, as in pink flowers of Antirrhinum, the underside of the cap (e.g. Boletus). and also in characters such as yield, which Compare Agaricales. are determined by many genes. Until the re- discovery of Mendel’s work, it was bolting Premature flowering and seed thought that all characteristics become production. Biennial plants will sometimes blended in the offspring. See Mendel’s bolt in their first year; this seriously re- laws. duces their yield. blepharoplast See kinetosome. borax carmine A dye used to stain nu- clei for light microscopy. blight A plant disease in which serious leaf damage appears suddenly. It is usually Boreal floral region (Holarctic floral due to fungi or oomycetes, such as the province) A floral region that consists of potato blight (Phytophthora infestans), or northern Eurasia and North America. See occasionally to bacterial infections, as in floral province. fireblight of pears, which is caused by Er- winia amylovora. boron A MICRONUTRIENT found in very low concentrations in plant tissues but es- bloom A sudden growth of microorgan- sential for plant growth. It is involved in isms in a freshwater or marine ecosystem, phenol metabolism and membrane func- often known as an algal bloom. This may tion. It is implicated in the uptake of cal- occur naturally in spring, when growth of cium, the development of apical meristems, microorganisms is for a time unchecked by and pollen germination. herbivores. It may also be brought on by nutrient enrichment of the water, often due botanical 1. Relating to the study of to fertilizers being washed off agricultural plants. land, or to seawater entering the marine 2. A cultivated plant of an unaltered wild environment. species. 3. An insecticide derived from a plant, e.g. blue-green bacteria See Cyanobacte- pyrethrum (from Chrysanthemum flow- ria. ers). BOD See Biochemical Oxygen Demand. botany The scientific study of plants. bog A plant community found in wet bracket fungi A member of the basid- acid areas where rainfall is high, the at- iomycote order Aphyllophorales, whose mosphere is continually moist, and decom- fruiting bodies (basidiomata) typically pro- position rates are slow, favoring PEAT ject from the trunks of trees like shelves or formation. Bogs are common in uplands domed brackets. These fruiting bodies are and western areas of north temperate and usually tough, sometimes woody. The some southern temperate regions. Typical spores are produced on small teeth or bog plants of north temperate regions in- lamellae rather than gills. clude the bog mosses (Sphagnum), grasses (Eriophorum), rushes (Juncus), A modified leaf that develops sedges (Carex), and certain heaths (e.g. below a flower or an inflorescence on its Calluna vulgaris, Erica tetralix). axis. It may be reduced or highly colored, as in the scarlet of Poinsettia. In the Boletales (boletes) An order of basid- daisy family the numerous bracts which

31 bract scale subtend the inflorescence are known as an Brown, Robert (1773–1858) British involucre. In the grasses bracts are borne in botanist. Brown studied medicine and pairs below the and are called served as a military medical officer. During glumes. Each individual floret of the a visit to London he was introduced to Sir is also subtended by two bracts, , who encouraged him to the lower being called the lemma and the apply for the post of naturalist on an expe- upper the palea. dition (1801–06) to survey the coast of Australia in the Investigator. Having re- bracteole A small leaflike organ on the turned with some 4000 plant specimens, he of a flower in an inflorescence that spent the next few years on their classifica- is itself subtended by a bract, as in the blue- tion. His painstaking observations led him bell. Compare bract. to discover many important features of plants. He was the first to observe and bract scale The structure found in large name the nucleus of cells and to recognize numbers in the female cone of gym- its importance. He also observed that gym- nosperms, each bearing an ovuliferous nosperm were not enclosed in an scale in its axil. ovary wall, leading to the establishment of one of the basic differences between an- branch A lateral stem arising from an- giosperms and gymnosperms. In 1827, other stem of the same form. upon observing a suspension of pollen grains in water, he discovered BROWNIAN Brassica A genus of plants of the cab- MOTION. bage family () that includes See Phaeophyta. many vegetables and plants. brown algae brown earth The type of soil found Brassicaceae (Cruciferae)A family of under forests in temperate cli- annual, biennial, or perennial plants, mates. It is rich in humus and slightly acid, mainly herbs, that includes many commer- due to leaching of lime. When cleared, it cially important crops, especially the BRAS- provides good fertile agricultural soil. SICA species; salad vegetables, such as cress Sinapis Eruca ( ), rocket ( ), and watercress Brownian motion The random motion (Rorippa); and ornamental plants, e.g. of microscopic particles due to their con- Alyssum, Aubretia, wallflower (Erysi- tinuous bombardment by the much smaller mum), stock (Matthiola), and honesty (Lu- and invisible molecules in the surrounding naria annua). liquid or gas. Particles in Brownian motion can often be seen in colloids under special breeding system The way in which a conditions of illumination. species breeds, including the method of mate selection, genetic of Bryophyta A phylum of simple, mainly sex and incompatibility, method of mate terrestrial, plants called mosses, commonly selection or pollination, and other factors found in moist habitats. They show a het- that place limits on the genetic diversity of eromorphic alternation of generations, the the offspring. See self-incompatibility; sex gametophyte being the dominant genera- determination; sexual reproduction. tion. When mature the gametophyte shows differentiation into stem and leaves but breeding true (pure breeding) The pro- there are no roots or vascular tissues, but duction of offspring whose genetic makeup mosses do possess elongated conducting for particular alleles is identical to that of cells. The sporophyte, which is wholly de- their parents. This requires both parents to pendent on the gametophyte, is simply a be homozygous for same alleles in ques- spore capsule borne on a stalk. The spores tion. germinate to produce a slender, filamen-

32 buttress root tous haploid , which produces reproduction. The stem is reduced to a buds that develop into leafy plants. disklike structure, bearing concentric lay- Formerly, the Bryophyta also included the ers of fleshy leaf bases that comprise the liverworts and hornworts, which in the food store. Each leaf has a bud at its base Five Kingdoms classification are now that is able to develop into a subsidiary placed in separate phyla (HEPATOPHYTA and . The whole bulb is protected by scale ANTHOCEROPHYTA). The collective name leaves and adventitious roots arise from for mosses, liverworts, and hornworts is the base of the stem. In spring or summer bryophytes. one or more buds grow and produce leaves and flowers, exhausting the food supply. bud 1. A compacted undeveloped shoot The new leaves photosynthesize and food consisting of a shortened stem and im- is stored in their bases thus giving rise to a mature leaves or floral parts. The young new bulb. If more than one bud develops, leaves are folded about the growing tip, eventually more than one bulb develops. and the outermost leaves may be scaly and See perennating organ; asexual reproduc- reduced to protect the growing point. A tion. bud has the potential to develop into a new shoot or flower. Terminal buds are formed bulbil A small bulblike organ of asexual at the stem or branch tip. Axillary or lat- reproduction that may form in a leaf axil, eral buds develop in the leaf axils. Buds can as in some lilies, or may replace flowers develop adventitiously on other parts of a in an inflorescence, as in the wild leek (Al- plant and are sometimes a means of asex- lium amploprasum). It may detach itself ual reproduction. from the parent plant and become inde- 2. The swelling produced by a unicellular pendent. organism that is reproducing asexually by BUDDING. bundle sheath The ring of parenchyma- tous or sclerenchymatous tissue, usually budding 1. The production of buds on one cell thick, that surrounds the vascular plants. bundle in an angiosperm leaf. The indi- 2. A type of grafting in which the grafted vidual cells are closely packed with no part is a bud. apparent intercellular spaces, and con- 3. A type of asexual reproduction in which duct water and solutes from the vas- a new individual is produced as an out- cular bundle to the surrounding tissues. growth (bud) of the parent organism. It oc- Chloroplasts may be present in the bun- curs in some unicellular fungi, such as the dle sheath, and are thought to be con- yeasts. nected with starch storage in the tropical grasses. buffer A solution that resists any change in acidity or alkalinity. Buffers are impor- bunt (stinking smut)A seed-borne dis- tant in living organisms because they guard ease of wheat (Avena) and other grasses, against sudden changes in pH. They in- caused by the basidiomycote Tilletia. volve a chemical equilibrium between a weak acid and its salt or a weak base and bur A kind of pseudocarp (false fruit) in its salt. In biochemistry, the main buffer which the fruit is encased in a barbed in- – systems are the phosphate (H2PO4 / volucre. The barbs catch in the fur of pass- 2– HPO4 ) and the carbonate (H2CO3/ ing animals and the fruits are thus – HCO3 ) systems. dispersed. Examples include burdock (Arc- tium) and cocklebur (Xanthium). bulb A modified shoot that acts as an organ of and often vegetative butenedioic acid See succinic acid.

33 C

Cactaceae (cacti)A large family of suc- (Ca2+) cause cross-linking of adjacent culent xerophytic dicotyledonous plants, chains of pectic acid, thus strengthening found mainly in warm dry parts of the the cell wall. Americas; some species, notably the prickly pears (Opuntia), have been intro- callose An insoluble that duced to the Mediterranean and Australia. is laid down around the perforations in sieve plates, at wound or infection sites, Caenozoic See Cenozoic. and by growing pollen tubes. As sieve tubes age, the callose layers become thicker, Cainozoic See Cenozoic. eventually blocking the SIEVE ELEMENT ei- ther seasonally or permanently. At wound calcareous soil (basic soil; alkaline and infection sites callose provides a phys- soil) A soil with a pH greater than 6.0. ical barrier. When pollen germinates on a Calcareous soils occur where lime accumu- stigma the growing pollen tubes deposit lates in the surface layers, usually as a re- callose as small plugs, but if the plant is sult of a calcareous bedrock such as self-incompatible, tubes from its own limestone, or a calcareous sand deposit pollen will deposit more callose, which near coasts. They are most common in re- then blocks the tube, preventing fertiliza- gions of light rainfall and are usually rich tion. See self-incompatibility. in ions and often other elements. See rendzina. callus 1. A mass of undifferentiated parenchyma. If a plant is injured, the sur- calcicole Describing plants that thrive rounding uninjured parenchyma cells form on neutral to alkaline soils rich in calcium a suberin-impregnated callus, sometimes, carbonate, i.e. chalk or limestone soils, sealing off the wound. such as marls. Compare . 2. In tissue cultures, cells are first de- differentiated to form a callus that can then calcifuge Describing plants that grow be induced to form adventitious shoots and on soils containing very little calcium car- roots. See also graft. bonate, such as sandy or peaty soils and cannot grow on basic substrates such as calmodulin A Ca2+-binding protein most chalk and limestone soils. Compare found in high levels in eukaryote cells. It calcicole. mediates many calcium-regulated pro- cesses in cells. When bound to calcium calcium Symbol: Ca. An essential min- ions, calmodulin changes shape and binds eral for and plant growth. One of to other proteins, such as kinases and phos- its most important functions in plants is as phatases, that are involved in signaling an intracellular messenger with steep gra- pathways, activating or inactivating them. dients of calcium concentration forming See cascade. across membranes with a system of cal- cium pumps. It is an important constituent Calvin, Melvin (1911–97) American of the middle lamella between plant cell biochemist noted for his investigations of walls as calcium pectates. The calcium ions the DARK REACTIONS of photosynthesis

34 Candolle, Augustin Pyrame de

(using radioactive carbon). He was bark. See ; wood. See awarded the 1951 Nobel Prize for Chem- also annual ring; meristem. istry. Cambrian The earliest period of the Pa- Calvin cycle See photosynthesis. leozoic era, about 590–510 million years ago. It is characterized by the appearance 1. The root cap: a conical- of algae and a proliferation of marine in- shaped structure that is formed by the ac- vertebrate animal forms, but no true tivity of the meristem at the root apex and plants. In Britain, Cambrian rocks are forms a protective cap around the root tip. found in Wales and North-West Scotland. It is constantly replaced by newly formed See also geological time scale. cells as the older tissue is sloughed off with growth of the root through the soil. Camerarius, Rudolph Jacob (1665– 2. A layer of cells derived from the venter 1721) German botanist. Camerarius was of the archegonium that covers the devel- professor of medicine and director of the oping sporophyte. In mosses and liver- botanic gardens at Tubingen. In his exper- worts it ruptures as the seta elongates, iments on sexuality in dioecious plants being taken up as a hood over the capsule (with male and female flowers on separate in mosses, and forming the sheath of tissue plants) and monoecious plants (with male at the base of the seta in liverworts. The and female flowers on the same plant), he presence of the calyptra is necessary for the demonstrated that if the female flowers proper development of the capsule in were isolated from the male flowers or the mosses and the embryo in ferns. male flowers were removed, the seeds pro- duced by the female flowers were always calyptrogen A layer of meristematic sterile. His results were published in De cells covering the root apical meristem in sexu plantarum (On the Sex of Plants) in some plants (e.g. grasses) that gives rise to 1694. He also described the process of pol- and maintains the root cap. lination and showed that the stamens were the male organs and that the stigma and calyx (pl. calices) The outermost part of style were the female organs of a plant. a flower, enclosing the other floral parts during the bud stage. It consists of , campylotropous (orthocampylotropous) often green but sometimes brightly col- See ovule. ored. The symbol K denotes the calyx in the . If the sepals are joined Canada balsam A yellowish with at their lateral margins a calyx tube is similar optical properties to glass, used as a formed, the mouth of which may be ex- mounting medium for microscope slides. tended into lobes or teeth. Candolle, Augustin Pyrame de (1778– CAM See crassulacean acid metabolism. 1841) Swiss botanist. In 1806 the French government invited Candolle to carry out a cambium The ring of dividing cells botanical and agricultural survey of France (meristem) responsible for lateral growth that took him six years to complete. in plants. The primary cambium is found in His Théorie élémentaire de la botanique the stem and root between the phloem and (Elementary Theory of Botany; 1813) in- xylem cells, and by division gives rise to the troduced the term ‘taxonomy’ for classifi- secondary phloem and xylem in woody di- cation. Candolle based his classification on cotyledonous plants. There are two zones resemblances between the plan of symme- of cambium: the VASCULAR CAMBIUM, which try of the sex organs in various plants, even produces secondary xylem and phloem, though these were often disguised by loss, and the PHELLEM (), which fusion, or degeneration. His idea was that produces secondary cortex and cork or even when these parts appeared to be dif-

35 cane sugar ferent, they were in fact homologous and pores as in snapdragon (Antirrhinum), or had a common ancestry. This classification by complete splitting of the capsule (e.g. replaced that of LINNAEUS, eventually being Iris). improved upon by BENTHAM and HOOKER. 2. The structure within which spores of the He is also noted for having recognized the sporophyte generation of mosses and liver- influence of the soil on the distribution of worts are formed. It is borne at the end of vegetation. a stalk (the seta), and ruptures to release the spores. cane sugar See sucrose. 3. The mucilaginous covering often found around the cell membrane in bacteria. canker A localized plant disease in which there is considerable NECROSIS of the A class of compounds cortex tissue, especially in woody plants, occurring widely in nature and having the which becomes surrounded by layers of general formula type Cx(H2O)y. (Note that callus tissue. An example is apple canker, although the name suggests a hydrate of caused by the Nectria galligena. carbon these compounds are in no way hy- drates and have no similarities to classes of canopy In a woodland, forest, or hydrates.) Carbohydrates are generally di- community, the uppermost layer of vegeta- vided into two main classes: SUGARS and tion, formed from the branches of the . Simple sugars are mono- tallest plants. Compare ground layer. saccharides – single sugar units, usually with five (pentoses) or six (hexoses) carbon capillarity (capillary action) The rise or atoms. Ribose and glucose are examples. fall of liquids in narrow tubes as a result of Oligosaccharides are of two to the surface tension of the liquid, which ten monosaccharide units. Polysaccharides causes the water to adhere to solid sur- are long linear or branched chains of faces, such as soil particles or the walls of monosaccharide units, which have a struc- xylem vessels. When water in the soil or in tural or storage role in the plant, since they the xylem is put under tension, due to the tend to be insoluble and relatively unreac- of water from a particular site tive. (which creates a water potential gradient in Carbohydrates are both stores of en- the direction of the evaporation), capillar- ergy and structural elements in living sys- ity enables water to overcome the cohesive tems. Living cells are able to build up forces within the water and move against polysaccharides from simple units (an- gravity in response to the water potential abolism) or break the larger units down to gradient. It is a key feature in the move- more simple units for releasing energy (ca- ment of water through the xylem and tabolism). through the finer pores of the soil. carbon Symbol: C. An essential element capitulum (pl. capitula) See inflores- in plant and animal nutrition that occurs in cence. all organic compounds and thus forms the basis of all living matter. It enters plants as capsid The protein coat of a VIRUS, carbon dioxide and is assimilated into car- which surrounds its nucleic acid. Capsid bohydrates, proteins, and fats, forming the proteins are involved in recognition and in- backbones of such molecules. The element fection of the host and can be used to iden- carbon is particularly suited to such a role tify the virus by serological means or as it can form stable covalent bonds with amino acid sequencing. other carbon atoms, and with hydrogen, oxygen, nitrogen, and sulfur atoms. It is capsule 1. A dry dehiscent fruit that is also capable of forming double and triple formed from several fused carpels. The nu- bonds as well as single bonds and is thus a merous seeds may be released through particularly versatile building block. Car-

36 carotene bon, like hydrogen and nitrogen, is far carboxylic acid An organic compound more abundant in living materials than in of general formula RCOOH, where R is an the Earth’s crust, indicating that it must be organic group and –COOH is the carboxy- particularly suitable to fulfill the require- late group. Many carboxylic acids are of ments of living processes. See also carbon biochemical importance. Those of particu- cycle. lar significance are: 1. The lower car- boxylic acids (such as citric, succinic, The circulation of carbon fumaric, and malic acids), which partici- between living organisms and the environ- pate in the respiration in the KREBS CYCLE, ment. The carbon dioxide in the atmos- and in CRASSULACEAN ACID METABOLISM. phere is taken up by autotrophic organisms Some plants accumulate high concentra- (mainly green plants) and incorporated tions of certain carboxylic acids: citrus into carbohydrates. The carbohydrates so fruits contain a lot of citric acid, and produced are the food source of the het- rhubarb contains oxalic acid. erotrophs (mainly animals). All organisms 2. The higher acids, which are bound in return carbon dioxide to the air as a prod- LIPIDS. These are also called fatty acids, al- uct of respiration and of decay. The burn- though the term ‘’ is often used to ing of fossil fuels also releases CO2. In describe any carboxylic acid of moderate- water, carbon, combined as carbonates to-long chain length. The fatty acids con- and bicarbonates, is the source for photo- tain long hydrocarbon chains, which may synthesis. be saturated (no double bonds) or unsatu- rated (C=C double bonds). Plant fatty carbon-14 dating See radiometric dat- acids are often unsaturated: oleic acid is the ing. commonest example.

Carboniferous The second most recent carcerulus (pl. carceruli) A type of cap- period of the era, some 355–280 sular fruit that breaks up at maturity into million years ago. It is characterized by the one-seeded segments or nutlets. The evolution on swampy land of amphibians, carcerulus is typical of the Lamiaceae fam- a few primitive early reptiles, and trees. ily, e.g. deadnettle (). Aquatic life included sharks and coela- canths. The period is named after the ex- (insectivorous plant) tensive coal deposits that formed from the A plant adapted to supplementing photo- remains of vast forests of swamp plants. synthesis by obtaining nutrients, especially These were dominated by tree-sized lyco- nitrates, from insects and other small ani- phytes (related to present-day clubmosses) mals. The plants have various means of and sphenophytes (horsetails), ferns, and trapping and killing the insects, which are some early seed plants (mainly pteri- then digested by enzymes secreted by the dosperms or ‘seed ferns’) some of which plant. Examples include butterworts (Pin- when rapidly buried and compressed over guicula), with slippery inrolling leaves; many millions of years formed the coal sundews (Drosera), with sticky glandular measures. See also geological time scale. hairs that close over the trapped insects; Venus’ fly trap (Dionaea muscipula), with carboxylase An enzyme that catalyzes hinged leaves that snap shut over the vic- the incorporation of carbon dioxide into a tim, triggered by sensitive hairs; and substrate molecule. Carboxylases are found pitcher plants, with leaves modified to in yeasts, bacteria, plants, and animals. form pitchers into which small animals fall The most important carboxylase in plants and drown. is ribulose bisphosphate carboxylase (ru- bisco), which catalyzes the carboxylation carotene A pigment, exam- of ribulose bisphosphate, the ‘carbon- ples being lycopene, and a- and b-carotene. fixing’ first step in the Calvin cycle. The latter compounds are important in an-

37 imal diets as a precursor of vitamin A. Red, funicle and are seen in castor oil seeds yellow, or orange carotene pigments occur (Ricinus) at the micropyle and in violet in all photosynthetic cells, where they func- seeds (Viola) at the hilum. The oily carun- tion as accessory pigments by screening cles of violet seeds attract ants, which help other photosynthetic pigments from photo- to disperse them. oxidation. Some carotenes give bright colors to animal-dispersed fruits, e.g. lyco- caryopsis (grain)(pl. caryopses) A dry pene in tomatoes. indehiscent fruit, typical of the grasses. It is similar to an ACHENE except that the ovary carotenoids A group of yellow, orange, wall is fused with the seed coat. or red pigments comprising the carotenes and xanthophylls. They are found in all cascade A sequence of biochemical reac- photosynthetic organisms, where they tions, each triggered by the previous one. function mainly as accessory pigments in Cascades are important in signaling path- photosynthesis. They contribute, with an- ways, where extracellular signals (such as thocyanins, to the autumn colors of leaves hormones or ions) bind with receptors on since the green pigment chlorophyll, which the cell membrane, triggering changes in normally masks the carotenoids, breaks membrane proteins that activate proteins down first. They are also found in some on the cytoplasmic side of the membrane, flowers and fruits, e.g. tomato. Carot- setting off a chain of reactions. enoids have three absorption peaks in the blue–violet region of the spectrum. Casparian strip An impervious band of thickening on the radial and transverse carpel The female reproductive organ of walls of the endodermis. It consists of de- a . It consists of an ovary, posits of suberin and cutin and ensures that containing one or more ovules, a stalk, or all the water and solutes entering the stele style (absent in some plant groups), and a pass through the cytoplasm of the endo- terminal receptive surface, the stigma. dermal cells, thus allowing for selective fil- Each flower may have one or more carpels tering of solutes and control of flow rate. borne singly giving an apocarpous gynoe- cium or fused together giving a syncarpous catabolism Metabolic reactions in- . The carpel is homologous with volved in the breakdown of complex mol- the megasporophylls of certain spore-bear- ecules to simpler compounds as, for ing vascular plants () and the example, in respiration. The main function ovuliferous scales of gymnosperms. It has of catabolic reactions is to provide energy, evolved by fusion of the two edges of the which is stored as ATP, for use in the syn- megasporophyll. This development can be thesis of new structures, cell-to-cell sig- most clearly seen in simple carpels such as nalling, or the movement of motile cells. those of the Fabaceae. See also metabolism. Compare anabolism. carrier 1. An organism that carries a re- catalase An enzyme present in both cessive, often harmful, gene masked in the plant and animal tissues that catalyzes the phenotype by a normal dominant gene. breakdown of hydrogen peroxide, a toxic 2. An individual infected with pathogenic compound produced during metabolism, microorganisms without showing symp- into oxygen and water. It also catalyzes the toms of disease. Such carriers can transmit oxidation of substrates by hydrogen perox- the infection to others. ide. Catalase is found in the cytoplasm in small organelles called peroxisomes. caruncle A fleshy outgrowth from a seed, similar to, but smaller than, an aril. catalyst A substance that increases the Caruncles are often waxy or oily. They rate of a chemical reaction without being may arise from the placenta, micropyle, or used up in the reaction. Enzymes are highly

38 efficient and specific biochemical catalysts. phases through which a cell passes leading They lower the activation energy needed to up to and including cell division (mitosis). initiate reaction by combining temporarily It is divided into four phases G1, S, and G2 with the reactants. (collectively representing interphase), and M-phase, during which mitosis takes place. cation A positively charged ion, formed Synthesis of messenger RNA, transfer by removal of electrons from atoms or RNA, ribosomes, and protein and general molecules. In electrolysis, cations are at- growth of the cell occurs in G1, and repli- tracted to the negatively charged electrode cation of DNA and synthesis of histones (the cathode). Compare anion. occurs during the S phase, as chromosomes divide to form chromatids. The materials See inflorescence. required for spindle formation are formed in G2, when there is a further burst of C2 cycle See . growth before prophase begins. The time taken to complete the cell cycle varies in cdl See critical day length. different tissues. cell The basic unit of structure of all liv- cell division See meiosis; mitosis; ami- ing organisms, excluding viruses. Cells tosis. were discovered by in 1665, but SCHLEIDEN and SCHWANN in 1839 were cell fractionation The separation of the the first to put forward a clear , different constituents of the cell into ho- stating that all living things were cellular. mogenous fractions. This is achieved by Prokaryotic cells (typical diameter 1 m m) breaking up the cells in a mincer or grinder are significantly smaller than eukaryotic and then centrifuging the resultant liquid. cells (typical diameter 20 m m). The largest The various components settle out at dif- plant cells are the fibers of false nettles ferent rates in a centrifuge and are thus sep- (Boehmeria nivea), up to 550 mm in length arated by appropriately altering the speed and stinging nettles (Urtica dioica) up to and/or time of centrifugation. The compo- 75 mm long; the smallest are the epider- nents are thus separated according to their mal cells of roses (0.04 mm in diameter). size and weight. Among algae, the internode cells of stoneworts (Chara spp.) reach lengths up cell lineage The theory stating that cells to 80 mm, while the individual indepen- arise only from pre-existing cells. The cell dent cells of Chlamydomonas are only lineage of a structure traces the successive 0.092 mm in diameter. All cells contain, or stages that the cells pass through from the once contained genetic material in the form time of their formation in the zygote to of DNA, which controls the cell’s growth their appearance in the mature functional and some of its activities; in eukaryotes this structure. is enclosed in the membrane-bounded nu- cleus. All contain cytoplasm, containing cell membrane See plasma membrane. various organelles and are surrounded by a plasma membrane. This controls entry and cellobiose See cellulase. exit of substances. Plant cells and most prokaryotic cells are surrounded by rigid cell plate A structure that appears in cell walls. In multicellular organisms cells late anaphase in dividing plant cells and is become specialized for different functions; involved in formation of a new cell wall at this is called differentiation. Within the the telophase stage of mitosis. It is formed cell, further division of labor occurs be- by fusion of vesicles from the Golgi appa- tween the organelles. ratus, resulting in a flattened membrane- bounded sac spreading across and cell cycle The ordered sequence of effectively dividing the cell. Cell-wall poly-

39 cell theory saccharides and pectic substances con- careous algae. Fungi differ, with their walls tained in the vesicles contribute to growth usually containing chitin. The walls of of the new wall and middle lamella inside some fungal spores contain the pigment the sac. The process begins at the phrag- melanin, which protects against the dam- moplast, a barrel-shaped region at the for- aging effects of ultraviolet radiation. Bacte- mer site of the spindle equator, where rial walls are more complex, containing microtubules remain and the vesicles clus- peptidoglycans – complex polymers of ter, possibly oriented by the microtubules. amino acids and polysaccharides. Cell Endoplasmic reticulum and ribosomes are walls are freely permeable to gases, water, also present. The cell-plate membranes and solutes. They have a mechanical func- eventually form the two new plasma mem- tion, allowing the cell to become turgid by branes of the daughter cells. See cytoki- , but preventing bursting. This con- nesis. tributes to the support of herbaceous plants. Plant cell walls can be strengthened cell theory The theory that all organ- for extra support by addition of lignin (as isms are composed of CELLS and cell prod- in xylem and sclerenchyma) or extra cellu- ucts and that growth and development lose (as in collenchyma). They are an im- results from the division and differentia- portant route for movement of water and tion of cells. mineral salts. Other modifications include the uneven thickening of guard cells, the cellular slime molds See Acrasiomy- sieve plates in phloem, the suberization of cota. endodermal cells, the waterproof coverings of epidermal and cork cells. cellulase An enzyme that hydrolyzes The cellulose in cell walls takes the 1,4-glycosidic linkages in cellulose, yield- form of microfibrils. At cell division in ing cellobiose (a disaccharide) and glucose. plants the primary wall is laid down on the It is important in the degradation of plant middle lamella of the cell plate as a loose cell walls in living plants (e.g. in leaf ab- mesh of cellulose microfibrils. This gives scission). an elastic structure that allows cell expan- secondary cellulose A polysaccharide forming the sion during growth. Later the wall grows and acquires greater rigidity framework of the CELL WALLS of all plants, many algae and other protoctists, and and tensile strength. New cellulose fibers some fungi. Cellulose molecules are un- are laid down in layers, parallel within branched chains that together form a rigid each layer, but orientated differently in dif- structure of high tensile strength. Bundles ferent layers. The Golgi apparatus provides of molecules form microfibrils, which may polysaccharide-filled vesicles that deposit be aligned in the primary cell wall either wall material by exocytosis, guided by mi- transversely or longitudinally. Cellulose crotubules. forms an important source of carbohydrate in the diets of herbivores, and is a major Cenozoic (Caenozoic; Cainozoic) The constituent of dietary in human diets. present geological era, beginning some 65 The individual units in cellulose are b-1,4 million years ago, and divided into two pe- linked D-glucose molecules. riods, the Tertiary and the Quaternary. It is characterized by the rise of modern organ- cell wall A rigid wall surrounding the isms, especially mammals and flowering cells of plants, fungi, bacteria, and algae. plants. During this era, the world’s tallest Plant cell walls are made of cellulose fibers mountain ranges such as the Rockies, in a cementing matrix of other polysaccha- Andes, European Alps and Himalayas rides. The walls of some algae differ, e.g. were formed, with dramatic effects on cli- the silica boxes enclosing diatoms, and the mate and plant distribution. See also geo- calcareous layer on the cell walls of cal- logical time scale.

40 chaparral centrarch Denoting a protostele in the centromere. The centromere is a spe- which protoxylem is at the center of the cific genetic locus and remains relatively axis. Compare endarch; exarch; mesarch. uncoiled during prophase, appearing as a primary constriction. It does not stain with centric bundle See concentric bundle. basic dyes. centrifugal Developing from the center centrosome A structure found in all eu- outwards so the youngest structures are at karyotic cells, except fungi, that forms the the outer edge. Centrifugal xylem differen- SPINDLE during cell division. It lies close to tiates from the center outwards, and a cen- the nucleus in nondividing cells. Micro- trifugal inflorescence (e.g. the dichasial tubules extend across the cell from the cen- cyme) is one in which the progression of trosome, which controls the distribution flower opening is from the center to the and assembly of the microtubules that periphery. Compare centripetal. See also make up the cytoskeleton, thus controlling acropetal; basipetal. the cell’s shape and, in some cells, move- ment. The centrosome probably plays an centriole A cell consisting of important role in the reorganization of the two short tubular structures orientated at cell that occurs during differentiation. At right angles to each other. Each tubular the start of cell division the centrosome di- structure is about 500 nm long and 150 nm vides, and the sister centrosomes move to in diameter. The centriole lies outside the opposite ends of the cell, organizing the nucleus of animal cells and many fungal spindle. In animal and protoctist cells the and protoctist cells, but is absent in cells of centrosome contains two short barrel- most higher plants. Prior to cell division it shaped structures, the CENTRIOLES, but replicates, and the sister centrioles move to these are not directly involved in spindle opposite ends of the cell to lie within the formation. In fungi, the function of the spindle-organizing structure, the centro- centrosome is served instead by the spindle some. At the start of cell division, fibrils pole body. appear to radiate from the centrioles, forming starlike ASTERS. These form the chalaza (pl. chalazae or ) The poles of the developing spindle. However, region of an angiosperm ovule where the the centriole is not essential for spindle for- nucellus and integuments merge. When mation, although an analogous structure, ovule orientation is orthotropous the cha- the basal body, is responsible for organiz- laza corresponds to the point where the fu- ing the microtubules of UNDULIPODIA. nicle is attached but in anatropous and campylotropous ovules the chalaza is some centripetal Developing from the outside distance from the funicle. inwards so the youngest structures are at the center. Centripetal xylem differentiates chalazogamy A method of fertilization from the outside toward the center, and a in angiosperms in which the pollen tube en- centripetal inflorescence (e.g. the capitu- ters the ovule by the chalaza instead of lum) is one in which the outer flowers open through the micropyle, penetrating the pla- before those in the center. Compare cen- centa en route. Chalazogamy is seen in cer- trifugal. See also acropetal; basipetal. tain trees and shrubs, e.g. beech. Compare porogamy. centromere The region of the chromo- some that contains the kinetochore, the chamaephyte A perennial plant that is structure that becomes attached to the nu- able to produce new growth from resting clear spindle during mitosis and meiosis. It buds near (within 0.25 m of) the soil sur- contains DNA of no known significance. face. Chamaephytes are usually small Following the replication of chromosomes, bushes (e.g. heather (Calluna vulgaris)). resultant chromatids remain attached at See also Raunkiaer’s plant classification.

41 chaparral Sclerophyllous scrub vegeta- and porphyrins) are often the only evi- tion of coastal and inland mountain re- dence for life in rocks of PRECAMBRIAN age. gions of southwestern North America, maintained by burning and and, in chemiosmotic theory The theory pro- some areas, possibly from past land clear- posed by Peter Mitchell in 1961 to explain ance. It consists of dense of broad- how electron transport and ATP synthesis leaved evergreen bushes and shrubs and are coupled during photosynthesis and res- twisted dwarf trees up to 2.5 meters (8 ft) piration. It requires a membrane that per- tall. The dominant plants are sages (Lami- mits the movement of protons from inside aceae) and evergreen and, in drier to outside, but not vice versa. An energy- areas, chamise (Adenostoma fascicula- requiring ‘proton pump’ (the ELECTRON- tum), manzanita (Arctostaphylos pun- TRANSPORT CHAIN) causes movement of gens), and species of Ceanothus. See protons (H+ ions) from the internal matrix maquis. Compare garrigue. of the organelle to the outside, creating a pH gradient across the membrane. As the Charophyceae A class of the Chloro- H+ ions diffuse back across the membrane. phyta (green algae) that includes the This gradient of potential energy drives re- stoneworts, formerly placed in a separate actions at specific sites on the membrane, phylum . They resemble true where the enzyme ATPase uses the energy plants in many ways and are thought to be to drive ATP synthesis, so the energy can the most closely related algal group to be transferred to reaction sites elsewhere. plants. chemoautotrophism (chemosynthesis) Charophyta See Charophyceae. See autotrophism; chemotrophism. chasmogamy The production of flow- chemoheterotrophism See chemotro- ers that open their petals so that cross pol- phism; heterotrophism. lination is possible. Compare cleistogamy. See also anemophily; entomophily. chemoreceptor An organ or structure that responds to a particular chemical chelation The process of forming a sta- . ble chemical complex (chelate) with a given substance, usually a metal ion, that chemosynthesis See autotrophism; che- involves the formation of at least two motrophism. chemical bonds. Chelates are usually or- ganic ring compounds, the ion being chemotaxis (chemotactic movement)A chelated to form the closure of the ring. TAXIS in response to a chemical concentra- Chelating agents can be used to remove tion gradient. The spermatozoids of primi- toxic metal ions from soil or water. Some tive plants are often positively chemotactic, plants achieve tolerance to toxic ions by swimming toward the female organs in re- chelating them, usually by combination sponse to a chemical secreted by the latter. with an organic compound, such as citrate For example, the archegonium (female or malate; the chelate is then deposited in organ) of the moss Funaria secretes su- the vacuole. Chlorophyll is also a chelate, crose. Motile organisms may also be nega- the porphyrin part of the molecule being tively chemotactic, swimming away from bound to iron. concentrations of toxic chemicals. chemical fossils Particularly resistant chemotaxonomy (biochemical systemat- organic chemicals present in geological ics; biochemical taxonomy) A system of strata that are thought to indicate the exis- classification of plants based on the nature tence of life in the period when the rocks of their metabolic products (metabolites). were formed. Chemical fossils (e.g. alkanes Techniques involved include chromatogra-

42 chlorinated hydrocarbon phy and , and the sequenc- material between homologous, nonsister ing of nucleic acids and proteins. chromatids (CROSSING OVER) and provide one mechanism by which RECOMBINATION chemotrophism A type of nutrition in occurs, through the splitting of linkage which the source of energy for the synthe- groups. Chiasmata also help to hold ho- sis of organic requirements is chemical. mologs together until the start of Most chemotrophic organisms are het- anaphase, thus helping to ensure the cor- erotrophic (i.e. chemoheterotrophic) and rect distribution of chromosomes into the their energy source is always an organic daughter nuclei. compound; animals, fungi, and most bac- teria are chemoheterotrophs. If auto- chimera An individual or part of an in- trophic (i.e. chemoautotrophic or chem- dividual in which the tissues are a mixture osynthetic) the energy is obtained by of two genetically different tissues. It may oxidation of an inorganic compound; for arise naturally due to in a cell example, by oxidation of ammonia to ni- of a developing embryo, producing a line trite or a nitrite to nitrate (by nitrifying of cells with the mutant gene, and bacteria), or oxidation of hydrogen sulfide hence different characteristics compared to to sulfur (by colorless sulfur bacteria). surrounding cells. It may also be induced Only certain specialized bacteria are chemo- experimentally. In plants, chimaeras pro- autotrophic. Compare phototrophism. duced from two different species are known as graft hybrids. For example, a (chemotropic movement) chemotropism bud may develop at the junction between A TROPISM in which the stimulus is chemi- the scion and stock with a mixture of tis- cal. The hyphae of certain fungi (e.g. sues from both. Mucor) are positively chemotropic, grow- ing toward a particular source of food. chi-squared test A statistical test used Pollen-tube growth down the style is both to compare observed frequencies of exper- positively and negatively chemotropic: the imental sampling results with expected tube grows toward chemicals secreted by (hypothetical) frequencies, usually calcu- the pistil, and away from atmospheric oxy- gen. lated from the results assuming no effect of the experimental variable. It is suitable for chernozem (black earth)A freely drain- observations that fall into discrete classes. The value: ing CALCAREOUS SOIL found in regions with 2 high summer rainfall, characterized by (observed – expected) /expected humus distributed fairly evenly throughout is calculated for each class, then the values the profile. The upper horizon of the soil is are added together. The sum is checked 2 very thick and black with humus; it merges against a table of c values at various de- into a brown lime-rich horizon below. grees of freedom to determine the degree of Chernozems are rich in plant nutrients and significance of the difference between ob- have a good crumb structure, allowing served and expected values is significant. drainage and aeration. They are associated with grasslands in temperate climates such chitin A nitrogen-containing polysac- as the prairies of North America, the pam- charide found in the cell walls of most pas of South America, and the Russian fungi and in some animals. Chitin is a poly- steppes, where they support highly produc- mer of N-acetylglucosamine. It consists of tive agricultural systems. many glucose units, in each of which one of the hydroxyl groups has been replaced by chiasma (pl. chiasmata) A connection an acetylamine group (CH3CONH). between homologous chromosomes seen during the prophase stage of meiosis. Chi- chlamydospore A thick-walled resting asmata represent a mutual exchange of spore produced by certain fungi (e.g.

43 chlorine

Chlorophyll: the structural formula of chlorophyll a smuts) and funguslike protoctists (e.g. habitats. Only the gametes and zoospores Mucor). are motile. They reproduce asexually by means of zoospores or nonmotile spores Chlorella See Chlorococcales. and sexually by fusion of similar-looking gametes (isogamy). chlorenchyma A form of parenchyma in which the cells contain many chloro- chlorophylls A group of photosynthetic plasts and relatively large intercellular pigments. They absorb blue-violet and red spaces, as in the mesophyll of the leaf. light and hence reflect green light, impart- ing the green color to green plants. The chlorinated hydrocarbon A hydrocar- molecule consists of a hydrophilic (water- bon in which one or more hydrogen atoms loving) head, containing at the are replaced by chlorine atoms. Some chlo- center of a porphyrin ring, and a long hy- rinated hydrocarbons are used as insecti- drophobic (water-hating) hydrocarbon tail cides, e.g. aldrin, dieldrin, and DDT. (the chain), which anchors the mol- ecule in the lipid of the membrane. Differ- chlorine Symbol: Cl. An element found ent chlorophylls have different chemical in trace amounts in plants. The chloride groups attached to the head. Chlorophylls ion (Cl-) can pass easily through cell mem- are located in the chloroplasts, and are in- branes, and is involved in the maintenance volved in the light reactions of photosyn- of osmotic and ionic balance in the plant. thesis, absorbing light energy and initiating electron transport. There are four kinds of Chlorococcales An order of Chloro- chlorophyll, a, b, c, and d, which differ in phyta (green algae) that contains single- their molecular structure. Chlorophyll a is celled and colonial algae that lack universal in all photosynthesizing eukary- undulipodia, e.g. Chlorella, Volvox, found otes. It converts the absorbed light energy in freshwater plankton and most terrestrial to chemical energy. Chlorophylls b, c, and

44 chromatin loop d are accessory pigments. On the thylakoid membranes are spherical occurs in plants and in the ; structures partially embedded in the mem- the other chlorophylls occur in certain brane. These are the photosynthetic units groups of algae. See absorption spectrum; (formerly called quantasomes), which con- photosynthetic pigments. See also bacteri- tain the pigments and associated lipids and ochlorophyll. proteins of I and II. Photo- system I units are about 17.5 nm in diame- Chlorophyta (green algae) A phylum of ter, and II units about 11 nm. protoctists comprising mainly freshwater Other, smaller particles on the thylakoid algae with some marine and terrestrial membranes are ATPases. The may forms. They contain the pigments chloro- contain storage products of photosynthe- phyll a and b together with carotenes and sis, e.g. starch grains. Apart from enzymes xanthophylls. The Chlorophyta store food of the dark reactions, the stroma also con- as starch and fat and have cell walls con- tains typical prokaryotic protein-synthesiz- taining cellulose and . Some ing machinery including circular DNA (see of the commoner orders include: Volvo- DNA) and smaller ribosomes. cales, unicellular and colonial plants, e.g. There is now strong evidence that chloro- Chlamydomonas, Volvox; Chlorococcales, plasts and other cell organelles, such as including unicellular and colonial plants, mitochondria, represent prokaryotic or- e.g. Chlorella, Pediastrum; Ulotrichales, ganisms that invaded heterotrophic eu- filamentous, and thallose plants, or coeno- karyotic cells early in evolution and are cytic tubes, e.g. Ulothrix, Ulva, Entero- now part of an indispensable symbiotic morpha; and Oedogoniales, e.g. the union (see theory). In C4 filamentous Oedogonium. plants there are two types of chloroplast. chloroplast A photosynthetic PLASTID chloroplast DNA (cp-DNA) The DNA containing chlorophyll and other photo- found in a chloroplast. It is in circular synthetic pigments. It is found in all photo- form, rather like that of prokaryotes. It synthetic cells of plants and protoctists but codes for only some of the proteins needed not in photosynthetic prokaryotes. It has a by the chloroplast. membrane system containing the pigments and on which the light reactions of PHOTO- The loss of chlorophyll from SYNTHESIS occur. The surrounding gel-like plants resulting in yellow (chlorotic) ground substance, or stroma, is where the leaves. It may be the result of the normal dark reactions occur. The typical higher- process of senescence, lack of key minerals plant chloroplast is lens-shaped and about for chlorophyll synthesis (particularly iron 5 m m in length. Various other forms exist and magnesium), or disease. in the algae, e.g. spiral in Spirogyra, stellate in Zygnema, and cup-shaped in Chlamy- chlor-zinc-iodide See Schultze’s solu- domonas. The number per cell varies, e.g. tion. one in Chlorella and Chlamydomonas and in most hornworts (Antherocyta), two in chromatid One of a pair of replicated Zygnema, and about one hundred in pal- chromosomes found during the prophase isade mesophyll cells of leaves. and metaphase stages of mitosis and meio- Chloroplast membranes form elon- sis. During mitosis, sister chromatids re- gated flattened fluid-filled sacs called main joined by their centromere until thylakoids. The sheetlike layers of the thy- anaphase. In meiosis it is not until lakoids are called lamellae. In all plants ex- anaphase II that the centromere divides, cept algae, the thylakoids overlap at the chromatids being termed daughter intervals to form stacks, like piles of coins, chromosomes after separation. called grana. In this way the efficiency of the light reactions seems to be improved. chromatin The loose network of

45 threads seen in nondividing nuclei that rep- chromomere A small beadlike deeply resents the chromosomal material, consist- staining structure seen in chromosomes ing of DNA, protein (mainly histone), and during prophase of mitosis and meiosis. It small amounts of RNA. It is classified as is thought to represent an area where the euchromatin or heterochromatin on the chromosome is relatively tightly coiled. basis of its staining properties, the latter Homologous chromosomes have chromo- staining much more intensely with basic meres in identical places along their length. stains because it is more coiled and com- pact. Euchromatin is thought to be actively chromoplast (chromatophore) A col- involved in transcription and therefore ored plastid, i.e. one containing pigment. protein synthesis, while heterochromatin is They include chloroplasts, which contain inactive. Euchromatin stains more inten- the green pigment chlorophyll and are sively than heterochromatin during nuclear therefore photosynthetic, and nonphoto- division. See also chromosome. synthetic chromoplasts. The term is some- times confined to the latter, which are best chromatin loop A region of uncoiled known in flower petals, fruits (e.g. DNA, usually about 200 nm long, that ex- tomato), and carrot roots. They are yellow, tends out in a loop from the chromosome. orange, or red owing to the presence of This uncoiled condition is thought to facil- carotenoid pigments. itate TRANSCRIPTION and probably indicates an area where genes are active. chromosome 1. One of a group of threadlike structures of different lengths and shapes in nuclei of eukaryotic cells. chromatography A method of analyz- They consist of DNA with RNA and pro- ing materials involving the separation by tein (mostly histones) and carry the GENES. selective absorption of the various com- During nuclear division the chromosomes pounds as identifiable bands. For instance, are tightly coiled and are easily visible a mixture of substances in solution is through the light microscope. After divi- passed slowly down a long packed sion, they uncoil and may be difficult to with alumina. The different compounds see, the long narrow threads of uncoiled move at different rates and separate into DNA being only about 2 nm in diameter. bands. The medium, together with the Along the length of the uncoiled chromo- bands of separated material, are collec- somes are beadlike structures called nu- tively called a chromatogram. cleosomes, highly organized aggregations In general chromatography involves a of DNA and histones. test material being carried by a moving The number of chromosomes per nu- phase (liquid or gas) through a stationary cleus is characteristic of the species except phase (solid or liquid). Different sub- in aneuploid plants. Normally one set stances move at different rates (depending (haploid) or two sets (diploid) of chromo- on their absorption–desorption) and are somes are present in the nucleus. However, therefore spatially separable, the least many or perhaps a majority of plants are readily absorbed being carried the farthest. polyploid in origin, i.e. they have multiple Colorless materials can be used if some sets of chromosomes, though most act as means of detecting them is used (electronic diploids in their reproduction. In some detection, radioactive labeling, or ninhy- species there are diploid and polyploid va- drin developer). See also paper chromatog- rieties that look almost identical, though raphy; thin-layer chromatography; gas sometimes they may be referred to as sepa- chromatography; gel filtration. rate species. In bryophytes the main plant body is haploid, as is the case in many chromatophore A name generally ap- algae (see alternation of generations). In plied to a pigment-bearing structure, usu- early prophase of mitosis and later ally a membrane lamella or vesicle. prophase of meiosis, the chromosomes

46 cilium split lengthwise into two identical chro- linkage map of all restriction sites, i.e. sites matids held together by the CENTROMERE. that are cleaved by restriction enzymes. The length of the arms of the chromosome These restriction sites can then be used as is determined by the position of the markers for closely neighboring genes. The centromere. The centromere contains the last step is to construct a physical map of kinetochore, a structure involved in the at- the base sequence of the chromosomal tachment of the chromosomes to the spin- DNA. One approach uses cloned DNA dle during nuclear division. In diploid cells, segments obtained from a gene library of there is a pair of sex chromosomes; the re- the chromosome. These clones can be fit- mainder are termed autosomes. Each chro- ted together to form a series of overlapping mosome contains one DNA molecule, segments (contig) that corresponds to a which is folded and coiled. Histones play a particular region of the chromosome. The major architectural role, holding the coiled base sequence of the contig is then deter- (helical) DNA in supercoils or beads, about mined, and hence the base sequence of the 10 nm in diameter and consisting of about chromosomal DNA. See gene library; re- 170 base pairs. These in turn are coiled to striction map. form a hollow fiber, 30 nm in diameter, which itself forms a hollow coil measuring chromosome mutation A change in about 240 nm in diameter. It is this that is the number or arrangement of genes in a visible as the typical looped threadlike chromosome. If chromosome segments chromatin material of the nondividing break away during nuclear division they chromosome. See chromomere; chromo- may rejoin the chromosome the wrong some map. way round, giving an inversion. Alterna- 2. See bacterial chromosome. tively, they may rejoin a different part of the same chromosome, or another chromo- chromosome map (genetic map) A dia- some, giving a translocation. If the segment gram showing the order of genes along a becomes lost, this is termed a deficiency or chromosome. Such maps have traditionally deletion; it is often fatal. A part of a chro- been constructed from information gained mosome may be duplicated and occur ei- by linkage studies to give a linkage map. ther twice on the same chromosome or on The techniques employed differ according two different nonhomologous chromo- to the type of organism being studied. somes: this is a duplication. Chromosome Many plants and animals can be crossed mutations can occur naturally but their fre- experimentally to study inheritance pat- quency is increased by the effect of x-rays terns of particular genes. However, the ad- and chemical mutagens. Such aberrations vent of new molecular techniques has may be visible by microscopy, especially dramatically changed the nature of chro- with the use of certain stains. See also gene mosome mapping in all organisms. The mutation. growing science of seeks to an- alyze the by starting with the pro- chromosome number The number of tein products of genes and working back to chromosomes possessed by a particular the genome. species. It may be expressed as either the Mapping a genome involves several haploid (n) or diploid (2n) number. For steps. The first is to assign each gene to many species, chromosome number is a a particular chromosome. This can be useful taxonomic character. Exceptions in- achieved by, for example, somatic-cell clude species exhibiting APOMIXIS. Chro- hybridization or using a gene probe. The mosome number also gives an indication of next step is to determine the relative polyploidy, where numbers vary by exact positions of the genes on a particular chro- multiples between related species. mosome. This involves comparing RESTRIC- TION FRAGMENT LENGTH POLYMORPHISMS Chrysomonada A phylum of golden- between individuals and constructing a brown algae, formerly called the Chryso-

47 circadian rhythm phyta, common in fresh temperate waters, for example, Convolvulus always rotates with a few species in marine waters. in an anticlockwise direction. For climbers Chrysomonads are mainly motile unicellu- nutation increases the likelihood of a solid lar algae, but there are some colonial forms support being found. It is also marked in and a few complex filamentous forms. many leaf and to a lesser extent in roots, flower stalks, and the sporangio- ciliate 1. Describing a structure, such the phores of some fungi. Gravity has been margin of a leaf, fringed with fine hairs. shown to be involved in the response: if a 2. A unicellular protoctist of the phylum plant is turned upside down, the tip and the Ciliophora, whose members are usually last few spirals unwind and straighten out covered in short undulipodia (cilia) and before winding up in the characteristic way have two different kinds of nuclei, mi- in the new direction. cronuclei and macronuclei. cisterna (pl. cisternae) A flattened mem- cilium (pl. cilia) A whiplike extension of brane-bounded sac of the endoplasmic certain eukaryotic cells that beats rapidly, reticulum or the Golgi apparatus, being the thereby causing locomotion or movement basic structural unit of these organelles. of fluid over the cell. Cilia are short UN- DULIPODIA, 2–10 m m long. cis–trans effect (complementation test) The beat of each cilium comprises an ef- The phenomenon resulting from recombi- fective downward stroke followed by a nation within a gene (cistron), in which a gradual straightening (limp recovery). Cilia mutation is only expressed in the pheno- beat in such a way that each is slightly out type if the mutant pseudoalleles are on dif- of phase with its neighbor (metachronal ferent homologous chromosomes. rhythm), thus producing a constant rather than a jerky movement or flow of fluid. cis–trans test A test that determines The basal bodies of cilia are connected by whether two mutations that have the same threadlike strands (neuronemes), which co- effect occur in the same gene or in different ordinate the beating of neighboring cilia. genes. The mutations may be in either the cis position (i.e. on the same chromosome) circadian rhythm (diurnal rhythm)A or the trans position (one on each ho- daily rhythm of various metabolic activi- molog). If the mutations are in different ties in animals and plants. Such rhythms genes then a normal phentotype results persist even when the organism is not ex- whether the mutations are in the cis or posed to 24-hour cycles of light and dark, trans position, since they are masked by and are thought to be controlled by an corresponding dominant genes on the endogenous biological clock. Circadian other homolog. However, if the mutations rhythms are found in the most primitive are in the same gene then a normal pheno- and the most advanced of organisms. Thus type will result only if the mutations are in Euglena shows a diurnal rhythm in the the cis position. In the trans position the speed at which it moves to a light source. mutant phenotype is expressed, since both Stomata open and close in a daily cycle, alleles of the locus are mutants. which persists for several days in total darkness. Experiments have failed to reveal cistron A unit of function, i.e. a segment the type and location of the control mech- of DNA that determines a single polypep- anisms involved. See also biorhythm. tide chain of a protein molecule or a mes- senger RNA molecule. Its boundaries may circinate See vernation. be defined by the cis–trans test. circumnutation The spontaneous spi- CITES The Convention on Interna- ral growth of the shoot tips of certain tional Trade in of Wild plants, particularly climbers. The direction Fauna and Flora: an agreement signed be- of rotation is often constant in a species; tween some 160 countries, prohibiting

48 cleavage commercial trade in endangered species phyta (green algae), generally called and products derived from them. As more siphonaceous algae. They are multicellular species have become endangered, the list of saclike forms or branched filaments with protected species has lengthened. CITES branching rhizoidlike holdfasts, e.g. Cla- sets out degrees of protection for different dophora. species, allowing threatened but not en- dangered species to be exported and im- class A that is subordi- ported under license. nate to a phylum (or sometimes a division in plant taxonomy) and superior to an citric acid A six-carbon carboxylic acid, order. The Latin names of plant classes end occurring in the juice of citrus fruits, par- in –ae, e.g. Dicotyledonae. Classes may be ticularly lemons, and present in many divided into subclasses, with the Latin end- other fruits. Citric acid is biologically im- ing –idae, e.g. Rosidae. portant because it participates in the KREBS CYCLE, where it is formed by the conden- classification The grouping and arrange- sation of acetyl CoA and oxaloacetate, ment of organisms into a hierarchical a rate-limiting step of the cycle. The sys- order. The aim is usually to aid in identifi- tematic name is 2-hydroxypropane-1,2,3- cation of organisms or represent their phy- tricarboxylic acid logenetic relationships, or, ideally, both. An important aspect of classifications is citric acid cycle See Krebs cycle. their predictive value. For example if a characteristic is found in one member of a A method of classification in group of plants, then it is also likely to be which the relationships between organisms found in the other members of that group are based on selected shared characteris- even though the characteristic in question tics. These are generally assumed to have was not used in the initial construction of been derived from a common ancestor, in the classification. the evolutionary process of cladogenesis, Artificial classification systems are de- although the ‘transformed cladists’ believe signed to aid identification, and may not that shared characteristics alone provide reflect true relationships. e.g. LINNAEUS’s a logical basis for classification without system, which was based on the number of postulating evolutionary relationships. The stamens and carpels in the flower. Natural patterns of these shared characteristics are classification systems are based on more demonstrated in a branching diagram characteristics, and thus thus are better for called a . The branching points predicting relationships. Phenetic classifi- of the cladogram may be regarded either as cation systems are based on many charac- an ancestral species (as in an evolutionary ters, and may also be weighted in favor of tree) or solely as representing shared char- particular characters. Phylogenetic classifi- acteristics. cation systems attempt to reflect the evolu- tionary relationships of taxa. The validity cladode A modified internode of the of these systems is now being tested by stem that functions as a leaf, being flat- mRNA studies. Compare cladistics. See tened and highly photosynthetic. It may be ; phylum; class; order; family; genus; a xerophytic adaptation and is seen in species. butcher’s broom (Ruscus aculeatus) and prickly pear (Opuntia). The true leaves clay Extremely fine-textured SOIL made may be reduced to spines or scales. Unlike up of small mineral particles, less than a leaf, a cladode has buds on its surface. 0.002 mm in diameter, formed mainly from aluminum and magnesium silicates. cladogram See cladistics. Clay soils become very sticky and diffi- cult to work when wet and can easily be- Cladophorales An order of Chloro- come waterlogged. Nutrient availability to

49 cleistogamy plants can be a problem as the nutrients climatic factors Aspects of climate, may become chemically bound to the sur- such as temperature, rainfall, humidity, air faces of the particles. movement, and light, that affect living or- ganisms. Compare biotic environment; clearing 1. In the preparation of perma- edaphic factors. nent microscope slides, the stage between dehydration and embedding, the purpose climax The final community in a succes- of which is to remove the dehydrating sion of natural plant communities in one agent and replace it by a substance that is area under a particular set of conditions. miscible with the embedding substance. Mixed woodland with oak is the natural Clearing also renders the tissues transpar- climax vegetation in much of lowland ent. Clearing agents include benzene and Britain, but on chalk it is often beechwood. xylene. A climax is self-perpetuating, at least for a 2. In the preparation of cell material for time, although there are constant small staining, the dissolving away of cell con- changes, e.g. when a tree dies pioneering tents so that the distribution of tissues may plants may invade or the tree may be re- be better observed, using reagents such as placed by a different species. Succession to sodium hypochlorite. a climax can be held at any stage by human 3. A treeless, usually grassy area in a wood- intervention, such as grazing on chalk land or forest. downland. Such an equilibrium is called a subclimax, plagioclimax, disclimax, or bi- otic climax. See clisere. cleavage polyembryony The splitting of an embryo into several identical parts, climbing ferns Ferns of the family each of which may develop into a mature Schizaeaceae, found in most parts of the embryo. It is a form of natural cloning. world in warm and tropical regions, in- Cleavage polyembryony occurs in some cluding North America. There are four gymnosperms, e.g. in Pinus, but usually genera: Anemia, Lygodium, Mohria, and only one embryo develops to maturity. See Schizaea. Many climbing ferns have long clone. creeping . cleistogamy The production of closed cline A graded series of characters, usu- flowers. It is a method of ensuring self- ally morphological, exhibited by a species pollination and occurs toward the end of or other related group of organisms, usu- the flowering season in certain plants, e.g. ally along a line of environmental or geo- wood sorrel (Oxalis acetosella) particu- graphical transition. The populations at larly when little or no seed has been set by each end of the cline may be substantially the cross-pollinating flowers. Compare different from one another. chasmogamy. clinostat See klinostat. cleistothecium (pl. cleistothecia) See as- coma. clisere A succession of climax communi- ties in a given area, each giving way to the The rise in respiration rate, next as a result of climatic changes. See found in the fruits of some species, associ- also climax; sere. ated with fruit ripening and senescence. It may be induced artificially by treatment clone A group of organisms or cells that with ethylene (ethene). are genetically identical. In nature, clones are derived from a single parental organ- climatic climax A plant community ism or cell by mitotic cell division, asexual that is in equilibrium with a stable climate. reproduction, or parthenogenesis. In ge- It represents the CLIMAX of a succession. netic engineering, multiple identical copies

50 of a gene are produced in cloning vectors, ribosomes, its string of codons is paired such as plasmids and phages. See gene with the anticodons of transfer RNA mol- cloning. See also apomixis. ecules, each of which is carrying one of the amino acids necessary to make up the pro- clubmoss See Lycophyta. tein. Most amino acids are coded for by more than one codon. The genetic code is clubroot A serious disease of the cab- therefore said to be degenerate. The term bage family (Brassicaceae) caused by Plas- codon is sometimes also used for the modiophora brassicae (phylum Plasmo- triplets on DNA, which are complemen- diophora), in which the roots become tary to those on mRNA, but contain the swollen and misshapen, with characteristic base uracil instead of thymine. See transfer gall-like swellings. Above-ground symp- RNA. toms include in bright sunlight and a reddish, bluish, or yellowish tinge to the coenocyte An area of cytoplasm con- leaves. taining many nuclei and enclosed by a cell wall, typically found in many fungi, certain CoA See coenzyme A. green algae (e.g. Cladophora), and some members of the Xanthophyta. Such a cell is coadaptation The evolution and main- described as coenocytic. tenance of certain genetic characteristics that increase the effectiveness of a relation- coenzyme See cofactor. ship between two species. Flower/pollina- tor relationships are an example, in which coenzyme A (CoA) A coenzyme that is of flowers for pollination by important in the synthesis and reactions of animals is to the mutual advantage of both fatty acids. In the KREBS CYCLE it combines species (coevolution). See mutualism. with pyruvic acid, leading to loss of carbon dioxide. cobalt See micronutrient. coenzyme Q (ubiquinone) A coenzyme coccus (pl. cocci) A spherical-shaped that is an essential component of the respi- bacterium. Cocci may be found singly, in ratory electron-transport chain. pairs (e.g. Diplococcus), or chains (e.g. Streptococcus), or in regularly or irregu- cofactor (coenzyme)A nonprotein sub- larly packed clusters. stance that helps an enzyme to carry out its activity. Cofactors may be cations or or- co-dominance See incomplete domi- ganic molecules, known as coenzymes. nance. Many enzymes require metal cofactors, e.g. ferredoxin requires ferrous and ferric codon A group of three nucleotide bases ions. The metal ion may be the catalytic (i.e. a nucleotide triplet) in a messenger center of the cofactor, or it may serve to RNA (mRNA) molecule that codes for a bind the enzyme and substrate together to specific amino acid or signals the beginning hold the enzyme in a catalytically active or end of the message (start and stop conformation. Unlike enzymes cofactors codons). Since four different bases are are, in general, stable to heat. When a cat- found in nucleic acids there are 64 (4 ´ 4 ´ alytically active enzyme forms a complex 4) possible triplet combinations. The with a cofactor a holoenzyme is produced. arrangement of codons along the mRNA An enzyme without its cofactor is termed molecule constitutes the genetic code. an apoenzyme. See also prosthetic group. When synthesis of a given protein is neces- sary the segment of DNA with the appro- cohesion theory A theory to explain priate base sequences is transcribed into how sap rises up the xylem. It suggests that mRNA. When the mRNA migrates to the the lowering of water potential in the leaf

51 coleorhiza due to an increase in evaporation causes collenchyma A specialized type of water to flow toward the leaf down a water parenchyma, usually located just beneath potential gradient. Strong cohesive forces the , that functions as supporting between the water molecules in the xylem tissue. The cell walls are irregularly thick- prevent the water column breaking. ened with cellulose and , the thicken- ing giving distinct patterns to the cells in colchicine An alkaloid drug, obtained cross-section. The thickening is usually from the autumn crocus Colchicum au- greatest in the corners of the cells or adja- tumnale, which is used experimentally to cent to the intercellular spaces. Col- prevent spindle formation in mitosis or lenchyma is the first strengthening tissue to meiosis by preventing tubulin polymerizing be formed in young plants and is able to into microtubules. It has the effect of halt- expand as the young tissues continue de- ing cell division at metaphase, the stage at velopment. Compare sclerenchyma. which the chromosomes have duplicated to give four homologs for each chromosome. colloid A substance that is composed of See anther culture. particles dispersed through another sub- stance. Such particles range from 10–7 to –3 cold hardening (cold acclimation) 10 cm; they are too small to see with a Processes taking place in plants before the light microscope, yet cannot pass through onset of winter that prepare them to with- a selectively permeable membrane. Pro- stand low temperatures. For example, teins in the cytoplasm are an example. plants may accumulate solutes, such as colony A body form in which many cells oligosaccharides and the amino acid pro- with similar form and function are line, to make their tissues frost-resistant. grouped together, as in many algae, e.g. Volvox. Such a colony is often surrounded coleoptile A sheathlike structure that by mucilage. protects the developing plumule in grasses. Some think that, together with the SCUTEL- columella 1. The structure present in LUM, it represents the COTYLEDON, while sporangia of many zygomycete fungi (e.g. others believe it is the first plumular leaf. Mucor) produced by formation of a dome- The coleoptile contains very little chloro- shaped septum cutting off the sporangium phyll and is usually light-sensitive. from the sporangiophore. 2. The central column of sterile tissue in the coleorhiza (pl. coleorhizae) The protec- sporangium of liverworts and mosses. tive sheath surrounding the in grasses. commensalism An association between two organisms in which one, the commen- coliform bacteria Gram-negative rod- sal, benefits and the other remains unaf- shaped bacteria able to obtain energy aero- fected either way, e.g. an epiphyte on the bically or by fermenting sugars to produce branch of a tree. Compare amensalism; acid or acid and gas. Most are found in the mutualism; parasitism; symbiosis. vertebrate gut (e.g. Escherichia coli), but some are present in soil, water, or as plant community A general term covering pathogens. Many are pathogenic to hu- any naturally occurring group of different mans (e.g. Salmonella). organisms living together in a certain envi- ronment, and interacting with each other. collateral bundle A kind of vascular See association; consociation. bundle in which the phloem is external to the xylem and on the same radius. Com- community biomass See biomass. pare bicollateral bundle; concentric bun- dle. companion cell An elongated thin-

52 confocal microscopy walled cell cut off longitudinally from the same or different species living together in same meristematic cell as the SIEVE EL- a community, when the resources are not EMENT with which it is closely associated. It sufficient to fill the needs of all the organ- has a nucleus, dense cytoplasm, and many isms. Plants, particularly the dominant mitochondria, and is thought to provide, plants of a community, compete mainly as via PLASMODESMATA, the needs of the less seedlings are establishing, but once this has metabolically active, enucleated sieve el- been achieved, the adult plant often out- ement and perhaps plays a role in regulat- competes any potential competitor unless ing the flow of metabolites through the affected by herbivores or pathogens. sieve tube. competitive inhibition A form of en- compass plant A plant with its leaf zyme inhibition in which the inhibitor edges permanently aligned due north and competes with the substrate to bind with south. Such plants thus avoid receiving the the active site of the enzyme. A competitive strong midday rays of the sun directly on inhibitor is usually structurally similar to the leaf blades, but are positioned to use the substrate, e.g. malonate is a competi- fully the weaker rays of the morning and tive inhibitor of the enzyme succinate de- evening sun from the east and west. The hydrogenase. It may bind to the site but best-known example is the compass plant will not react to produce products. of the prairies (Silphium laciniatum). complementary DNA (cDNA) A form compatibility The coexistence of differ- of DNA synthesized by genetic engineering ent bacterial plasmids in the same host cell. techniques from a messenger RNA tem- Plasmids that cannot exist in the same host plate using a reverse transcriptase enzyme. cell belong to the same incompatibility It is used in cloning to obtain gene se- group. quences from mRNA isolated from the tis- sue to be cloned. It differs from the original compensation depth See photic zone. DNA sequence in that it lacks intron and promoter sequences. Labeled single- compensation point The concentra- stranded cDNA is used as a gene probe to tion of carbon dioxide at which, in a closed identify common gene sequences in differ- system, the rate of photosynthesis is ex- ent tissues and species. See gene cloning. actly balanced by the combined rates of respiration and photorespiration, so that complementary genes Genes that can net exchange of oxygen and carbon diox- only be expressed in the presence of other ide is zero. genes; for example if one gene controls the The point at which photosynthesis does formation of a pigment precursor and an- not increase with increased light intensity other gene controls the transformation of is termed the light saturation point. This that precursor into the pigment, then both point occurs at much higher light intensi- genes must be present for the color to de- ties in C4 plants than C3 plants. velop in the phenotype. Such interactions between genes lead to apparent deviations competence A characteristic of embry- from the 9:3:3:1 dihybrid ratio in the F2. onic cells and meristematic cells whereby they have the potential to differentiate into Compositae See Asteraceae. any of several different types of cell. Once plant cells have differentiated, most lose composite fruit See multiple fruit. competence, except under special condi- tions. See totipotency. compound fruit See aggregate fruit. competition The utilization of the same concentric bundle (centric bundle)A resources by one or more organisms of the in which the xylem and

53 conformation phloem are arranged in rings, one com- tiveness as enzymes. Their conformation is pletely surrounding the other. Compare bi- affected by the sequence of amino acids collateral bundle; collateral bundle. and the type, location, and nature of the bonds between different parts of the mol- conceptacle A flask-shaped reproduc- ecule, or between the polypeptide chain tive cavity that develops on the swollen tips and various cofactors and prosthetic (receptacles) of the thalli of certain brown groups. algae, (e.g. bladder wrack). Female concep- tacles are lined with unbranched sterile conidiophore See conidium. hairs (paraphyses) and the oogonia de- velop on short stalks projecting from the conidiospore See conidium. chamber wall. Male conceptacles contain branched paraphyses that bear the an- conidium (conidiospore)(pl. conidia) theridia. Both female and male concepta- An asexual spore of certain fungi, espe- cles open to the exterior via a pore, the cially of the Ascomycota and Fungi ostiole. Anamorphici e.g. Pythium and Albugo. Conidia are cut off externally in chains condensation reaction A chemical re- at the apex of a specialized hypha, the action in which two molecules are joined conidiophore. together with the elimination of a simpler molecule or group. Many condensation re- Coniferophyta (conifers) The largest actions involve the elimination of water, phylum of gymnosperms, comprising e.g. the formation of peptide bonds be- about 560 species of evergreen trees and tween amino acids in the formation of pep- shrubs, with many important species, e.g. tides. Compare hydrolysis. Pinus (pine), Picea (spruce), Taxus (yew), and Abies (fir). They dominate the vast bo- conduplicate See ptyxis. real forests of the northern hemisphere. Most are evergreen, but Larix (larches) cone See strobilus. and (swamp cypresses) are de- ciduous. confocal microscopy A form of light microscopy involving the use of two mi- conjugated protein A protein that on croscopes, one on each side of the speci- hydrolysis yields not only amino acids but men. The first microscope focuses an also other organic and inorganic sub- illuminated aperture onto the specimen, stances. They are simple proteins com- while the other also focuses on the speci- bined with nonprotein groups (prosthetic men, but receives the light transmitted groups). See also glycoprotein; lipoprotein. through it. This produces an image of a sin- gle plane in the specimen. It can be re- conjugation 1. A type of sexual repro- peated at different depths of focus to build duction found in the conjugating green up a three-dimensional picture. algae (phylum Gamophyta), and certain fungi, in which a conjugation tube forms conformation The spatial arrangement between cells of two individuals and the of atoms in a molecule. In affecting the gamete formed in one cell (the male ga- molecule’s shape and the distribution of mete) moves through the tube and fuses chemical groups and electrostatic charges with the gamete of the other cell (the fe- on its surface, the conformation deter- male gamete) to form a zygote. Spirogyra mines the potential energy and reactivity and Rhizopus are examples. of the molecule that results from the at- 2. In prokaryotes, a means of transferring tractive and repulsive forces between the genetic information from one bacterium to different parts of the structure. The confor- another across a bridgelike structure. In mation of PROTEINS is critical to their effec- Escherichia and related genera, two indi-

54 Convention on Biological Diversity

Conjugation: the stages occurring in Spirogyra viduals join by a conjugation bridge and conspecific Describing individuals that part of the genetic material of one, the belong to the same species. donor (or male) cell, is transferred to the recipient (or female) cell. consumer An organism that feeds upon another organism, e.g. all animals and par- asitic and insectivorous plants. Compare connective The parenchymatous tissue producer. See also . that joins the two lobes of the anther and contains the vascular strand. See stamen. continental drift The theory that pre- sent-day continents have arisen by the consociation A climax of natural vege- breaking up and drifting apart of a previ- tation dominated by one particular species, ously existing ancient land mass (Pangaea). such as oakwood, dominated by the oak There is much evidence to support the tree, or Calluna heathland dominated by theory, and it serves to explain the distrib- the heather, Calluna vulgaris. Many conso- ution of contemporary and fossil plants and animals. Continental drift is now be- ciations together may form an association, lieved to reflect the movement over geolog- for example oakwood, beechwood, and ical time of underlying plates in the earth’s ashwood consociations together make up a crust – the theory of plate tectonics. deciduous forest association. See also asso- ciation. continuous variation (quantitative vari-

55 ation; multifactorial inheritance) A form ement (micronutrient), essential for plant of variation in which the characters show growth. It is found in the enzyme cy- continuous variation within a certain range tochrome c oxidase, which is part of the of values. Examples are grain yield in respiratory electron-transport chain, and wheat and height in humans. Such traits in the protein plastocyanin, which is in- are typically controlled by many different volved in photosynthetic electron trans- genes (polygenes). The identification and port. Copper deficiency results in chlorosis, manipulation of genetic loci determining suggesting that copper may be involved quantitative traits (quantitative trait loci in the synthesis of chlorophyll. See also (QTLs)) is important in plant and animal micronutrient. breeding. coppice (copse) A woodland managed contractile root A specialized root de- for wood production by cutting trees back veloped by certain bulb- and -forming to ground level at regular intervals (usually plants that serve to pull the bulb or corm 10–15 years) and allowing adventitious down to the appropriate depth in the soil, shoots to grow up from the base. The e.g. Crocus. young shoots, which are often very straight, are used for fencing, Convention on Biological Diversity burning, and firewood. Occasional trees An international convention, signed at the may be left to mature: these are termed United Nations Rio Summit in 1992, that standards. aims to conserve biodiversity, the sustain- able use of natural resources, and the equi- coralloid roots A type of root regularly table sharing of the benefits arising from produced by , which contains sym- exploitation of genetic resources. It covers biotic nitrogen-fixing cyanobacteria in all aspects of biodiversity, from genetic root nodules, giving the roots a knobbly resources to ecosystems, and aims to con- coral-like appearance. serve resources while promoting sustain- able development and to encourage the Heart-shaped, usually referring sharing of costs and benefits between de- cordate veloping and developed countries. The just to the base of leaves, e.g. the leaves of United Nations Environment Program the common blue violet (Viola papil- (UNEP) responded by commissioning the ionacea). Global Biodiversity Assessment, to collect and review data, theories, and opinion re- cork See phellem. lating to current issues worldwide, which provided valuable information on the cork cambium See phellogen. rapid rate at which environmental change is taking place and natural ecosystems are corm An organ of perennation and veg- being modified. etative reproduction, consisting of a short erect fleshy swollen underground stem, convergent evolution (convergence) usually broader than high and covered The development of similar (analogous) with membranous scales. It stores food structures in unrelated organisms as a re- material in the stem and bears buds in the sult of living in similar ecological condi- axils of the scalelike leaf remains of the tions. An example is the evolution of previous years’ growth. One or more new similar succulent forms in the Cactaceae form from these buds, and lie above and the Euphorbiaceae, in which the leaves the shrivelled remains of the previous are reduced to pale spines. See also analo- year’s corm. The corm’s position in the soil gous. is often maintained with the aid of con- tractile roots. Examples of corms are cro- copper Symbol Cu. A metal trace el- cus and gladiolus. Compare bulb.

56 Cretaceous corolla A collective term for the petals which one or more pairs of electrons are of a flower. The corolla is denoted in the shared between two atoms. It is often rep- floral formula by the symbol C. Where the resented by a single line between the sym- margins of the petals are partly or com- bols of the two atoms that are bonded pletely fused, a corolla tube is formed. together. Compare hydrogen bond; ionic bond. corona 1. In flowering plants, any type of outgrowth from the petals or sepals, cp-DNA See chloroplast DNA. such as the trumpet of the daffodil flower (Narcissus). C3 plant A plant in which the first prod- 2. A group of cells at the tip of the oogo- uct of photosynthesis is a 3-carbon acid, nium in certain green algae (Chlorophyta), glycerate 3-phosphate (phosphoglyceric such as the stonewort Chara and the mer- acid). Most plants are C3 plants. They are maid’s cup (Acetabularia). characterized by high carbon dioxide com- pensation points owing to PHOTORESPIRA- corpus The central region of the meri- TION. Compare C4 plant. stem below the tunica where cell divisions are in all directions giving both increased C4 plant A plant in which the first prod- width and length to the apex. The tissues of uct of photosynthesis is a 4-carbon dicar- the stele and cortex are derived from the boxylic acid, oxaloacetic acid. C4 plants corpus. See tunica–corpus theory. have evolved from C3 plants by a modifica- tion in carbon dioxide fixation, leading to cortex (pl. cortices or cortexes) A pri- more efficient carbon dioxide uptake. The mary tissue in roots and stems of vascular modified pathway is called the Hatch– plants derived from the corpus meristem, Slack pathway or the C4 dicarboxylic acid that extends inwards from the epidermis pathway. In the leaves, the mesophyll cells to the phloem. It usually consists of surrounding the vascular bundles (bundle PARENCHYMA cells but other tissues (e.g. sheath cells) contain the carbon dioxide- collenchyma and sclerenchyma) may be fixing enzyme phosphoenolpyruvate car- present. boxylase (PEP carboxylase) in their cytoplasm. This has a higher affinity for See inflorescence. carbon dioxide than ribulose bisphosphate carboxylase (RUBP carboxylase). The prod- cothallus See thallus. uct of carbon dioxide fixation is oxalo- acetate, which is rapidly reduced to the C4 cotyledon (seed leaf) The first leaf of the acid malate or transaminated to the C4 acid embryo of seed plants, which is usually aspartate. These C4 acids are transported simpler in structure than later-formed to the bundle-sheath cells encircling the leaves. Cotyledons play an important part leaf veins, where they are decarboxylated, in the early stages of seedling development. releasing CO2, which is then refixed as in For example they act as storage organs in C3 plants. some seeds, such as and when C4 plants are mainly tropical or sub- mature. They form the first photosynthetic tropical, including many tropical grasses organ in seeds showing germina- (e.g. , sugar cane, and sorghum). tion, e.g. sunflower (Helianthus) but in They are more efficient than C3 plants, others they remain underground. Mono- producing more glucose per unit leaf area. cotyledons and dicotyledons are so termed See also bundle sheath; crassulacean acid because they normally contain one and metabolism. two cotyledons respectively although there are exceptions. crassulacean acid metabolism (CAM) A form of photosynthesis first discovered covalent bond A chemical bond in in the family Crassulacaeae and since

57 crista found in many other succulent plants, such day plant may receive and still flower, and as cacti. It allows plants of arid environ- conversely, the minimum a long-day plant ments to keep their stomata closed to con- needs to flower. Cocklebur, a short-day serve water during the day, opening them plant, will not flower if given more than at night to exchange gases. Carbon dioxide 15fi hours light per day; i.e. its cdl is 15fi taken in through the stomata at night is hours. Henbane, a long-day plant, will converted into an organic acid for storage. only flower if given more than 11 hours Instead of combining with ribulose bispho- light per day; i.e. the cdl is 11 hours. In re- sphate, it combines with the 3-carbon ality, however, it is the length of the dark phosphoenolpyruvate (PEP) to form ox- period that it critical, short-day plants re- aloacetic acid, which is then converted to quiring more than a critical length of dark malic acid for storage in the vacuole. The period before they will flower, and vice following day, when the stomata are versa. See photoperiodism. closed, the malic acid is broken down and the carbon dioxide is released to take part cross 1. The act of cross-fertilization. in the light reactions of photosynthesis. 2. The organism resulting from cross- When stressed by drought, plants with fertilization. crassulacean metabolism (CAM plants) can last for days or even weeks without crossing over The exchange of material opening their stomata, conserving water between homologous chromatids by the and living on stored resources. Compare formation of chiasmata. This results in the C3 plant; C4 plant. recombination of linked genes. Such reas- sortment of genes is the main source of ge- cremocarp A dry fruit splitting into two netic variation during sexual reproduction. one-seeded portions. The portions are , in which one chro- termed mericarps. matid ends up with one copy of a chromo- some segment while the other has three Cretaceous The most recent period of copies, results from crossing over after the Mesozoic era, 145–66 million years pairing between homologous chromo- ago. The dominant vegetation on land in somes that are not properly aligned. See the early Cretaceous was forests of cycads, also chiasma; independent assortment; conifers, gingkgoes and ferns. Most mod- tetrad. ern-type ferns, gymnosperms, and an- giosperms arose during the Cretaceous, crossover frequency See chromosome and by the end of the period the flowering map. plants had replaced the gymnosperms as the dominant terrestrial vegetation, form- cross-pollination The transfer of pollen ing vast broad-leaved forests with magno- from the anthers of one individual to the lias, figs, poplars, sycamores, , and stigma of another individual, usually of the herbaceous plants and a great variety of in- same species, with subsequent germination sect . The Cretaceous is named of the pollen and growth of the pollen tube. after the large amounts of chalk (fossilized This transfer is achieved by intermediary plankton) found in rocks of the period. See agents such as birds (ornithophily), , also geological time scale. insects (entomophily), wind (anemophily), or water (hydrophily). See incompatibility. crista (pl. cristae) The structure formed Compare self-pollination. by folding of the inner mitochondrial membrane, giving a large surface area for crown gall A disease of plants caused by electron transport. the soil-borne bacterium Agrobacterium tumefasciens. It causes galls both and critical day length (cdl) The amount of above and below ground on a wide range light per day that is the maximum a short- of plants, especially fruit trees.

58 cypsela

Cruciferae See Brassicaceae. A waterproof, protective layer of wax secreted by the epidermis that covers crumb structure The texture of a soil in the external surface of the plant, with the terms of the size of the soil particles and exception of the stomata and lenticels. how they interact, often determined simply Some seeds also have a cuticle. Its thickness by rubbing the soil with fingers. Some soils, varies with the species and environment. such as sandy soils, have a loose crumb Xerophytes tend to have thick , and structure with good drainage, whereas in dry conditions often induce cuticular thick- clay soils the particles are small and stick ening. Its main function appears to be re- together to give a dense crumb structure striction of water loss from the plant. On and poor drainage average, only about 5% of the water lost from a plant is via the cuticle (cuticular In early classifications, any transpiration). plant that reproduces by spores or gametes rather than by seeds. were cutin A group of substances chemically thus named because early botanists consid- related to fatty acids forming a continuous ered their method of reproduction to be layer called the cuticle on the epidermis of hidden (cryptic). They included the algae, plants, interrupted only by stomata or fungi, mosses, liverworts, hornworts, lenticels. Being fatty in nature, cutin is ferns, clubmosses, and horsetails, the vas- water-repellent, therefore helping to re- cular plant group often being termed vas- duce transpiration. It is also protective, for example preventing invasion by parasites. cular cryptogams. Compare phanerogam. See suberin. cryptophyte (geophyte)A plant in cutinization The impregnation of a which the resting buds are below the soil plant cell wall with cutin. surface or in water. See also Raunkiaer’s plant classification. cutting A part of a plant that is removed from the parent and encouraged to grow Any agricultural or horticul- into another plant. Cuttings are a means of tural ‘variety’. The term is derived from the asexual propagation and may vary in size words cultivated variety. from buds, leaves or root segments to large shoots. culture A population of microorganisms or dissociated cells of a tissue grown on or Cyanobacteria A phylum of Eubacteria within a solid or liquid medium for exper- containing the blue-green bacteria (for- imental purposes. This is done by inocula- merly called blue-green algae) and the tion and incubation of the nutrient green bacteria (chloroxybacteria). Both medium. See also tissue culture. groups convert carbon dioxide into or- ganic compounds using photosynthesis, culture medium A mixture of nutrients generally using water as a hydrogen donor used, in liquid form or solidified with agar, to yield oxygen, like green plants. Cyano- to cultivate microorganisms, such as bacte- bacteria are an ancient group, and their ria or fungi, or to support tissue cultures. fossils (stromatolites) have been dated at up to 3500 million years old. They are A plant that forms a tight spherical (coccoid) or form long micro- low hummock as an adaptation to cold and scopic filaments of individual cells. dry or windy situations. Cushion plants usually have small thick leaves to minimize Cycadophyta (cycads) A phylum of water loss, or hairy leaves to reduce tran- cone-bearing gymnosperms (about 75 liv- spiration. Cushion plants are common in ing species) with palmlike compound –alpine habitats. leaves and special CORALLOID ROOTS at or

59 cyst near the ground surface, which contain cyst A thick-walled resting spore. symbiotic nitrogen-fixing cyanobacteria. cysteine A sulfur-containing AMINO ACID cyclic AMP (cAMP; adenosine-3¢ ,5¢ - synthesized from and serine. It monophosphate) A form of adenosine is involved in the synthesis of biotin. It also monophosphate (see AMP) formed from acts as a store of sulfur for biosynthesis, ATP in a reaction catalyzed by the enzyme and can be broken down to pyruvate. adenyl cyclase. It has many functions, acting as an enzyme activator, genetic cystine A compound formed by the join- regulator, chemical attractant, secondary ing of two cysteine AMINO ACIDS through a messenger, and as a mediator in the activ- –S–S– linkage (a cystine link). Bonds of this ity of many hormones, including epineph- type are important in the conformation of rine, norepinephrine, vasopressin, ACTH, proteins, acting as cross-links between dif- and the prostaglandins. ferent parts of a polypeptide or between different polypeptide chains, thus deter- cyclic photophosphorylation In PHO- mining the pattern of folding. It is also re- TOSYNTHESIS, the flow of electrons from quired to the synthesis of the coenzyme , after excitation by ab- thiamin pyrophosphate, which is involved sorbed light energy, along the ferredoxin/ in many decarboxylation reactions. cytochrome b/f/plastocyanin electron- transport chain and back to Photosystem I A deposit of calcium carbon- ate arising internally on a stalk from the to be reenergized. Cyclic photophosphory- cell walls of large modified epidermal cells lation drives ATP synthesis, but does not in some flowering plants, e.g. stinging net- produce NADPH. tle (Urtica dioica). cyclins Regulatory proteins whose con- cytidine (cytosine nucleoside)A NUCLE- centration varies with certain stages of the OSIDE formed when cytosine is linked to D- CELL CYCLE. Cyclins help control progress ribose via a b-glycosidic bond. through the various stages of the cell cycle by activating protein kinases (cyclin- cytochrome oxidase (cytochrome aa3) dependent protein kinases). The enzyme that catalyzes the final step in the respiratory electron transport chain, in cyclosis (cytoplasmic streaming) The which water is formed from hydrogen ions streaming of cytoplasm in a circular mo- and oxygen. See respiration. tion around the cell observed in some plants, particularly young sieve tube el- cytochromes Conjugated proteins con- ements. taining heme, that act as intermediates in the electron-transport chains of photosyn- cycyanocobalamin (vitamin B12) One thesis and respiration. The iron ion in the of the water-soluble B-group of VITAMINS. porphyrin prosthetic group of cytochromes changes its oxidation state from ferric cymose inflorescence (cyme; definite in- (Fe3+) to ferrous (Fe2+) when the cy- florescence) See inflorescence. tochrome is reduced. There are four main classes, designated a, b, c, and d, most cypsela An achenelike fruit that devel- bound to membranes. ops from an inferior ovary and contains some tissues not derived form the carpel. It cytogamy The fusion of gametes during is common in the Asteraceae, e.g. dande- fertilization. Compare karyogamy. lion (Taraxacum), whose fruits bear a ring of hairs (the pappus) derived from the The division of the cyto- calyx. plasm after nuclear division (mitosis or

60 cytotaxonomy meiosis). In the cells of plants and most the phloem and be important in sequential algae it involves formation of a new plant leaf senescence up the stem. cell wall by means of a structure called the cell plate. In primitive protoctists, as in an- cytology The study of cells; . imals, the cell membrane ingrows under the control of a ring of actin microfila- cytoplasm The living contents of a cell, ments, and a furrow constricts the cell excluding the nucleus and large vacuoles, equator until it pinches the cell in two. See in which many metabolic activities occur. cell plate. It is contained within the plasma mem- brane and comprises a colorless substance cytokinin One of a class of plant hor- (hyaloplasm) containing organelles and mones concerned with the stimulation various inclusions (e.g. crystals and insolu- ble food reserves). The cytoplasm is about of cell division, nucleic acid metabolism, 90% water. It is a true solution of ions (e.g. and root–shoot interactions. , sodium, and chloride), small are often purine derivatives: e.g. kinetin (6- molecules (e.g. sugars, amino acids, and furfuryl aminopurine), an artificial cy- ATP), and a colloidal solution of large mol- tokinin commonly used in experiments to ecules (e.g. proteins, lipids, and nucleic promote cell division in tissue cultures; and acids). It can be gel-like, usually in its outer zeatin, found in maize cobs. Natural cy- regions, or sol-like. A complex cytoskel- tokinins are all derivatives of adenine. eton of F-actin filaments and microtubules Cytokinins are produced in roots, is often present, and appears to control the where they stimulate cell division. They are structure of the cytoplasm, cell move- also transported from roots to shoots in the ments, and movements of the cytoplasm , where they are essen- and conformation of the membranes. See tial for healthy leaf growth. Subsequent cytoskeleton; organelle; protoplasm. movement from the leaves to younger leaves, buds, and other parts may occur in cytoplasmic inheritance (extrachromo-

61 D

2,4-D (2,4-dichlorophenoxyacetic acid) evolution by . Darwin A synthetic AUXIN used as a potent selec- first became interested in natural history at tive weedkiller. Monocotyledenous species Cambridge . He was inspired by with narrow erect leaves (e.g. and the geologist Adam Sedgwick and encour- grasses) are generally resistant, while di- aged by the professor of botany at Cam- cotyledonous plants are often very suscep- bridge, John Henslow, to apply for the tible. The compound is thus particularly position of naturalist on a survey of the Pa- effective in controlling weeds in cereal cific and South America on HMS Beagle in crops and lawns. 1831. On this voyage, he observed many different animal species and was struck by dark-ground illumination A light mi- the way in which they gradually changed croscopy technique used to study transpar- from one location to another, in particular, ent material, such as living cells or the finches of the Galápagos Islands. He microbes. The specimen is lighted from the recorded 14 species of finches on these is- side, so that only diffracted light from the lands, each species dominating a particular specimen passes through into the objective. part of the islands in different conditions. The specimen thus appears bright against a He speculated that it was probably un- dark background. The difference in refrac- likely that they could all have arisen spon- tive index between organelles and the sur- taneously as separate species and thought rounding cytoplasm causes the boundaries that it was more probable that they had between organelles and cytoplasm to re- evolved from a mainland species of finch, flect more light. possibly from Ecuador. It took 20 years for Darwin to collect enough evidence to pub- dark reactions (light-independent reac- lish his seminal work On the Origin of tions)A group of reactions that follow the Species by Means of Natural Selection LIGHT REACTION in PHOTOSYNTHESIS and (1859), having been stimulated to publish form glucose and other reduced products by an unexpected letter in 1858 from the from carbon dioxide. They are not depen- British naturalist Alfred Russel WALLACE, dent on light, although they can take place outlining theories similar to Darwin’s own. in the light. The dark reactions use energy Darwin and Wallace read a joint paper on (as ATP) and reducing power (as NADPH) their theories to the Linnaean Society in generated by the light reaction of photo- London in the summer of 1858. The publi- synthesis to reduce carbon dioxide. In eu- cation of Darwin’s book led to a storm of karyotes, the dark reactions take place in controversy among and church- the stroma of the chloroplast by one of two men as his theory challenged the biblical pathways, according to whether the plant account of the Creation. Darwin continued is a C3 PLANT or a C4 PLANT. The reduced to publish more works, including his The carbon dioxide is incorporated into carbo- Descent of Man (1871), in which he ap- hydrates in the Calvin cycle. See also cras- plied his theory of evolution to humans, sulacean acid metabolism. implying that they had descended from apelike creatures Darwin, Charles Robert (1809–82) British naturalist noted for his theory of Darwinism Darwin’s explanation of

62 deficiency disease the mechanism of evolutionary change, ing them enabled him to work out their de- namely, that in any varied population of velopment and life cycles, thus laying organisms only the best adapted to that en- down the foundations of modern mycol- vironment will tend to survive and repro- ogy. In his Researches on Fungal Blights duce. Individuals that are less well adapted (1853) outlining certain rust and smut dis- will tend to perish without reproducing. eases, he explained that fungi are the cause Hence the unfavorable characteristics, pos- and not the effect of diseases. He also sessed by the less well-adapted individuals, showed that a was an association be- will tend to disappear from a species, and tween an alga and a fungus and coined the the favorable characteristics will become term ‘symbiosis’ for dissimilar plants living more common. Over time the characteris- together to their mutual benefit. tics of a species will therefore change, even- tually resulting in the formation of new decarboxylase An enzyme that cat- species. Darwin called this process of selec- alyzes the decarboxylation of carboxylic tive birth and death NATURAL SELECTION. acids, including the conversion of amino See also neo-Darwinism. acids to amines. dating techniques Methods used to de- deciduous Denoting plants that season- termine the age of rocks, fossils, or archae- ally shed all their leaves, for example be- ological remains. There are two main fore the winter, or dry season. It is an methods. Relative dating assesses the age adaptation to prevent excessive water loss of a specimen in comparison to other spec- by transpiration when water is scarce. imens. Absolute dating involves assessing Compare evergreen. the actual age of a specimen by using some reliable measure of time. See also den- decomposer An organism that feeds drochronology; radiometric dating. upon dead organisms, breaking them down into simpler substances. Decom- day-neutral plant A plant that requires posers recycle nutrients, making them no particular photoperiod to flower. See available to producer organisms. Bacteria photoperiodism. and fungi are important decomposers in most ecosystems. See also ; deamination A type of chemical reac- trophic level. tion in which an amino group (NH2) is re- moved from a compound. Amino acids can decussate Describing leaves in opposite be broken down by oxidative deamination. pairs and with alternating pairs at right an- Transamination of many amino acids gles to each other up the stem. yields glutamate, which can be deaminated to a-ketoglutarate, which can then enter dedifferentiation The reversion of a the Krebs cycle to be further broken down differentiated cell to the meristematic (un- with the release of energy. See transamina- differentiated) state, which is capable of tion. differentiating again. It occurs naturally just before the production of secondary De Bary, Heinrich Anton (1831–88) meristems or in response to wounding or German botanist. De Bary abandoned a ca- stimulation by growth factors. See callus; reer in medicine in to devote the totipotency. rest of his life to his botanical studies, be- coming professor of botany at a number of deficiency disease A disease caused by German . His work on fungi deficiency of a particular essential nutri- represents a major landmark in the under- ent (such as a MICRONUTRIENT or trace el- standing of and fight against plant dis- ement), usually with a characteristic set of eases. His observations of fungi in their symptoms. For example, iron deficiency re- natural and his methods of cultur- sults in CHLOROSIS between the veins, par-

63 definite growth ticularly in young leaves that are still syn- alyzes the removal of certain hydrogen thesizing chlorophyll. Nitrogen deficiency atoms from specific substances in biologi- causes stunting of growth and chlorosis of cal systems. Dehydrogenases are usually older leaves, as chlorophyll pigments are called after the name of their substrate, e.g. broken down so their components can be lactate dehydrogenase. Some dehydroge- reused in the young growing parts of the nases are highly specific, with respect to plant. both their substrate and coenzyme, whilst others catalyze the oxidation of a wide definite growth See determinate growth. range of substrates. Many require the pres- ence of a coenzyme, which is often definite inflorescence See cymose in- involved as a hydrogen acceptor. Dehydro- florescence. genases catalyze the transfer of two hydro- gen atoms from substrates to NAD and definitive nucleus See polar nuclei. NADP. See also alcohol dehydrogenase. deforestation (disafforestation) The per- deletion See chromosome mutation. manent removal of forests, especially by means of logging for commercial timber deme A subpopulation of a species: a and clearing for agriculture and human set- discrete interbreeding group of organisms, tlement. A major cause of loss of natural with recognizable cytological or genetic habitats and biodiversity worldwide, it can characters, that is spatially distinct from lead to soil , flooding downstream, other such groups, although it may be ad- and increasing drought in neighboring jacent to them. Compare . areas. denaturation A process that causes un- degenerate code A part of the genetic folding of the peptide chain of proteins or code in which some amino acids are coded of the double helix of DNA. These changes for by more than one triplet of nucleotide may be brought about by a variety of phys- bases (codons). For some amino acids, only ical factors: change in pH, temperature, vi- the first two bases appear to be important, olent shaking, and radiation. The primary and it matters little what the third base is. structure remains intact. Denatured pro- For example, leucine is coded for by CUU, teins and nucleic acids show changes in CUC, CUA, and CUG, and by UUA and physical and biological properties; dena- UUG. tured proteins, for example, are often in- soluble in solvents in which they were dehiscent Describing a fruit or fruiting originally soluble. body that opens at maturity to release the seeds or spores. Dehiscence may be violent dendrochronology A method of ar- to aid . Compare indehiscent. chaeological dating by the ANNUAL RINGS of trees, used when the lifespans of living and dehydration A process followed when fossil trees in an area overlap. Exact dates tissues are prepared for permanent micro- for sites can be calculated and the method scope slides. Water is removed by immers- is more accurate than radiometric dating ing the tissue in increasing strengths of techniques. Bristlecone pines, which can ethyl alcohol. The alcohol concentration live for up to 5000 years, have been used in must be increased gradually as otherwise such work. the cells would dehydrate too quickly and shrink. Dehydration is necessary because denitrification The chemical reduction water does not mix with the chemicals used of nitrate by soil bacteria. Denitrifying bac- in clearing and mounting sections. teria such as Pseudomonas, Micrococcus, and Clostridium use nitrate as the terminal dehydrogenase An enzyme that cat- electron acceptor in anaerobic respiration.

64 De Vries, Hugo

The process is important in terms of soil determinate growth (definite growth)A fertility since the products of denitrifica- form of growth that has a finite limit, i.e. tion (e.g. nitrites and ammonia) cannot be once a maximum size is reached, no further used by plants as a nitrogen source. Com- growth occurs. It occurs, for example, in pare nitrification. See nitrogen cycle. annual and biennial plants and plant or- gans such as internodes. The term is some- dentate Toothed: describing leaf mar- times applied to cymose inflorescences, in gins that have a series of outward-pointing which growth ceases with the production notches. of a terminal flower bud. Compare indefi- nite growth. denticulate Describing a leaf margin that is finely toothed. detritus Fragments of dead material, such as leaf litter and products of the deoxyribonuclease See DNase. breakdown of organic material by decom- posers. deoxyribonucleic acid See DNA. Deuteromycota See Fungi Anamorphici. deoxyribose A pentose (five-carbon) sugar that is a component of the nu- The geological period known cleotides that make up DNA. as the ‘Age of Fish’, some 405–355 million years ago, between the and the deoxy sugar A SUGAR in which oxygen Carboniferous periods of the Paleozoic era. has been lost by replacement of a hydroxyl It was also the time when the first major in- group (OH) with hydrogen (H). The most vasion of the land by plants occurred. The important example is DEOXYRIBOSE. very first land plants date from the late Sil- urian, but during the Devonian many vas- dermatogen (protoderm) See histogen cular land plants appeared, such as the theory. (all now extinct), clubmosses (Lycophyta), horsetails (Sphenophyta), An area in which the rate of evap- and eusporangiate ferns (early members of oration exceeds the rate of precipitation for most of the time. A desert may form in the Filicinophyta. The gymnosperms prob- any climate where average annual precipi- ably arose in the Late Devonian. See also tation is less than 250 mm and intermit- geological time scale. tent. A ‘true desert’ has no plant life at all. The term is also used for semideserts in De Vries, Hugo (1848–1935) Dutch which plant and animal life is sparsely dis- plant physiologist and geneticist. An expert tributed, and adapted to long periods of on Netherlands flora, De Vries showed drought. EPHEMERAL plants are common that of the fluid in plant and there may also be succulent or xero- cells was partially responsible for growth phytic perennials. Cold include and coined the term ‘plasmolysis’ for the parts of the TUNDRA and may also form in effect on cells of water loss during wilting. hot regions with annual rainfall of less than His work in this area formed a basis for 400 mm. theories of osmosis. In the 1880s he developed an interest in desiccation The drying out of an organ- heredity, proposing that there were struc- ism due to evaporation. tures in the nucleus (which he called ‘pan- genes’) that determine characteristics in the desmids A group of green algae of the offspring. He carried out phylum GAMOPHYTA that are basically sin- experiments, and the results of his crosses gle-celled but in some species form fila- showed segregation of characters in the mentous or irregular colonies. progeny in a ratio of 3:1. He came across

65 dextrin

MENDEL’s work in 1900 and immediately sediments on the floors of oceans and lakes published his results. called diatomaceous earth or kieselguhr, In The Mutation Theory (1901–03), De which is used in insulating materials, fire- Vries postulated that new species could proof cements, filters, and in the manufac- arise from a single dramatic mutation. He ture of explosives. had observed different types of evening primrose within a single colony and be- diatropism (diageotropism) See tropism; lieved them to be the result of mutations. It . has subsequently been discovered that the plants De Vries believed to be mutants dicaryon See dikaryon. were triploids or tetraploids, but his essen- tial premise continues to be important, dichasial cyme (dichasium) See inflo- showing that variation is essential for evo- rescence. lution within a species. dichogamy The condition in which the dextrin Any of a class of intermediates anthers and stigmas mature at different produced by the hydrolysis of starch. Fur- times thus helping prevent self-pollination. ther hydrolysis eventually produces the Compare homogamy. See protandry; pro- monosaccharide glucose. togyny. dextrose (grape sugar) An outdated dichotomy Forked branching produced name for GLUCOSE. by division of the growing point into two equal parts, seen for example in Fucus. diakinesis The last stage of the PROPHASE in the first division of MEIOSIS. Dicotyledonae A class of flowering Chiasmata are seen during this stage, and plants (phylum Angiophyta) characterized by the end of diakinesis the nucleoli and by having two cotyledons in the seed. They nuclear membrane have disappeared. Dur- include herbs, shrubs, and trees and sec- ing diakinesis the sister chromatids of ho- ondary growth is normal. Examples are the mologous pairs of chromosomes complete sunflower and the oak. The flower parts their separation, and the chromosomes coil are arranged in fours or fives, or multiples tightly, shortening and thickening. thereof, and the leaf veins are branched. The vascular bundles of the stem are dialysis The separation of large mol- arranged in a ring within a single endoder- ecules from smaller ones by mis and , giving a eustele. Com- through a selectively permeable membrane pare Monocotyledonae. that allows through only molecules up to a certain size. For example, proteins can be dictyosome A stack of membrane- separated from amino acids or glucose in bounded sacs (cisternae) that, together this way. Compare osmosis. with associated vesicles (Golgi vesicles), forms the GOLGI APPARATUS. The term is Diatoms A phylum of the Protoctista usually only applied to plant cells, where whose members are unicellular algae found many such stacks are found. In contrast, in freshwater, the sea, and soil. Much of the Golgi apparatus of most animal cells is plankton is composed of diatoms and they a continuous network of membranes. The are thus important in FOOD CHAINS. Di- term dictyosome is little used today. atoms are diploid unicells with silica cell walls (frustules) composed of two overlap- dictyostele A modified solenostele that ping valves ornamented with perforations, is broken up by large leaf gaps so crowded which are arranged differently in each together that they overlap. The tube of a species. The frustules of dead diatoms are stelar tissue is thus broken up into a mesh, highly resistant to decay, and form deep each small segment of remaining vascular

66 diplobiontic tissue being called a meristele. It is found in dikaryon (dicaryon)A fungal hypha or certain stems (e.g. Dryopteris). mycelium whose cells each contain two dif- ferent nuclei, arising from the fusion of two differentially permeable membrane compatible cells, each with one nucleus. See osmosis. The nuclei do not fuse immediately, in- stead dividing independently but simulta- differentiation A process of change neously. Dikaryosis is common in during which cells with generalized form ascomycetes and basidiomycetes, and re- become morphologically and functionally sults from the fusion of two monokaryotic specialized to produce the different cell mycelia of different mating types. types that make up the various tissues and organs of the organism. See also toti- dimorphism The existence of two dis- potency. tinct forms. Examples include male and fe- male plants in dioecious species, aerial and diffuse-porous Describing wood in submerged leaves, alternation of gameto- which vessels of approximately equal di- phyte and sporophyte stages of a life cycle, ameter tend to be evenly distributed so and mesophyll and bundle-sheath chloro- there is no obvious growth ring. Diffuse- plasts. porous wood is seen for example in yellow birch (Betula lutea). Compare ring-porous. dinitrogenase The enzyme that cat- alyzes . diffusion The movement of molecules along a concentration gradient from areas See Dinomastigota. of high concentration to areas of lower concentration as a result of random move- Dinomastigota () A phy- ment. Gaseous exchange in plants take lum of the Protoctista containing mainly place by diffusion, and some of the move- single-celled marine or freshwater algae ment of ions through plant fluids is also that swim in a twirling manner by means of due to diffusion. See dialysis; transpiration. two UNDULIPODIA that lie at right angles to Compare active transport; osmosis. each other in two grooves within the or- ganism’s rigid body wall (test). The body diffusion pressure deficit (DPD) The wall is often stiffened by cellulose plates net force that causes water to enter a plant under the cell membrane. Many dinoflagel- cell. It is the difference between OSMOTIC lates possess stinging organelles (tricho- PRESSURE (P) and TURGOR PRESSURE: cysts) that they discharge to catch prey; DPD = P – TP. and some produce potent toxins that are See water potential. capable of killing fish. Planktonic dinofla- gellates can multiply rapidly to form red dihybrid (dihybrid cross) An organism tides. The phosphorescent seas of the trop- that is heterozygous at two loci, formed by ics are usually due to bioluminescent di- crossing homozygous parents with differ- noflagellates. Roughly half of all known ent alleles at two given loci: for example, dinoflagellates are capable of photosynthe- Mendel’s cross between yellow round sis. Many form symbioses with marine in- (YYRR) and green wrinkled (yyrr) vertebrates such as corals and clams. peas to give a yellow round dihybrid (YyRr). When a dihybrid is selfed, a char- dinucleotide A compound of two nu- acteristic dihybrid ratio of 9:3:3:1 is ob- cleotides linked by their phosphate groups. tained in the offspring. Nine plants exhibit Important examples are the coenzymes both dominant characters, six plants show NAD and FAD. one dominant and one recessive character, and one plant exhibits both recessives. dioecious Denoting a plant species in Compare monohybrid. which male and female reproductive or-

67 diploid gans are borne on separate individuals. whose distribution is not continuous Compare hermaphrodite; monoecious. within its range. Examples are species that are found in the Alps, arctic regions, and diplobiontic Describing life cycles show- north European mountains and ing a typical alternation of generations that occur in eastern North America, Cen- with haploid and diploid somatic bodies. tral America, and southeast Asia, but not Ferns and mosses are diplobiontic organ- in the intervening regions. Such disjunc- isms. Compare haplobiontic. tions may arise where re- duces the range of a species, restricting diploid A cell or organism containing them to certain isolated areas (refugia) twice the haploid number of chromosomes where the local climate is more suitable for (i.e. 2n). In plants exhibiting an alternation them, i.e. since the last Ice Age for the cold- of generations the sporophyte is diploid, loving arctic–alpine species now confined and the gametophyte haploid. In lower to isolated mountain tops. The other major plants such as mosses and liverworts the cause of disjunction is CONTINENTAL DRIFT. persistent vegetative plant is haploid, the sporophyte being a relatively short-lived disk floret See inflorescence. phase. Higher plants are normally always diploid. Exceptions are those species in distal Denoting the part of an organ, which polyploidy occurs. limb, etc., that is furthest from the origin or point of attachment. Compare proximal. diplont A diploid organism that repre- sents the vegetative stage in life cycles in distely Having two steles, e.g. the stem which haploidy is restricted to the gametes. of kraussiana. The steles are Such diplontic life cycles are found in some joined only at branches. See also polystely. protoctists, such as the diatoms and certain Phaeophyta (brown algae), including the disulfide bridge In PROTEINS, a covalent wracks. Compare haplont; alternation of bond formed between the sulfhydryl (–SH) generations. groups of different cysteine molecules. Such bridges are important in stabilizing diplotene In MEIOSIS, the stage in late PRO- the tertiary structure of peptides and pro- PHASE I when the pairs of chromatids begin to separate from the tetrad formed by the teins. association of homologous chromosomes. Chiasmata can often be seen at this stage. diurnal 1. Describing an event or activity occurring during the day or a species or process that is active only by day and not disaccharide A SUGAR with molecules composed of two monosaccharide units. by night. Sucrose and maltose are examples. These 2. Occurring at daily intervals. See circa- are linked by a –O– linkage (glycosidic dian rhythm. link). See also glycosidic bond. divergent evolution The evolution of discontinuous variation (qualitative different forms from a single basic struc- variation; qualitative inheritance) A form ture in response to different selection pres- of variation in which a character has two sures. For example, the basic angiosperm or more distinct forms in a population. Ex- stem is vertical and elongated, but some amples are Mendel’s characters. It gen- cacti have swollen water-storing stems, erally occurs when there are two or more while other xerophytic plants that have allelic forms of a major gene in a popula- lost their leaves during the course of evolu- tion. See . tion have evolved flattened leaflike stems for photosynthesis, e.g. butcher’s broom disjunct Denoting a species or genus (Ruscus aculeatus).

68 DNA division The equivalent of a phylum in plants (and hence phylum names too) end the Five Kingdoms classification. The term in -phyta (e.g. Bryophyta, Filicinophyta), division was retained for the plant king- dom until recently. Division names of while those of fungi end in –mycota (e.g.

DNA

69 DNA hybridization

Ascomycota, Basidiomycota). Divisions each other to give a double helix. Phos- may be divided into subdivisions. phate molecules alternate with deoxyri- bose sugar molecules along both chains, DNA (deoxyribonucleic acid) A nucleic linked by phosphodiester bonds. Each acid, mainly found in the CHROMOSOMES, sugar molecule is also joined to one of four that contains the hereditary information of nitrogenous bases – adenine, guanine, cy- organisms. The molecule is made up of two tosine, or thymine. The two chains are helical polynucleotide chains coiled around joined to each other by hydrogen bonding between bases. The sequence of bases along the chain makes up a code – the ge- netic code – that determines the precise se- quence of amino acids in proteins (see messenger RNA; protein synthesis; tran- scription). In DNA, the two purine bases (adenine and guanine) always bond with the pyrim- idine bases (thymine and cytosine), and the pairing is quite specific: adenine with thymine and guanine with cytosine. DNA is the hereditary material of all organisms with the exception of RNA viruses. To- gether with RNA and histones it makes up the chromosomes of eukaryotic cells. Some DNA viruses contain single-stranded DNA rather than the double helix. See also junk DNA; replication; selfish DNA; RNA.

DNA hybridization See nucleic acid hybridization.

DNA ligase An enzyme, used in DNA REPLICATION and the in vitro synthesis of DNA, that joins together segments of newly synthesized polynucleotide chains using energy from ATP. See replication. DNA polymerase See polymerase. DNA probe (gene probe)A nucleic acid consisting of a single strand of nucleotides whose base sequence is complementary to that of a particular DNA fragment being sought, for example a gene on a chromo- some or a restriction fragment in a DNA digest. The probe is labeled (e.g. with a ra- dioisotope or a fluorescent compound) so that when it binds to the target sequence, both it and the target can be identified (by autoradiography or FLUORESCENCE MI- CROSCOPY). DNA probes are used to ana- lyze fragments of DNA or RNA generated by restriction enzymes and determine their DNA: the double helix

70 dwarfism sequence in the original nucleic acid, a dormin (abscisin II)A former name for process called restriction mapping (see re- ABSCISIC ACID. striction map). They are also used to detect gene and chromosome mutations. See also dorsal 1. Designating the lower or abax- FISH. ial surface of a lateral organ such as a leaf. 2. Designating the upper surface away DNase (deoxyribonuclease) Any en- from the substrate in thallose plants such zyme that hydrolyzes the phosphodiester as liverworts. Compare ventral. bonds of DNA. DNases are classified into two groups, according to their site of ac- dorsifixed A stamen in which the fila- tion in the DNA molecule (see endonucle- ment is fused to the back of the anther. ase; exonuclease). Compare basifixed; versatile. domain 1. A discrete part of the tertiary dorsiventral Describing a structure that structure of a PROTEIN that has a particular has distinct upper and lower surfaces. A function, e.g. as the binding site for the dorsiventrally flattened structure has flat substrate of an enzyme. dorsal and ventral surfaces and very nar- 2. A taxonomic grouping of organisms that row lateral surfaces. ranks higher than kingdom. There are gen- erally considered to be three domains: AR- double fertilization The fusion of one CHAEA, BACTERIA, and EUKARYA. The FIVE pollen nucleus with the egg to form the zy- KINGDOMS CLASSIFICATION recognizes only gote and of the other pollen nucleus with a two ‘superkingdoms’: Prokarya (which in- polar nucleus to form the triploid en- cludes the single kingdom Bacteria com- dosperm nucleus. The process is restricted prising both archaea and the bacteria) and to certain angiosperms but a primitive Eukarya. form is known from some gymnosperms. domatium (pl. domatia) See ant plant. double helix See DNA. dominant 1. An allele that, in a het- double recessive An organism contain- erozygote, prevents the expression of an- ing both recessive alleles of a particular other (recessive) allele at the same locus. gene and thus expressing the recessive form Organisms with one dominant and one re- of the gene in its phenotype. Double reces- cessive allele thus appear identical to those sives, being of known , are often with two dominant alleles, the difference in used in test crosses to establish whether the their becoming apparent only on organism to which it is crossed is heterozy- examination of their progenies. The domi- gous or homozygous for the same gene. See nant allele usually controls the normal backcross. form of the gene, while mutations are gen- erally RECESSIVE. DPD See diffusion pressure deficit. 2. A plant species that is the most impor- tant member of a particular community drupe (pyrenocarp) A fleshy indehiscent and affects the structure of that commu- fruit containing one or more seeds each nity. surrounded by a hard stony wall, the endo- carp. with one seed include plums dormancy A period of minimal meta- and cherries while many-seeded drupes in- bolic activity of an organism or reproduc- clude holly and elder fruits. Blackberries tive body. It is a means of surviving a and raspberries are collections of small period of adverse environmental condi- drupes or drupelets. tions, e.g. cold or drought. Seeds, spores, cysts, and perennating organs of plants are duplex Double, or having two distinct potentially dormant structures. See estiva- parts. The term is particularly used to de- tion. scribe the double helix of the Watson–

71 E

ecocline A CLINE that is due to a specific within a species adapted genetically to the environmental factor, such as a gradient of combination of environmental factors in heavy metal concentration in the soil. their local habitat, but still able to repro- duce with other ecotypes belonging to The functional role of the same species and produce fertile off- an organism in a community. It includes spring. Differences between ecotypes may the habitat in which the organism , the be physiological or morphological. Com- resources it uses, and the periods of time in pare biotype. See also adaptive radiation; which it is present and/or active there and . its interactions with other organisms. It can normally be defined only by the pres- ectomycorrhiza (ectotrophic ) ence of the organism and is used more ex- (pl. ectomycorrhizae) See mycorrhiza. tensively for animals than for plants. If two species occupy the same niche, then com- ectoparasite See parasitism. petition may occur until either one has re- placed the other or the two divide the niche ectoplasm The outer gel-like layer of between them to some extent. A similar the cytoplasm in the cells of plants and niche may be occupied by different species some protoctists, which lies immediately in different areas. For example different beneath the cell membrane and contains a species of grasses are dominant in grass- dense array of microtubules. lands in different parts of the world. ectotrophic mycorrhiza See mycor- ecology The study of the relationships rhiza. of organisms to one another and to their living (biotic) and nonliving (abiotic) envi- edaphic factors The physical, chemical, ronment. and biological characteristics of the soil that together form an important compo- ecosystem (ecological system) A unit nent of the habitat because of their influ- made up of all the living and nonliving ence on plant distribution. The main components of a particular area that inter- edaphic factors are water content, pH, or- act and exchange materials with each ganic matter, and soil texture. other. The concept of the ecosystem differs from that of the community in that more EDTA Ethylenediamine tetra-acetic acid, emphasis is placed on abiotic factors. Var- a compound that acts as a chelating agent, ious studies have been made to attempt to reversibly binding iron, magnesium, and itemize the energy flow of an entire eco- other positive ions. It is used as a competi- system, taking into account factors such as tive inhibitor of certain enzymes that have incoming radiation, photosynthetic effi- metal ion cofactors, and as a source of ciency, etc. The term can be applied on var- iron, which it releases slowly into the solu- ious scales, from small ponds to the whole tion. planet. effector A molecule that can combine ecotype A population of individuals with a repressor in an OPERON, making the

72 elongation

REPRESSOR inactive and thus enabling MES- In eukaryotic cells, the enzymes and SENGER RNA and protein to be produced. other components of the respiratory elec- tron-transport chain are located in the egg apparatus The three haploid nuclei inner membrane of the MITOCHONDRIA. that are situated at the micropylar end of During aerobic respiration the reduced the embryo sac in most flowering plants. coenzyme NADH, produced by the KREBS The central nucleus is the female gamete CYCLE in the mitochondrial matrix gives up and those to either side of it are called the two electrons to the first component in the synergids. See embryo sac. See also antipo- chain, NADH dehydrogenase, and two hy- dal cells. drogen ions (H+) are discharged from the matrix of the into the inter- egg cell See ovum. membrane space. The electrons are trans- ferred along the chain to a carrier molecule (lipidoplast; oleoplast) A col- (COENZYME Q), and then in sequence to a orless plastid (leucoplast) storing lipids series of cytochromes, finally acting with (fats or ). the enzyme cytochrome oxidase to reduce an oxygen atom, which combines with two elaiosome See oil. H+ ions to form water. During this electron transfer, a further two pairs of H+ ions are elater One of many elongated spirally pumped into the intermembrane space, thickened cells that are formed within making a total of six per molecule of the capsule of certain liverworts (e.g. NADH. If FADH2 is the electron donor, Marchantia and Pellia). When the elaters only four H+ ions are pumped across. are exposed to the air they dry out un- The function of electron transport in evenly because of the differential thicken- the mitochondrion is to phosphorylate ing. The resulting twisting movements of ADP to ATP. the cells help in dispersing the spores. electrophoresis The migration of elec- electron carrier A molecule that can trically charged particles toward oppo- function as both an acceptor and a donor sitely charged electrodes in solution under of electrons and/or protons in an electron- an electric field – the positive particles to transport system. See coenzyme Q; cy- the cathode and negative particles to the tochromes; electron-transport chain; fer- anode. The rate of migration varies with redoxins. molecular size and shape. The technique can be used to separate or analyze mixtures electron micrograph See micrograph. (e.g. of proteins or nucleic acids). See microscope. elicitors Molecules that initiate cell sig- naling pathways leading to the activation electron-transport chain (respiratory of plant defense genes and the production chain) A chain of membrane-linked oxi- of PHYTOALEXINS and other defensive dation–reduction reactions involving pro- chemicals in response to infection by a teins and enzymes, during which electrons pathogen. are transferred from an initial electron donor through a series of intermediates to elongation The enlargement of cells a final electron acceptor, resulting in the after mitotic division that results from the formation of ATP. The main electron- uptake of water by OSMOSIS until the cells transport systems in plants are involved in become turgid. Elongation also involves respiration and the light reaction of PHO- the enzyme-mediated stretching of the cell TOSYNTHESIS. Heterotrophic bacteria have walls. Hormones, especially AUXINS, are in- a similar respiratory electron-transport volved in the promotion of elongation. chain. TROPISMS are due to differential elongation

73 elongation factor of cells on different sides of root or shoot, the of higher plants. See embryo resulting in curvature. sac. elongation factor See translation. embryogeny (embryony) 1. The devel- opment of an EMBRYO from the moment elution The removal of an adsorbed the ZYGOTE divides. substance in a CHROMATOGRAPHY column 2. The development of from or ion-exchange column using a solvent diploid somatic cells in embryo culture. (eluent), giving a solution called the eluate. Such embryos are often termed embryoids, The chromatography column can selec- to distinguish them from embryos formed tively adsorb one or more components from . They are capable of giving from the mixture to ensure efficient recov- rise to normal plants. Embryoids that de- ery of these components graded elution is velop from pollen grains grow into haploid used. The eluent is changed in a regular plants. manner starting with a nonpolar solvent and gradually replacing it by a more polar The study of the develop- one. This will wash the strongly polar com- ment of embryos, usually from fertiliza- ponents from the column. tion. emasculation The removal of the sta- embryony See embryogeny. mens of a plant before the anthers burst to prevent self-pollination or unwanted embryo sac A large oval thin-walled sac crosses to nearby plants. in the nucellus of flowering plants in which egg fertilization and subsequent embryo Embden–Meyerhof–Parnas pathway development occurs. It corresponds to the See glycolysis. female GAMETOPHYTE of lower plants and contains a number of nuclei derived by di- embedding The sealing of tissue pre- vision of the MEGASPORE nucleus. pared for permanent microscope slides in a There are commonly eight nuclei in the solid block of paraffin wax prior to sec- embryo sac: the egg apparatus at the mi- tioning. After CLEARING, tissues are placed cropylar end, made up of an egg nucleus in two or three baths of molten paraffin and two synergid nuclei; three antipodal wax. When the tissue is completely infil- cells at the opposite chalazal end that prob- trated by the wax it is allowed to harden. ably aid embryo nourishment; and two As the wax is opaque the block must be polar nuclei in the center that fuse to form marked to insure correct orientation when the primary endosperm nucleus. Other sectioning. Tissue prepared for electron plants have four nuclei or sixteen nuclei in microscopy may be embedded in tougher their embryo sacs. At fertilization one male epoxy such as Araldite and cut with a diamond knife. See also fixation. embryo The organism that develops from the zygote of a after the proembryo has differentiated into em- bryo and SUSPENSOR but before germina- tion, i.e. while the young plant is still nutritionally dependent on parental tis- sues. In the Filicinophyta, Sphenophyta, and Lycophyta the embryo sporophyte produces a footlike structure that remains embedded in the tissues of the parent ga- metophyte plant, absorbing nutrients like Embryo sac

74 endoplasmic reticulum nucleus fuses with the egg nucleus to form from lysosomes. Compare exocytosis. See the zygote, while the second male nucleus lysosome. fuses with the primary endosperm nucleus to form a triploid cell, or more rarely a sec- endodermis The innermost part of the ond diploid or a pentaploid cell that later cortex, consisting of a single layer of cells gives rise to the ENDOSPERM. In the gym- that controls the passage of water and nosperms the megaspore gives rise to a cell solutes between the cortex and the stele. A that is termed the embryo sac because of its clearly defined endodermis is seen in all similarity to the angiosperm structure. See roots and in the stems of the non seed-bear- nucellus; micropyle. ing vascular plants and some dicotyledons. See Casparian strip; passage cells. Emerson effect The observation by Robert Emerson in 1957 that PHOTOSYN- endogenous Produced or originating THESIS proceeds much faster when chloro- within an organism. Compare exogenous. plasts are illuminated with light at a wavelength of 650 nm as well as at 700 nm endomitosis The duplication of chro- (the absorption wavelength of chlorophyll mosomes without division of the nucleus, a molecules). It contributed to the discov- causing polyploidy and leading to an in- ery of PHOTOSYSTEMS I AND II. crease in nuclear and cytoplasmic volume. Endomitosis occurs regularly in certain tis- endangered species See CITES; Red sues, such as the phloem cells of some legu- Data Book. minous plants. Compare amitosis; mitosis. endarch Denoting a stele in which meta- endomycorrhiza (endotrophic mycor- xylem develops to the outside of pro- rhiza)(pl. endomycorrhizae) See mycor- toxylem. Compare centrarch; exarch; mes- rhiza. arch. endonuclease An enzyme that catalyzes endemic Describing a population or the hydrolysis of internal bonds of polynu- species that is restricted geographically. cleotides such as DNA and RNA, produc- Some endemics have evolved in geographi- ing short segments of linked nucleotides cal isolation on islands or mountain tops; (oligonucleotides). See also DNase; restric- others may be relics of once widespread tion endonuclease. species that have become restricted owing to climate change, geological change, or endophyte A bacterium, protoctist, or human activity. fungus that lives inside a plant or other photosynthetic organism, but is not a par- endocarp See pericarp. asite. Compare epiphyte. endocytosis The bulk transport of ma- endoplasm The inner layer of cyto- terials into cells across the plasma mem- plasm in plant cells, which contains the brane by processes not involving diffusion main organelles. Compare ectoplasm. or active transport. It is described as pinocytosis (cell drinking) or phagocytosis endoplasmic reticulum (ER) A system (cell eating) depending on whether the ma- of membranes forming tubular channels terial is fluid, containing molecules in solu- and flattened sacs (cisternae), running tion, or solid respectively. The process through the cytoplasm of all eukaryotic involves extension and invagination of the cells and continuous with the nuclear enve- plasma membrane to form small vesicles in lope. Although often extensive, it was only pinocytosis (pinocytotic vesicles) or vac- discovered with the advent of electron mi- uoles in phagocytosis (food vacuoles). The croscopy. Its surface is often covered with contents are often digested by enzymes ribosomes, forming rough ER. The pro-

75 endopolyploidy teins they make can enter the cisternae for cyanobacteria, into larger bacterial cells transport to other parts of the cell or for se- led to their permanent incorporation as cretion via the GOLGI APPARATUS. ER lack- forerunners of the plastids (e.g. chloro- ing ribosomes is called smooth ER and is plasts) seen in modern eukaryotes. Simi- involved with lipid synthesis, including larly, other symbiotic aerobic bacteria gave phospholipids and steroids. rise to the mitochondria. endopolyploidy See endomitosis. endotrophic mycorrhiza (pl. endo- trophic mycorrhizae) See mycorrhiza. endoscopic Describing the type of de- velopment of a plant embryo in which the entire Describing the margin of a leaf, inner cell formed by the first division of the , or that is smooth and undi- zygote develops into the embryo, while the vided. outer cell develops into the suspensor. It is seen in many ferns and in all seed plants. entomophily Pollination by insects. Compare exoscopic. Various structures and mechanisms have evolved to attract insects (e.g. showy petals endosperm The nutritive tissue that sur- and ) and to insure that they carry rounds the embryo in angiosperms. In pollen away on their bodies. nonendospermic seeds most of the en- dosperm is absorbed by the developing em- environment The complete range of ex- bryo and the food stored in the cotyledons. ternal conditions under which an organism In endospermic seeds the endosperm re- lives, including physical, chemical, and bi- places the nucellus and is often a rich ological factors, such as temperature, light, source of growth-regulating substances. the availability of food and water, and the Many endospermic seeds (e.g. cereals and effects of other organisms. oil seeds) are cultivated for their food re- serves. The endosperm is usually triploid enzyme A compound that catalyzes bio- and develops from the primary endosperm chemical reactions. Enzymes are proteins, nucleus, the triploid nucleus resulting from which act with a given compound (the sub- the fusion of one of the male gametes from strate) to produce a complex, which then the pollen tube with the polar nuclei in the forms the products of the reaction. The en- embryo sac. zyme itself is unchanged in the reaction; its presence allows the reaction to take place endospore A resting stage produced by by lowering the activation energy. The certain bacteria under unfavorable condi- names of most enzymes end in -ase, added tions. Endospores are formed within the to the substrate (e.g. lactase) or the reac- cell, one in each parent cell, and are sur- tion (e.g. hydrogenase). rounded by a thick coat containing dipicol- Enzymes are extremely efficient cata- inic acid. On germination the wall is lysed lysts for chemical reactions, and specific to and one vegetative cell is produced. En- particular reactions. They may have a non- dospores can remain viable for several cen- protein part (cofactor), which may be an turies and are resistant to heat, desiccation, inorganic ion or an organic constituent and x-rays. (coenzyme). The mechanism of action of most enzymes appears to be by active sites endosporium (intine) See pollen. on the enzyme molecule. These have very specific shapes and charge distributions. endosymbiont theory The theory that The substrate acting with the enzyme eukaryotic organisms evolved from symbi- changes shape to fit the active site, and the otic associations between bacteria. It pro- reaction proceeds. Enzymes are very sensi- poses that integration of photosynthetic tive to their environment – e.g. tempera- bacteria, for example purple bacteria and ture, pH, and the presence of other

76 Equisetales substances. See activation energy. See also sunflower (Helianthus). Compare hy- ribozyme. pogeal germination.

Eocene The second oldest epoch of the epigyny The type of flower structure in Tertiary period, 55–38 million years ago. It which the and androecium are in- was a period of widespread temperate and serted above the gynoecium, giving an infe- subtropical forests. The extensive grass- rior ovary, fused with the . It is lands of the late Tertiary had not yet devel- seen in the Asteraceae and Rosaceae. Com- oped, but large-hoofed grazing mammals, pare hypogyny; perigyny. such as horses and elephants, were evolv- ing rapidly. See also geological time scale. epilimnion In a stratified lake (a lake in which there are distinct layers of different eosin An acid stain that colors cellulose temperature densities), the upper layer of red and cytoplasm pink. See staining. warm water. See stratification. ephemeral A plant that has a very short epinasty (epinastic movements) A nastic life cycle and may complete more than one movement that involves the curving of a life cycle within a year. Such plants are plant organ away from the axis (i.e. down- often referred to as annuals. Examples are ward) as a result of greater growth on the shepherd’s purse (Capsella bursa-pastoris) upper surface, as in the opening of flowers. and certain desert plants that grow, flower, See ; leaf mosaic. and set seed in brief periods of rain. Com- pare annual; biennial; perennial. epiphylly 1. The growth of a plant on the leaf of another plant. Such a plant epicarp See pericarp. is termed an epiphyll, or is described as epiphyllous. Examples include many epicotyl The part of the plumule above mosses. the cotyledons. Compare hypocotyl. 2. The production of adventitious buds on a leaf, e.g. Bryophyllum, where the buds epidermis The outer protective layer of form along the leaf margins, developing cells in plants. In aerial parts of the plant the outer wall of the epidermis is usually into small plantlets with adventitious roots covered by a waxy cuticle that prevents that eventually become detached from the desiccation, protects the underlying cells leaf. from mechanical damage, and increases protection against fungi, bacteria, etc. The epiphyte Any plant growing upon or at- cells are typically platelike and closely tached to another plant or object merely packed together except where they are for physical support. Examples of tropical modified for a particular function, as are epiphytes are ferns, orchids, and bromeli- guard cells. The epidermis arises from the ads. In temperate regions vascular epi- tunica meristem. In damaged stems and phytes are rare: here most epiphytes are roots, and in those undergoing secondary lichens, mosses, liverworts, and algae. growth, it is replaced by a secondary layer, the periderm. The specialized epidermal epithelium (pl. epithelia) A tissue con- area of the roots from which the root hairs sisting of a sheet (or sheets) of cells that arise is termed the piliferous layer. The covers a surface. Epithelia may line resin term epiblem is sometimes used instead to canals in gymnosperms, or, sometimes, denote the outermost layer of cells in the gum ducts in dicotyledons, and usually root. have a secretory role. epigeal germination Seed germination equatorial plate An arrangement of the in which the cotyledons form the first pho- chromosomes in which the centromeres be- tosynthetic organs above the ground, e.g. come aligned in a single plane at the center

77 ER of the spindle during metaphase of mitosis denstion reaction of a carboxylic acid with and meiosis. an alcohol: RCOOH + HOR1 ® RCOOR1 + H2O. Equisetales (horsetails) The single order of the phylum SPHENOPHYTA, which con- estivation The way in which sepals and tains the horsetails. petals are folded in the flower bud before expansion. Compare ptyxis; vernation. ER See endoplasmic reticulum. ethanedioic acid See oxalic acid. era A unit of the geological time scale that is made up of several periods, e.g. ethanoic acid See acetic acid. the Mesozoic Era comprises the Triassic, Jurassic, and Cretaceous periods. When ethene See ethylene. used formally, the initial letter is capital- ized. ethylene (ethene)A gaseous hydrocarbon (C2H4), produced in varying amounts by ergot A fungal disease of many cereals. many plants, that functions as a plant HOR- The hard black sclerotia of Claviceps pur- MONE. Its production may be stimulated by purea, an ascomycete, replace the grains other plant hormones. The amino acid me- (see sclerotium). They contain thionine is a precursor. Ethylene is in- that can cause severe poisoning and even volved in the control of germination, cell death if taken in with food by humans and growth, fruit ripening, abscission, and other animals. senescence, and it inhibits longitudinal growth and promotes radial expansion. In Escherichia coli A rod-shaped Gram- ripening fruit a rapid rise in ethylene pro- negative bacterium, some strains of which duction precedes respiration to reach the are normally present in animal intestines, climacteric ripeness: production of ethyl- soil, or water. It is widely used in genetic ene stimulates further production. This is research and genetic engineering as a why ripe fruit stimulate other fruits to repository for fragments of DNA incorpo- ripen quickly and this process can be con- rated into plasmids in its cytoplasm. trolled to some extent for fruit storage and transport. essential amino acid See amino acids. etiolation The type of growth exhibited essential element An element that is in- by plants grown in darkness, usually from dispensable for the normal growth, devel- seed. They lack chlorophyll and therefore opment, and maintenance of a living appear white or yellow. They show less dif- organism. Some, the major elements, are ferentiation and contain reduced amounts required in relatively large quantities and of supporting material, such as lignin, con- may be involved in several different meta- centrating resources on elongation of in- bolic reactions (see carbon; hydrogen; ternodes. The plant can then reach the light oxygen; nitrogen; sulfur; phosphorus; faster and photosynthesize and synthesize potassium; magnesium; calcium). Others, more chlorophyll. such as iron, manganese, molybdenum, boron, zinc, copper, cobalt, iodine, and se- etioplast A modified chloroplast formed lenium, are required in only small or from proplastids in leaves grown in total minute amounts. See micronutrient. darkness. Any of a number of volatile Eubacteria A major subkingdom of the OILS secreted by aromatic plants that are Bacteria, containing a large and diverse the source of characteristic odors or tastes. group of bacteria, principally distinguish- able from the other major subkingdom, the ester A compound formed by the con- Archaea, by differences in the base se-

78 eutrophic quences of RNA subunits of the ribosomes, mitochondria, and, in photosynthetic eu- which are thought to reflect the very early karyotes, chloroplasts; a quite different evolutionary divergence between the two and much elaborated membrane structure, groups. Most are unicells that divide by bi- including internal membranes such as the nary fission. The cells can be spherical, endoplasmic reticulum and Golgi appara- rod-shaped, or helical, and some form tus; different-sized ribosomes; and com- assemblages of cells, such as branching plex UNDULIPODIA composed of arrays of filaments. Most are immotile, but some MICROTUBULES. Some of these features possess flagella. They are a ubiquitous probably arose through endosymbiosis of group, some being found in extreme condi- prokaryotes. See endosymbiont theory. tions. Cell-wall structure, morphology, and metabolic features are used to distin- Euphorbiaceae A large family of di- guish the different phyla. The Eubacteria cotyledonous plants that include the contain both Gram-negative and Gram- spurges. There are over 8000 species. They positive bacteria, and bacteria with no cell are mostly trees and shrubs, with but a few walls at all. herbaceous species, and are found world- wide, but especially in the tropics. Com- eucaryote See eukaryote. mercially important species include Hevea brasiliensis, the main source of natural euchromatin See chromatin. rubber; cassava (Manihot esculenta); and castor oil (Ricinus communis). Euglenophyta Formerly, a phylum of aquatic single-celled protoctists that swim euphotic zone See photic zone. using one or more UNDULIPODIA (flagella), now classified as a class, the Euglenida, of euploidy The normal state in which an the phylum Discomitochondria. It contains organism’s chromosome number is an both photosynthesizing and nonphotosyn- exact multiple of the haploid number char- thesizing members. For example, members acteristic of the species. For example, if the of the genus Euglena possess a flexible haploid number is 7, the euploid number pellicle surrounding the cell, and a single would be 7, 14, 21, 28, etc., and there undulipodium. Many species contain would be equal numbers of each different chloroplasts, with pigments similar to chromosome. Compare aneuploidy. those found in plants, although these or- ganisms may also consume dissolved or eusporangiate Describing the condi- particulate food from their surroundings. tion, found in clubmosses, horsetails, and certain ferns in which the sporangia de- Eukarya In taxonomy, a DOMAIN of liv- velop from a group of initial cells and do ing organisms characterized by having eu- not have an elaborate spore dispersal karyotic cells (See eukaryote). In the Five mechanism. Compare leptosporangiate. Kingdoms classification, the Eukarya con- See also Filicinophyta. stitutes a ‘superkingdom’ containing the kingdoms Animalia, Fungi, Plantae, and eustele The STELE arrangement found in Protoctista. most gymnosperms and dicotyledons in which the is arranged into a eukaryote (Eukaryotae) All the living ring of discrete bundles separated by kingdoms except the Bacteria (Archaea medullary rays, all contained within a sin- and Eubacteria) are the eukaryotes. They gle ring of endodermis and pericycle. It is a are defined by the presence of a much more form of SIPHONOSTELE. elaborate cell than the prokaryotes. Eu- karyotes are characterized by: the genetic eutrophic Describing lakes or ponds material packaged in chromosomes within that are rich in nutrients and consequently a membrane-bound nucleus; possession of are able to support a dense population of

79 evergreen plankton and littoral vegetation. Eutroph- creted by diffusion through the cell or body ication is the process that results when an surface into the external medium. excess of nutrients enters a lake, for exam- ple as sewage or from water draining off exine (exosporium) See pollen. land treated with fertilizers. The nutrients stimulate the growth of the algal popula- exocarp (epicarp) See pericarp. tion giving a great concentration or ‘bloom’ of such plants. When these die they exocytosis The bulk transport of mate- are decomposed by bacteria, which use up rials out of the cell across the plasma mem- the oxygen dissolved in the water, so that brane. It involves fusion of vesicles or aquatic animals such as fish are deprived of vacuoles with the plasma membrane in a oxygen and die from suffocation. Compare reversal of endocytosis. See also lysosome. oligotrophic. Compare endocytosis. evergreen Describing plants that retain An outermost layer of thick- their leaves throughout the year or through ened or suberized cortical cells that some- several years. Many tropical species of times replaces the epidermal layer in the broadleaved flowering plants are evergreen older parts of roots if the epidermal cells and their leaves are thicker and more leath- have died. ery than those of deciduous trees. Water conservation is not a problem in tropical exogamy The fusion of gametes pro- evergreen forests, so there is no need to duced by organisms that are not closely re- shed leaves in a particular season. In polar lated. See outbreeding. and cold temperate regions many of the are shrubs or trees with needle- exogenous Produced or originating out- like or scalelike leaves, adaptations that side an organism. Compare endogenous. prevent water loss by transpiration. Com- pare deciduous. exon A segment of a gene that is both transcribed and translated and hence car- evolution The process of genetic change ries part of the code for the gene product. that occurs in populations of organisms Most eukaryotic genes consist of exons over a period of time. It manifests itself as interrupted by noncoding sequences (see new characteristics in a species, and even- intron). Both exons and introns are tran- tually the formation of new species. See scribed to heterogeneous nuclear RNA Darwinism; natural selection. (hnRNA), an intermediary form of messen- ger RNA (mRNA); the introns are then re- exarch Denoting a stele in which the moved leaving mRNA, which has only the metaxylem develops to the inside of the essential sequences and is translated into protoxylem. Compare centrarch; endarch; the protein. Bacteria do not have introns. mesarch. exonuclease An enzyme that catalyzes excretion The process by which excess, the hydrolysis of the terminal linkages of waste, or harmful materials, resulting from polynucleotides such as DNA and RNA, the chemical reactions that occur within thereby removing terminal nucleotides. See the cells of living organisms, are eliminated also DNase; restriction endonuclease. from the body. In plants most waste prod- ucts accumulate in the vacuoles, whose se- exoscopic Describing the type of devel- lectively permaeable membranes prevent opment of a plant embryo in which the them interfering in cell reactions. Gaseous apex of the sporophyte develops from the excretion takes place by diffusion through outer cell formed by the first division of the the stomata and lenticels. In fungi and zygote, the inner cell giving rise to the foot. small protoctists waste products are ex- Compare endoscopic.

80 eyespot exosporic The formation of spores out- comes sigmoid (S-shaped). When the pop- side the spore-producing organ. In the ulation remains stable, it has reached the basidiomycete fungi, for example, the ba- stationary phase. If organisms are intro- sidiospores are borne on the tips of the duced into a new medium, there is often a sterigmata, which are outgrowths of the period during which the population re- basidium. mains constant before it begins to increase (called the lag phase). During this phase the exosporium (exine) See pollen. cells are increasing in size and synthesizing materials necessary for growth under the explantation The culture of isolated tis- new conditions. sues of adults or embryos in an artificial medium for the study of maintenance, extensin See glycoprotein. growth, and/or differentiation. Such a piece of transplanted tissue is called an explant, extracellular Occurring or situated out- and its placement on the culture medium is side a cell. called inoculation. Explants are also a means of , producing extrachromosomal DNA In eukary- clones and offering a more rapid means of otes, DNA found outside the nucleus of the multiplication than seed production. cell and replicating independently of the chromosomal DNA. It is contained within exponential growth A type of growth organelles in the cytoplasm, e.g. mitochon- in which the rate of increase in numbers at dria, chloroplasts, and plastids, and is re- a given time is proportional to the number sponsible for cytoplasmic inheritance. See of individuals present. Thus, when the cytoplasmic inheritance. population is small multiplication is slow, but as the population gets larger, the rate extrorse Denoting anthers in which de- of multiplication also increases. An expo- hiscence lines are to the outside of the nential growth curve starts off slowly and flower, promoting cross-pollination. Com- increases faster and faster as time goes by pare introrse. (called the log phase). However, at some point factors such as lack of nutrients, ac- eyespot (stigma) A light-sensitive struc- cumulated wastes, etc., limit further in- ture of certain protoctists and invertebrate crease, when the curve of number against animals. The eyespot of unicellular and time begins to level off and the curve be- colonial algae and their gametes and

81 F

F1 The first filial generation; i.e. the first ism that can grow under anaerobic condi- generation that results from a particular tions but is also able to survive in aerobic cross. conditions. Compare obligate.

F2 The second filial generation, obtained FAD (flavin adenine dinucleotide) A de- by crossing or selfing within the F1 genera- rivative of riboflavin that is a coenzyme in tion. If the characteristic is governed by electron-transfer reactions. It acts as a simple Mendelian genes, the typical mono- prosthetic group to various dehydrogenase hybrid and dihybrid ratios become appar- enzymes, the substrate being oxidized as ent in the F2 generation. FAD accepts electrons and is reduced to FADH2. See also flavoprotein. Fabaceae A large family of dicotyledo- nous flowering plants, commonly known Fagaceae A family of deciduous or ever- as . There are about 18,000 spe- green trees and shrubs that includes the cies, which include herbs, trees, shrubs, oaks (Quercus), beeches (Fagus), southern climbers, and some aquatics. Most species beeches (Nothofagus), and chestnuts (Cas- have root nodules containing nitrogen- tanea). Members of this family dominate fixing bacteria. The leaves are usually com- broad-leaved forests, especially of temper- pound and pinnate, with . The ate regions. Fabaceae have a worldwide distribution and many are of economic importance, false fruit See pseudocarp. including peas (Pisum sativum), beans (Phaseolus and Vicia), lentils (Lens culi- family A taxonomic category involving naris), and peanuts (Arachis hypogea). a collection of similar genera. Families may Others are used for forage, e.g. clovers be subdivided into subfamilies, tribes, and (Trifolium) and lucerne (Medicago sativa). subtribes. Plant family names generally end There are also ornamental species such as in aceae. Similar families are grouped into lupins (Lupinus). orders. facilitated diffusion A passive trans- far-red light Electromagnetic radiation port of molecules across a cell membrane of wavelength approximately 740 nm. See along a concentration gradient, mediated ; photoperiodism. by carrier molecules or complexes, usually proteins. No energy is expended in this fascicular cambium (pl. cambia) See process, but it enables the passage through intrafascicular cambium. the membrane of molecules that otherwise could not pass through. fast green See staining. facultative Describing an organism that fat A triglyceride of a long-chain car- can utilize certain conditions but is not de- boxylic acid (fatty acid) that is solid below pendent on them, i.e. an organism that can 20°C. Fats commonly serve as energy stor- adopt an alternative mode of living. For ex- age material in some plants, especially in ample, a facultative anaerobe is an organ- seeds. See also lipid.

82 field capacity fatty acid See carboxylic acid. They have also been isolated from mito- chondria. In photosynthesis ferredoxin is feedback inhibition The inhibition of involved in electron transfer between pho- the activity of an enzyme (often the first) tosystem I and the final electron acceptor, in a multienzyme reaction sequence by NADP. the product of that sequence. When the product accumulates beyond an optimal fertilization (syngamy) The fusion of a amount it binds to a site (ALLOSTERIC SITE) male gamete with a female gamete to form on the enzyme, changing the shape so that a zygote, the essential process of sexual re- it can no longer react with its substrate. production. External fertilization occurs However, once the product is utilized and when gametes are expelled into a watery its concentration drops again, the enzyme substrate from the parental bodies before is no longer inhibited and further forma- fusion; it is typical of sporebearing plants. tion of product results. The mechanism is Internal fertilization takes place in the seed used to regulate the concentration of cer- plants and some clubmosses and ferns, tain substances within a cell. often by means of a pollen tube. Internal fertilization is an adaptation to life in a ter- feedback loop (feedback mechanism)A restrial environment. regulatory mechanism in a system. This may involve negative feedback, for exam- Feulgen’s stain A test used to show the ple, an increase in the number of herbi- presence of DNA, especially in dividing nu- vores in an ecosystem leads to overgrazing clei. A sample of tissue is placed in dilute and reduction of the herbivores population hydrochloric acid at 60oC to hydrolyze the by starvation or increased susceptibility to DNA. This exposes the aldehyde groups of disease, which allows the plants to grow deoxyribose. The tissue is then soaked in back, and so on – the population of both Schiff’s reagent: the development of a ma- species fluctuates about a mean. Positive genta color indicates the location of DNA. feedback has the opposite effect, reinforc- See staining. ing a change in the system. In certain meta- bolic pathways, for example, the substrate of the first enzyme in the pathway stimu- fiber A form of SCLERENCHYMA cell that lates subsequent reactions in the sequence. is often found associated with vascular tis- sue. Fibers are long narrow cells, with fermentation The breakdown of or- thickened walls and finely tapered ends. ganic substances, particularly carbohy- Their function is more as supporting tissue drates, under anaerobic conditions. It is a than as conducting tissue. Where they form of anaerobic respiration and is seen in occur interspersed with the xylem they certain bacteria and yeasts, and also in may be distinguished from tracheids by plants under waterlogged conditions. See their narrower lumen. alcoholic fermentation. See also glycolysis; lactic acid bacteria. fiber- An elongated cell with bordered pits found in wood, intermediate ferns See Filicinophyta. in form between a fiber and a tracheid. ferredoxins A group of red-brown pro- fibrous root system A branching root teins found in green plants, many bacteria, system in which there is no main root. In and certain animal tissues. They contain fibrous root systems the radicle or pri- nonheme iron in association with sulfur at mary root either branches or dies and is re- the active site. They are strong reducing placed by adventitious roots, e.g. grasses agents (very negative potentials) and (Poaceae). Fibrous root systems are charac- function as electron carriers, for example teristic of . Compare tap- in photosynthesis and nitrogen fixation. root.

83 field layer field capacity The point at which the (Ophioglossales), Marattiales, and royal soil contains all the water it can hold by ferns (Osmundales), which are to some ex- capillary and chemical attraction. Any tent intermediate between leptosporan- more water added to soil at field capacity giate and eusporangiate ferns. The whisk would drain away by gravity. Soil at field ferns (Psilotales) may also be included capacity has a high water potential. Water here, but in the Five Kingdoms classifica- content at field capacity is usually ex- tion they have the status of a phylum pressed as a percentage of the weight or (Psilophyta). There are also a number of volume of oven-dry soil. This is affected by fossil genera, leptosporangiate ferns dating the soil texture, clay having a high field ca- back to the Cretaceous era, eusporangiate pacity and sand a low field capacity. ferns to the Devonian. Ferns do not show secondary thickening, but in large ferns field layer The layer of a plant such as tree ferns bands of sclerenchyma community. Compare canopy; ground and the overlapping bases of the layer. help to support the plant. filament 1. The stalk of the stamen fimbriae (sing. fimbria) See pili. bearing the anther in angiosperms. 2. The vegetative body of the filamentous fine structure See . algae (e.g. Spirogyra), composed of a line of similar cells joined by their end walls. FISH (fluorescence in situ hybridization) 3. A strand of protein of diameter 4–15 nm A technique for locating specific sequences found inside a cell. Such filaments are usu- of DNA using fluorescent probes that are ally composed of the contractile proteins viewed under the MICROSCOPE. The probes actin and/or myosin, and are thought to bind to (hybridize with) specific sequences play a role in cell motility, and perhaps also of DNA; different colors of probe may be change in shape. used to distinguish different DNA se- quences. See DNA probe. Filicinophyta (ferns) The largest phy- lum of spore-bearing vascular plants com- fission A type of asexual reproduction prising the ferns and including about in which a parent cell divides into two 12,000 species. It is divided into two main daughter cells (binary fission) genetically groups, the LEPTOSPORANGIATE ferns and identical to the parent. Binary fission oc- the EUSPORANGIATE ferns. In leptosporan- curs in many unicellular organisms, such as giate ferns, the sporangia are thin-walled protoctists, bacteria, and fission yeasts. and develop from one cell. In the Filicales Fission begins with division of the nucleus the leptosporangium has a specialized by mitosis, followed by cytoplasmic divi- spore dispersal mechanism involving an sion, and sometimes sporulation. Compare annulus of cells with thick walls that ejects budding; fragmentation. s pores explosively as it dries out. Lep- tosporangiate ferns include the typical In an evolutionary context, the ferns (Filicales) and the water ferns (Mar- ability of an organism to produce a large sileales and Salviniales). The Filicales is number of offspring that survive to a re- much the largest living group of ferns, and productive age. they normally have large spirally arranged leaflike fronds (MEGAPHYLLS) derived from Five Kingdoms classification A classi- lateral branches of the main axis, some or fication system that recognizes five king- all of which bear sporangia on their mar- doms containing at least 96 phyla. It gins or undersurfaces. In eusporangiate comprises two superkingdoms, the Pro- ferns, the sporangia are thick-walled and karya (PROKARYOTES) and Eukarya (EU- develop from a number of cells. Eusporan- KARYOTES). The Prokarya contains a single giate ferns include the adderstongues kingdom, the BACTERIA, which is divided

84 floral formula into two subkingdoms, the ARCHAEA and Flavins are light-sensitive yellow pigments the EUBACTERIA. The Eukarya contains four with an absorption peak around 370 nm. kingdoms, the Animalia, Plantae, Fungi, They are involved in , and in and Protoctista. PHOTOPERIODISM, where they are thought to affect phytochrome by transferring light fixation 1. The situation in a usually small population when an allele reaches a 100% frequency due to the complete loss of all other alleles of that gene. All mem- bers of the population are homozygous for this particular allele. 2. In the preparation of microscope slides, the process by which tissues are rapidly killed and preserved by chemicals to pre- vent decay or autolysis with minimal distortion of structure. Fixing agents (fixa- tives) should also render cell organelles and inclusions more visible and harden the tis- sue to prevent shrinkage and distortion Flavonoid structure during dehydration, embedding, and sec- tioning. Examples of fixatives are energy from blue to red wavelengths. See formaldehyde, osmium tetroxide, ethanol, phytochrome. and acetic acid. See also dehydration; em- bedding. flavin adenine dinucleotide See FAD. 3. The incorporation of inorganic materi- als such as carbon and nitrogen into or- flavin mononucleotide See FMN. ganic compounds by living organisms. An example is the incorporation (fixation) of flavonoid One of a common group of carbon from carbon dioxide into new or- plant compounds having the C6–C3–C6 ganic compounds during photosynthesis. chemical skeleton in which C6 is a benzene See also nitrogen fixation. ring. They are an important source of non- photosynthetic pigments in plants. flaccid Lacking turgor. When a plant loses water to the extent that its cells be- flavoprotein A conjugated protein in come flaccid, wilting occurs. See plasmoly- which a FLAVIN (FAD or FMN) is the pros- sis. thetic group joined to a protein compo- nent. Flavoproteins are enzymes and many flagellum (pl. flagella) A whiplike ex- are dehydrogenases. They act as electron- tension of prokaryote cells, with a basal transport agents and play an important body at its base, whose beat causes loco- role in the electron-transport chains of motion of the cell. Strictly the term is now respiration, photosynthesis, and the micro- reserved for the bacterial flagellum. The somal system responsible for the desatu- flagella and cilia of eukaryote cells have a ration of fatty acids in heterotrophic quite different structure and are called UN- bacteria. DULIPODIA. Bacterial flagella are much sim- pler than undulipodia, being hollow flocculation The aggregation of soil cylinders about 15 nm in diameter, consist- particles into crumbs. Compacted struc- ing of subunits of a protein (flagellin) tureless clay soils can be flocculated by the arranged in helical spirals. addition of neutral salts, particularly of calcium. The addition of lime to saturated flavin A derivative of riboflavin occur- clay soils (liming) is a common agricultural ring in the flavoproteins; i.e. FAD or FMN. practice, improving soil structure by en-

85 floral province couraging crumb formation and making photoperiod stimulus from the leaves to heayy soils more workable. the apex where flowering is induced. The stimulus can be transferred from one plant floral formula A series of symbols used to another by grafting, but attempts to iso- to describe the structure of a flower. The late florigen have so far been unsuccessful. various whorls of structures are abbrevi- See photoperiodism. ated as follows: K (calyx), C (corolla), P (perianth), A (androecium), and G (gynoe- flower The characteristic reproductive cium). These letters are each followed by a structure of an angiosperm. It usually con- number indicating the number of parts in sists of an axis or receptacle bearing the the . If the number exceeds 12 then sepals, petals, and stamens. The gynoecium the symbol ¥ is used to denote an indefinite is borne either above the receptacle or en- number. Fusion of parts of the whorl is in- closed within it. One or more of these dicated by placing the number of parts in whorls may be absent. Flower structures brackets. The position of the gynoecium is are extremely variable and show numerous shown by a line above or below the letter G adaptations to promote pollination and denoting an inferior or superior ovary re- seed dispersal. See also floral formula; in- spectively. The formula is preceded by Å to florescence. indicate actinomorphic flowers and ·|· for zygomorphic flowers. The floral formula flowering hormone See florigen. for the buttercup would be written: ÅK5 C5 A¥ G¥ flowering plants See angiosperms. floral province (floral kingdom; floral re- fluid-mosaic model See plasma mem- gion) One of the six main regions and a brane. few minor ones into which the world is di- vided on the basis of plant distribution. fluorescence in situ hybridization See Each province has one or more centers of FISH. diversity and each contains unique families and genera of plants. The main provinces fluorescence microscopy A form of are the Boreal, which covers all of the microscopy in which fluorescent probes north temperate; the Neotropical, covering are added to the material being investi- tropical Central and South America; the gated. These probes bind to specific parts Paleotropical, covering and of cells or to particular molecules, such as Asia; the Australian; the Cape Province, particular sequences of DNA or protein re- covering a small area around the Cape of ceptors. Good Hope; and the Antarctic, including New Zealand and temperate South Amer- FMN (flavin mononucleotide) A deriva- ica. Oceanic islands, particularly in the Pa- tive of riboflavin that is a coenzyme in cific, form minor provinces. There are electron-transfer reactions. It acts as a floral affinities and connections between prosthetic group to various dehydroge- the provinces, particularly near their nases, such as NADH dehydrogenase, boundaries. which catalyzes the transfer of electrons from NADH to coenzyme Q in the respira- floret 1. An individual small flower of a tory electron-transport chain. See also clustered inflorescence, e.g. daisy (Bellis). flavoprotein. 2. In grasses, the lemma, palea, and the flower they enclose. folic acid (pteroylglutamic acid) One of the water-soluble B-group of VITAMINS. florigen (flowering hormone) A hypo- Folic acid is important in metabolism in thetical plant hormone that has been pos- various coenzyme forms, all of which are tulated to account for the transfer of the specifically concerned with the transfer

86 freeze fracturing and utilization of the single carbon (C1) form The lowest taxonomic group, group. It is important in the growth and re- ranking below the variety level, normally production of cells, participating in the referring to the recognizably different mor- synthesis of purines and thymine. phology found in several members within a plant species. Subforms may also be recog- A dry dehiscent fruit formed nized. from one carpel that splits along one edge to release its seed, for example Delphinium formalin A mixture of about 40% fruit. formaldehyde, 8% methyl alcohol, and 52% water (the methyl alcohol is present food chain The chain of organisms ex- to prevent polymerization of the formalde- isting in any natural community, through hyde). It is a powerful reducing agent and which food energy is transferred. Each link is used as a disinfectant, germicide, and in the food chain obtains energy by eating and also as a general preserving the one preceding it and is in turn eaten by solution. the organisms in the link following it. At each transfer a large proportion (80–90%) fossil The remains of, or impressions left of the potential energy is lost as heat, there- in rocks by, long-dead animals and plants. fore the number of links in a sequence is Most fossils consist of hard skeletal mater- limited, usually to 4 or 5. The shorter the ial because soft tissues and organs rot away food chain, the greater the available en- very quickly. Plants seldom contain hard ergy, so total energy can be increased by mineral material, so they are preserved cutting out a step in the food chain, for ex- well only under certain conditions. With ample if people consume cereal grain in- rapid burial in a moist anoxic environ- stead of consuming animals that eat cereal ment, such as the coal swamps of the Car- grains. boniferous period, carbonaceaous films of Food chains are of two basic types: the plant material like fern fronds may survive. grazing food chain, which goes from green Alternatively, mineral salts from surround- plants, to grazing herbivores, and finally to ing rocks gradually replace the hard or- carnivores; and the detritus food chain, ganic material, to give a cast in a process which goes from dead organic matter, termed petrification, as in the various pet- to microorganisms, and then to detritus- rified forests dotted around the globe. The feeding organisms. The food chains in a hard coats of spores and pollen grains are community are interconnected with one preserved better, forming microfossils. Al- another, because most organisms consume ternatively the organic material dissolves more than one type of food, and the inter- away leaving an impression or mold in the locking pattern is referred to as a food web surrounding rocks. See also chemical fos- or food cycle. See trophic level. sils; . food web See food chain. fragmentation 1. A form of ASEXUAL REPRODUCTION in which the plant or alga foot The basal portion of an embryo, breaks into fragments, each fragment sporophyte, or spore-producing body, growing into a new, independent plant. which is embedded in the tissue of the par- Examples include filamentous algae such ent. as Spirogyra, and waterweeds like the Canadian pondweed (Elodea). Compare forest 1. A plant formation dominated budding; fission. by trees whose crowns touch, forming a 2. The separation through human activity continuous canopy. of once continuous habitats into fragments 2. A major plant community in which the that may or may not contain viable popu- dominant plants are trees. lations of the organisms present.

87 Fries, Elias Magnus frameshift mutation See gene muta- the furanose ring form. Fructose forms tion. polymers called fructosans, which include , an important storage polysaccha- freeze drying A technique for dehydrat- ride in the Asteraceae. ing a specimen by the sublimation of ice under a vacuum. It avoids damage that fruit The ripened OVARY of a flower that would result from heating. is usually formed following fertilization of the ovule. It may consist of the ripened freeze fracturing A method of prepara- ovary only or include other parts of the tion of material for electron microscopy, flower. Fruits vary according to the particularly useful for studying mem- method of seed dispersal, succulent fruits branes. Material is frozen rapidly (e.g. by normally being distributed by animals immersion in liquid nitrogen) thus preserv- while dry fruits may be dispersed by wind ing it in lifelike form. It is then fractured, or water, and dehiscent fruits split to re- usually with a sharp knife. The fracture lease the seeds with the fruit remaining on plane tends to follow lines of weakness, the plant. Fruits are classified according to such as between the two lipid layers of how the ovary wall (pericarp) develops, de- membranes, revealing their internal sur- pending on whether it becomes fleshy or faces. Carbon or platinum replicas made of hard. Fruits are further classified according the surfaces are shadowed for examination to whether or not the fruit wall opens to re- in the electron microscope. In freeze etch- lease the seeds. See also dehiscent; indehis- ing the fractured surface is etched, i.e. some cent; multiple fruit; pseudocarp. ice is allowed to sublime away before SHADOWING. This exposes more detailed Fucales An order of Phaeophyta (brown structure, such as the outer surface of the algae) in which the haploid generation con- membrane. sists only of the gametes. The gametes are produced in special cavities called concep- Fries, Elias Magnus (1794–1878) tacles, often in the inflated tips of branches Swedish mycologist. Fries, working at of the thallus, e.g. wracks (Fucus). See al- Lund University, published Systema Myco- ternation of generations. logicum, his three-volume work on the classification of fungi, in 1921–22. He also Fuchs, Leonhard (1501–66) German published flora covering all the Scandina- botanist and physician. His beautifully il- vian countries and an important work on lustrated manual De historia stir- European lichens. pum was published in 1542. It contained about four hundred German plants and A term usually applied to large about a hundred foreign plants, arranged well-divided leaves as found in ferns, in alphabetical order. The illustrations palms, and cycads. The leaflike thalli of were accompanied by a description of each certain algae and lichens may also be plant and its habitat and even information termed fronds. See also megaphyll. about the best time to collect them. The book also included a glossary of terms. fructification A seed- or spore-bearing Fuchs is best known for the genus of orna- structure. The term is used especially for mental shrubs , which was named the aerial fruiting bodies of fungi, e.g. for him. mushrooms. fucoxanthin A brownish xanthophyll fructose A sugar (C6H12O6) found in pigment found in Diatoms, Chrysomon- fruit juices, honey, and cane sugar. It is a ada, Haptomonada, Phaeophyta, and ketohexose, existing in a pyranose form some Dinomastigota. The light absorbed is when free. In combination (e.g. with glu- used with high efficiency in photosynthe- cose in the disaccharide sucrose) it exists in sis, the energy first being transferred to

88

Fruit: a classification of types of fruit

89 Fungi chlorophyll a. It has three absorption plants and animals, while others are peaks covering the blue and green parts of sources of antibiotics. Yeasts and other the spectrum, which penetrate to greater fungi are used in commercial fermentation depths under water than the red end of the processes such as brewing. There are three spectrum. See carotenoids. main phyla of Fungi, based on the nature of their fruiting structures: the Zygomy- fumaric acid (trans-butenedioic acid) An cota, Ascomycota, and Basidiomycota. In unsaturated dicarboxylic acid, which oc- the modern Five Kingdoms classification, curs in many plants. The fumarate ion par- certain funguslike organisms that were pre- ticipates in several important metabolic viously considered to be fungi, are now pathways, e.g. the Krebs cycle, purine assigned to phyla in the Kingdom Protoc- pathways, and the urea cycle. tista.

Fungi A kingdom of nonphotosynthetic Fungi Anamorphici (Deuteromycota; mainly terrestrial organisms that are quite Fungi Imperfecti; mitosporic fungi)A distinct from plants and animals, and phylum of so-called ‘imperfect’ fungi in placed in their own kingdom, Fungi. They which sexual reproduction is unknown or are characterized by having cell walls made has been lost during evolution. Its mem- chiefly of chitin, not the cellulose of plant bers, for example the Penicillium molds, cell walls, and they all develop directly are assigned as a taxonomic convenience, from spores without an embryo stage. Moreover, undulipodia are never found in rather than by any true criteria, and some any stage of their life cycles. Fungi are gen- authorities prefer to allocate the deutero- erally saprophytic or parasitic, and may be mycotes to either the Ascomycota (the ma- unicellular or composed of filaments jority) or the Basidiomycota, on the basis (termed hyphae) that together comprise the of available evidence. Indeed Penicillium is fungal body or mycelium. Hyphae may now known to have a sexual stage, for- grow loosely or form a compacted mass of merly regarded as a quite distinct asco- pseudoparenchyma giving well-defined mycete fungus (Talaromyces). Most of structures, as in toadstools. Fungi are these fungi reproduce by conidia produced major decomposers in soil, and fresh by mitosis (see conidium). There are about water. Some form symbiotic associations 15 000 species. with algae or cyanobacteria to form lichens, while others form extremely im- fungicide A chemical that inhibits fun- portant mycorrhizae with the roots of gal growth. They are used to prevent fun- many plants, including most forest trees. gal infection as well as to deal with existing Some parasitic fungi cause diseases in infections.

90 G

galactose A sugar found in lactose and cellular alga. The gametophyte is HAPLOID many polysaccharides. It is an aldohexose, and produces sex organs. It derives from isomeric with glucose. See also sugar. the germination of haploid spores pro- duced by the SPOROPHYTE generation. It is gall An abnormal outgrowth or swelling the main generation in the life cycle of the on a root, stem, or leaf in response to Bryata, in which the sporophyte is com- attack by a parasite or to invasion by a pletely or partially dependent upon it. The symbiont. ROOT NODULES induced by the PROTHALLUS of Lycophyta (clubmosses), nitrogen-fixing bacterium Rhizobium are Sphenophyta (horsetails), and Filicino- part of a symbiotic relationship (see sym- phyta (ferns), which may be a green struc- biosis). ture on the soil surface or a subterranean mycorrhizal structure, is the gametophyte. gametangium (pl. gametangia) A cell or In these groups and in the bryophytes, organ in which sexual cells (gametes) are some species produce unisexual gameto- produced. When the gametes are readily phytes and some produce hermaphrodite distinguished as male or female, other gametophytes. In the Coniferophyta the fe- terms may be used such as ANTHERIDIUM, male gametophyte is reduced to a nonpho- ARCHEGONIUM, or OOGONIUM. tosynthetic tissue with up to 2000 nuclei in the ovule, which produces archegonia (see gamete A cell capable of fusing with an- archegonium). While in the Angiophyta other cell to produce a zygote, from which the female gametophyte is simply the EM- a new individual organism can develop. BRYO SAC. The male gametophytes of Gametes may have similar structure and conifers and angiosperms are the POLLEN behavior (isogametes), as in many simple grains. See also alternation of generations. organisms, but are usually dissimilar in ap- pearance and behavior (anisogametes). Gamophyta A phylum of green algae in The typical male gamete is small, motile which sexual reproduction is by conjuga- (by means of UNDULIPODIA), and produced tion, e.g. Spirogyra and DESMIDS. Amoe- in large numbers. The typical female ga- boid gametes are produced, which cross to mete is large because of the food reserves it adjacent filaments through conjugation contains, and is produced in smaller num- bridges and fuse to form a zygote, which bers than the male gametes. In all except usually forms a resistant zygospore. On the most primitive organisms, the female germination, the zygospore undergoes gamete is nonmotile (OOGAMY). Gametes meiosis to produce haploid cells that give are usually haploid, so fusion of gametes rise to a new thallus. Asexual reproduction results in the nucleus of the zygote having is by FRAGMENTATION. the diploid number of chromosomes. garrigue Scrub woodland, a secondary gametogenesis The formation of sex formation derived from the original mixed cells or gametes. forest, characteristic of limestone areas of the Mediterranean region, with low rain- gametophyte The gamete-producing gen- fall and thin, dry soils. It is dominated by eration in the life cycle of a plant or multi- low-growing prickly dwarf shrubs and

91 gas chromatography aromatic herbs (most commonly of the better able to penetrate the hydrated pores Lamiaceae), such as lavender (Lavandula within the particles of the gel. Molecules vera), sage (Salvia officinalis), and thymes too large to penetrate the pores are ex- (Thymus), many with drought-resistant cluded, and thus flow more rapidly foliage. There are also bulbous plants such through the column. By analyzing the liq- as species of Iris. Compare chaparral; uid that drips from the bottom of the col- maquis. umn (the eluate) at set intervals and comparing it with a standard (obtained by gas chromatography A technique widely running a known through used for the separation and analysis of the column) information about the sizes mixtures. Gas chromatography employs a and molecular weights of the components column packed with either a solid station- of the mixture is gathered. The most fre- ary phase (gas–solid chromatography or quently used commercial gel is Sephadex. GSC) or a solid coated with a nonvola- tile liquid (gas–liquid chromatography or (pl. gemmae) 1. A vegetative GLC). The whole column is placed in a produced by mosses, liverworts, thermostatically controlled heating jacket. and certain Lycophyta (clubmosses). Gem- A volatile sample is introduced into the col- mae are small dispersal units consisting of umn using a syringe, and an unreactive car- one to several cells, often forming disks, rier gas, such as nitrogen, passed through plates, or filaments. They often form in it. The components of the sample will be groups in receptacles called gemma-cups carried along in this mobile phase. How- and eventually become detached from the ever, some of the components will cling parent to form new plants. more readily to the stationary phase than 2. See chlamydospore. others, either because they become at- tached to the solid surface or because they gemmation A type of asexual reproduc- dissolve in the liquid. The time taken for tion, seen in mosses and liverworts, involv- different components to pass through the ing the production of a group of cells (a column is characteristic and can be used to gemma) that develops into a new individ- identify them. The emergent sample is passed through a detector, which registers ual, before or after separation from the the presence of the different components in parent. the carrier gas. gene In classical genetics, a unit of gas–liquid chromatography See gas hereditary material located on a chromo- chromatography. some that, by itself or with other genes, de- termines a characteristic in an organism. It gas–solid chromatography See gas corresponds to a segment of the genetic chromatography. material, usually DNA (although the genes of some viruses consist of RNA). Genes gel filtration (gel-permeation chromatog- may exist in a number of forms, termed al- raphy) A chromatographic method using leles. For example, a gene controlling the a column packed with porous carbohy- characteristic ‘height’ in peas may have drate-gel beads of standard size and poros- two alleles, one for ‘tall’ and another for ity. It is a standard technique used for ‘short’. In a normal diploid cell only two separating and identifying alleles can be present together, one on each of various sizes, e.g. proteins or nucleic of a pair of homologous chromosomes: the acids. A solution of the mixture of macro- alleles may both be of the same type, or molecules is added to the top of the column they may be different. The segregation of and allowed to flow through by gravity. alleles at meiosis and their dominance rela- The smaller molecules are hindered in their tionships are responsible for the particulate passage down the column because they are nature of inheritance. Genes can occasion-

92 generative cell ally undergo changes, called MUTATIONS, to within and between populations of the new allelic forms. same species through interbreeding. Although the DNA molecules of the chromosomes account for the great major- gene frequency The proportion of an ity of genes, genes are also found as allele in a population in relation to other PLASMAGENES in certain DNA-containing alleles of the same gene. cytoplasmic bodies (e.g. mitochondria, plastids). A gene can be defined as the gene library A collection of cloned smallest hereditary unit capable either of DNA fragments derived from the entire recombination or of mutation. alterna- GENOME of an organism. The genetic ma- tively it may be described as a unit of func- terial of the organism is first broken up tion, a CISTRON, which may be much larger randomly into fragments using restriction than a unit of recombination or mutation, enzymes, for example. Then each fragment and controls the synthesis of a single is cloned using a VECTOR (e.g. plasmid or polypeptide or a messenger RNA molecule bacteriophage) inside a suitable host, such whose genetic code is contained within the as the bacterium E. coli. Particular genes or gene. Research with bacteria has shown DNA sequences are identified by a suitable that the smallest unit of recombination or DNA PROBE. See also gene bank; gene mutation is one base pair, while the size of cloning. a unit of function can be determined by the CIS–TRANS TEST. gene mutation An alteration in a single gene resulting from a change in the num- gene bank 1. A place where plant ma- ber, type, or sequence of nucleotide bases. terial is stored in a viable condition in Unlike CHROMOSOME MUTATIONS, gene mu- order to preserve plants that are in danger tations are not visible under the micro- of becoming extinct in the wild or scope. A point mutation is a change at a that are being lost from cultivation. The single locus. It may be an addition, dele- material also provides a source of genes for tion, inversion, or substitution of one or breeding new varieties. more bases. A substitution of one or two 2. See gene library. bases may still result in the same amino acid, or at worst to just one amino acid gene cloning (DNA cloning) A tech- change, and may have little effect on the nique of GENETIC ENGINEERING whereby a resulting protein. However, if the muta- gene sequence is replicated, giving many tion results in the loss or gain of one or identical copies. The gene sequence is iso- two bases, it causes the remainder of the lated by using restriction endonucleases, or gene to be misread during translation, as it by making a complementary DNA from a changes the ‘reading frame’; for example, messenger RNA template using a reverse CUC/CAG/GCA becomes UCC/AGG/CA. transcriptase. It is then inserted into the cir- Such mutations are called frameshift muta- cular chromosome of a cloning vector, i.e. tions, and are more likely to be deleterious. a plasmid or a bacteriophage. The hybrid is See mutation; chromosome mutation. used to infect a bacterium, usually Es- cherichia coli, and is replicated within the gene pool The total number and variety bacterial cell. A culture of such cells pro- of genes existing within a breeding popula- duces many copies of the gene, which can tion or species at a given point in time. subsequently be isolated and purified. gene probe See DNA probe. genecology The study of ecological ge- netics, or population genetics in relation to generative cell One of the haploid cells environment. in the POLLEN TUBE of seed plants. In an- giosperms it gives rise directly to the two The movement of alleles male gametes or generative nuclei. In gym-

93 generative nuclei nosperms it gives rise to the body cell and two bases, are necessary to insure the cod- stalk cell. ing of a specific amino acid. Three triplets, termed ‘nonsense triplets’, do not code for generative nuclei The two gametic nu- any amino acid and have other functions, clei formed by the division of the genera- e.g. marking the beginning and end of a tive cell in the pollen tube of angiosperms. polypeptide chain. One fuses with the egg cell to form a zygote while the other either degenerates or, in (Sewall Wright effect) The certain angiosperms, fuses with the polar fluctuation of allele frequencies in a small nuclei to give the primary endosperm nu- population due entirely to chance. If the cleus. number of matings is small, then the actual numbers of different types of pairing may gene sequencing Determination of the depart significantly from the number ex- order of bases making up a gene on a DNA pected on a purely random basis. Genetic molecule. Sequencing requires multiple- drift is one of the factors that can disturb cloned copies of the gene; long DNA se- the HARDY–WEINBERG EQUILIBRIUM. quences are cut into more manageable lengths using restriction enzymes. Since genetic engineering The direct intro- these cleave DNA at specific points, it is duction of foreign genes (from other indivi- possible to reconstitute the overall se- duals or species or artificially synthesized) quence once the constituent fragments into an organism’s genetic material by mi- have been analyzed individually. cromanipulation at the cell level. Genetic engineering techniques bypass crossbreed- gene splicing 1. The joining of EXONS ing barriers between species to enable gene after the INTRON sequences have been re- transfer between widely differing organ- moved, to produce functional messenger isms. The commonest method of genetic RNA that leaves the nucleus to undergo engineering is RECOMBINANT DNA TECHNOL- TRANSCRIPTION. In the nucleus this is per- OGY. Genes may be take from living tissues formed by a special assemblage of RNA by isolating the RNA produced when the and proteins called a spliceosome. gene is active, and recreating the DNA se- 2. In GENETIC ENGINEERING, the joining of quence of the gene using the enzyme re- DNA fragments by the action of the en- verse transcriptase. Gene transfer can be zyme DNA ligase. achieved by various methods, many of which employ a replicating infective agent, genetic code The sequence of bases in such as a virus, plasmid, or phage for DNA either DNA or messenger RNA that con- segments up to 24 000 base pairs in length, veys genetic instructions to the cell. The a cosmid for segments up to 45 000 base basic unit of the code consists of a group of pairs, or a YAC (yeast artificial chromo- three consecutive bases, the base triplet or some) for pieces up to 50 000 base pairs, as CODON, which specifies instructions for a a VECTOR (see gene cloning). Other meth- particular amino acid in a polypeptide, or ods include microinjection of DNA into acts as a start or stop signal for translation cell nuclei and direct uptake of DNA of the message into polypeptide assembly. through the cell membrane. Recognizing For example, the DNA triplet CAA (which whether or not transfer has occurred may is transcribed as GUU in mRNA) codes for be difficult unless the new gene confers an the amino acid valine. There are 64 differ- obvious visual or physiological characteris- ent triplet combinations but only 20 amino tic. Consequently the desirable gene may acids; thus many amino acids can be coded be linked to a marker gene, e.g. a gene con- for by two or more triplets. The code is ferring resistance to an antibiotic in the said to be degenerate, since some amino growth medium. The transferred gene acids are coded for by more than one must also be linked to appropriate regula- triplet: in certain cases only the first one or tory DNA sequences to insure that it works

94 genetic engineering in its new environment and is regulated been produced by transfer of antigen- correctly and predictably. coding genes to bacteria. Genes have been Initial successes in DNA transfer were introduced to crop plants for various achieved with bacteria and yeast. Human reasons, for instance to reduce damage genes coding for medically useful proteins during harvest or to make them resistant have been transferred to bacteria. Human to the used in controlling weeds. insulin, growth hormone, and interferon Genetically modified tomatoes and soya are now among a wide range of therapeu- beans are now widely available. There tic substances produced commercially is also hope that in future many genetic from genetically engineered bacteria. Ge- diseases will be treatable by manipulating netically engineered vaccines have also the faulty genes responsible. However, ge-

Genetic code

95 genetic fingerprinting netic engineering raises many legal and eth- genome, diploid organisms have two ho- ical issues, and the introduction of geneti- mologous sets, polyploid organisms have cally modified organisms into the many sets sometimes from the same ances- environment requires strict controls and tor (autopolyploids) and sometimes from monitoring. different ancestors (allopolyploids). Inter- specific hybrids also have two or more dif- genetic fingerprinting A technique for ference . identifying individuals by means of their DNA. The DNA being tested is extracted genotype The genetic make-up of an or- from cells (from blood, semen, tissue frag- ganism. The actual appearance of an indi- ments, etc.) and broken into fragments of vidual (the phenotype) depends on the 600–700 bases each, using restriction en- dominance relationships between alleles in zymes. The human genome contains many the genotype and the interaction between loci where short base sequences are re- genotype and environment. Compare phe- peated in tandem, with great variation be- notype. tween individuals in the number of such repeats. These so-called variable number genus (pl. genera)A taxonomic category tandem repeats (VNTR) can be identified involving a collection of similar species. using special DNA probes, thus providing Genera may be subdivided into subgenera, a virtually unique set of markers for any and also, especially in plant taxonomy, given individual. This technique is used in into sections, subsections, series, and sub- veterinary and human medicine to estab- series. The scientific name of a species al- lish the parentage of individuals, and in ways includes the genus name (or its forensic science to identify individuals abbreviation) as the first word of the bino- from traces of body tissue or fluids. Even mial. Similar genera are grouped into fam- minute amounts of DNA can now be am- ilies. See binomial nomenclature. plified, using the POLYMERASE CHAIN REAC- TION, to provide sufficient material for geological time scale A system of mea- genetic fingerprinting. suring the history of the earth by studying genetic map A map showing the se- the rocks of the earth’s crust. Since new quence of particular genes or segments of rocks are generally deposited on top of ex- DNA on a chromosome. Restriction en- isting material, those lower down are old- zymes and DNA probes are used to deter- est, although this is often disrupted by e.g. mine the exact sequence, but it is possible volcanic activity, tectonic movements and to get a rough estimate by analyzing the erosion. The strata of rock are classified ac- frequency of recombination between the cording to their age, and a time scale cor- alleles of linked genes. See chromosome responding to this can be constructed. The map; DNA probe; restriction endonucle- main divisions (eras) are the PALEOZOIC, ase. MESOZOIC, and CENOZOIC. These are fur- ther subdivided into periods and epochs. genetics The term coined by the British Bateson (1861–1926) to describe geophyte A plant whose perennating the study of inheritance and variation and buds are situated below ground as, for ex- the factors controlling them. Today the ample, in plants with , corms, , subject has four main subdivisions – or rhizomes. See Raunkiaer’s plant classifi- Mendelian genetics (classical genetics), cation. population genetics, , and mol- ecular or biochemical genetics. geotropism See gravitropism. cell genome A complete haploid chromo- germ cell Any cell in a cell lineage that some set. Haploid organisms have one eventually produces gametes.

96

Geological time scale

97 germ line germination The first outward sign of diterpenoids chemically related to gibberel- growth of a reproductive body, such as a lic acid; their molecules have the gibbane spore or pollen grain. The term is most skeleton. More than thirty have been commonly applied to seeds, in which ger- isolated, the first and one of the most com- mination involves the emergence of the mon being GIBBERELLIC ACID, GA3. Gib- radicle, plumule, or coleoptile through the berellins stimulate elongation of shoots of testa. Both external conditions (e.g. water various plants, especially the extension to availability, temperature, and light) and in- normal size of the short internodes of ge- ternal biochemical status must be appro- netically dwarf pea or maize plants. This is priate before germination can occur. Seed achieved by increasing cell-wall expansibil- germination may be either epigeal, in ity, but it requires the presence of auxin, which the cotyledons appear above too. Such dwarf varieties may be used in ground, or , in which the cotyle- bioassays for gibberellin. Increased gib- dons remain below ground. berellin levels can mimic or mediate the effect of long days. Thus they stimulate in- germ line The lineage of cells from ternode extension and flowering in LONG- which gametes arise, continuous through DAY PLANTS such as lettuce and . generations. They are also effective in inhibiting tuber development or breaking tuber dormancy, e.g. in the potato, and breaking bud dor- mancy in woody species. Gibberellins can induce , and are used in the production of seedless fruits. They may be produced in both shoots and roots and travel in both xylem and phloem. They work antagonistically with ABSCISIC ACID, and in some situations work cooperatively with AUXINS as, for example, in the control Gibberellic acid: the skeleton of the molecular structure of sex expression in dioecious plants.

gill One of many thin platelike spore- germ plasm The part of an organism producing structures radiating outward that, according to Weissmann at the begin- from the stalk on the undersurface of the ning of the 20th century, passed its charac- cap (see pileus) of agaric fungi such as ters on to the next generation. It is now mushrooms and toadstools (see Basid- known that most of this information is car- iomycota). The spores (basidiospores) are ried by DNA in the chromosomes. produced on the outer layer (hymenium). giant ferns See Marattiaceae. Ginkgophyta (ginkgoes)A phylum of deciduous gymnosperms with fan-shaped, gibberellic acid (GA3) A common gib- stalked leaves that have dichotomously berellin and one of the first to be discov- branching veins, a feature unique among ered. Together with GA1 and GA2 it was living gymnosperms. There is a single class, isolated from Gibberella fujikuroi, a fun- order, genus and species, the maidenhair gus that infects rice seedlings causing ab- tree or ginkgo (Ginkgo biloba) from normally tall growth. Gibberellic acid is a China, known only in cultivation. Fossil terpenoid synthesized from mevalonic remains of other species have been found in acid. Some growth retardants are thought many parts of the world, dating back to the to act by blocking this synthesis pathway. Middle Jurassic period. The ginkgo is dioe- See gibberellin. cious. gibberellin A plant hormone involved glabrous Smooth, with no hairs or other chiefly in shoot extension. Gibberellins are projections.

98 glycerate 3-phosphate gland A specialized cell or group of cells for synthesis of carbohydrates and nu- concerned with the of various cleotide sugars. Glucose is synthesized di- substances produced as byproducts of rectly from carbon dioxide in the Calvin plant metabolism. The may pass cycle, and from fatty acids via b-oxidation to the exterior or be contained in cavities to acetyl CoA, which then enters the GLY- or canals in the plant body. OXYLATE CYCLE (a synthetic pathway in germinating oil-rich seeds). Glucose is oxi- GLC See gas–liquid chromatography. dized in respiration through glycolysis and the KREBS CYCLE. See glycolysis; photosyn- gley (glei) A waterlogged soil lacking in thesis. oxygen, in which raw humus accumulates as a result of lack of decomposition by bac- glume See bract. teria. The defining feature is the gley hori- zon below the humus – blue-grey clay glutamic acid See amino acids. whose color is due to ferrous iron com- pounds that have been reduced by mi- glutamine An amino acid formed by the croorganisms, flecked with localized areas addition of ammonia to glutamic acid, of rust-colored oxidized ferric compounds. using energy from ATP. Glutamine is im- The formation of a gley is known as gley- portant in the storage and neutralization of ing. Gley soils are typical of tundra, mead- free ammonia. It also donates an amino ows, and boggy areas. group in synthesis reactions, e.g. the syn- thesis of purines and pyrimidines. See global warming See greenhouse effect. amino acids. globulin A kind of globular protein that gluten A mixture of proteins found in dissolves in dilute salt solutions but is rela- wheat flour. It is composed mainly of two tively insoluble in pure water. Globulins proteins (gliaden and glutelin), the proteins are found in plant seeds as storage pro- being present in almost equal quantities. teins, particularly in the seeds of legumes Certain people are sensitive to gluten (Fabaceae). (celiac disease) and must have a gluten-free diet. glucan A polysaccharide made up en- tirely of glucose units, e.g. cellulose, starch. glycan 1. A POLYSACCHARIDE made up of See polysaccharide. a single type of sugar unit (i.e. >95% ). As a class the glycans serve both as structural gluconeogenesis The synthesis of glu- units (e.g. cellulose in plants and chitin in cose from noncarbohydrate precursors, fungi) and energy stores (e.g. starch). The such as pyruvate, amino acids, and fatty most common homoglycans are made up acids (which are converted to glucose by of D-glucose units and called GLUCANS. the KREBS CYCLE and GLYOXYLATE CYCLE), 2. Any polysaccharide. glycerol, and intermediates of the Krebs cycle. In plants, glucose is also formed by glyceraldehyde (CHOCH(OH)-CH2OH) photosynthesis. A simple monosaccharide bearing an alde- hyde group. The phosphory- lated form, glucose A monosaccharide (C6H12O6) glyceraldehyde 3-phosphate is derived occurring widely in nature as a storage from fructose 1,6-bisphosphate during compound: it occurs as glucose units in su- GLYCOLYSIS, and from glycerate 3-phos- crose (which is hydrolyzed to glucose and phate in the Calvin cycle (See photosynthe- fructose) and GLYCOGEN (which is hy- sis). Glyceraldehyde 3-phosphate is drolyzed to glucose), as well as in STARCH involved in many metabolic pathways. and CELLULOSE. It is the main energy source of most organisms, and the starting point glycerate 3-phosphate (phosphoglyceric

99 glyceride acid) The phosphorylated form of glyc- glycogen A polysaccharide composed of eric acid, a 3-carbon atom molecule. Glyc- many GLUCOSE units, with a branching erate 3-phosphate is the first product of the structure similar to that of amylopectin. It dark reactions of PHOTOSYNTHESIS, formed is used as a storage compound in fungi and by the carboxylation and cleavage of ribu- certain bacteria as well as in animals, being lose bisphosphate. It is also formed from hydrolyzed to glucose with the aid of am- glyceraldehyde 3-phosphate in GLYCOLYSIS, ylase. with the release of ATP, after which it may be converted to phosphoenol pyruvate. glycolipid (glycosyldiacylglycerol) An Glycerate 3-phosphate is a precursor of the acylglycerol containing a carbohydrate amino acid serine. group or an amino sugar. Glycolipids are the major lipids in chloroplasts. See glyc- glyceride See acylglycerol. erolipid. glycerol (glycerin; 1,2,3-propanetriol) glycolysis (Embden–Meyerhof–Parnas An alcohol with three OH groups. Glyc- pathway) The conversion of glucose into erol is biologically important as the alcohol pyruvate, with the release of some energy involved in LIPID formation (these particu- in the form of ATP. Glycolysis occurs in lar lipids being called acylglycerols). It is cell cytoplasm. In ANAEROBIC RESPIRATION, also used to protect living tissues such as corneas and red blood cells for freezing, and as a mounting medium in microscopy. See acylglycerol; phosphoglyceride. glycerolipid Lipids derived from GLYC- EROL. Glycerolipids are synthesized from glycerol 3-phosphate by the addition of fatty acid chains and a head group. Plants synthesize glycerolipids in mitochondria, plastids and the endoplasmic reticulum. Glycerolipids include triacylglycerols (glyc- erol linked to three fatty acid chains), im- portant storage compounds in seeds, being broken down with the aid of lipases into fatty acids, which can then undergo b- oxidation. Compare glycolipid. glycine (aminoethanoic acid; aminoacetic acid) The simplest AMINO ACID, with the formula NH2CH2COOH. It may be syn- thesized directly from ammonia, carbon dioxide, and a donated methyl group; de- rived from serine; or derived from glycer- ate 3-phosphate by TRANSAMINATION of glyoxylate. It is broken down by a reversal of the first two pathways, or by deamina- tion to glyoxylate, followed by conversion to malic acid and further breakdown in the Krebs cycle. Glycine is necessary for the synthesis of purines, and is a basic compo- nent of PORPHYRINS. Glycolysis

100 Golgi apparatus breakdown proceeds no further and pyru- tion of a pyranose sugar with an alcohol or vate is converted into ethanol (or lactic phenol. acid in animals and some bacteria) for stor- age or elimination. In AEROBIC RESPIRATION, glycosidic bond (glycosidic link)A glycolysis is followed by the KREBS CYCLE. bond formed between monosaccharides by Glycolysis alone yields only two molecules removal of water (condensation reaction). of ATP per molecule of glucose in anaero- The most common glycosidic bond occurs bic respiration. In aerobic respiration there between the carbon-1 of one sugar and the is a net yield of six (the conversion of carbon-4 of the other (a 1-4 glycosidic NADH back to NAD yields a further four bond); sucrose has a 1-2 glycosidic bond ATP molecules, and can occur only when between the carbon-1 of glucose and car- oxygen is present). The main regulatory bon-2 of fructose. If the bond lies below enzyme in glycolysis is phosphofructoki- the plane of the glucose ring, the bond is nase, which catalyzes the phosphorylation termed an a-glycosidic bond (e.g. starch), if of fructose 6-phosphate to fructose 1,6-bis- above this plane, a b-glycosidic bond (e.g. phosphate, and is inhibited by high con- cellulose). centrations of ATP. See also respiration; alcoholic fermentation. glycosyldiacylglycerol See glycolipid. glycoprotein A conjugated protein glyoxylate cycle A modification of the formed by the combination of a protein KREBS CYCLE occurring in plants, protoc- with carbohydrate side chains. Certain tists, and some microorganisms, in regions antigens, enzymes, and hormones are gly- where fats are being rapidly metabolized, coproteins. One of the most important e.g. in germinating fat-rich seeds. Acetyl groups formed from the fatty acids are passed into the glyoxylate cycle, with the eventual formation of mainly carbohy- drates. The glyoxylate cycle bypasses the carbon dioxide-releasing steps of the Krebs cycle, allowing carbohydrates to be synthe- sized from fatty acids via succinate. See il- lustration overleaf.

glyoxysome See microbody.

Gnetophyta A phylum of gymnosperms with compound cones, cells in the sec- Glycosidic bond: the glycosidic bond in ondary xylem almost identical to an- sucrose giosperm vessels, and the tendency for the tip of the nucellus to be drawn out into a plant glycoproteins is extensin, a structural micropylar tube; there are no resin canals. component of cell walls, in which the side The embryo has two cotyledons. Often chains are tetrasaccharides of arabinose. more than one embryo is produced ini- tially, but usually only one reaches matu- glycoside A derivative of a pyranose rity. There are three very different orders, sugar (e.g. glucose) in which there is a each with only one genus: Gnetales (Gne- group (R) attached to the carbon atom tum), Ephedrales (Ephedra or joint-firs), that is joined to the –CHO group. In a gly- and Welwitschiales (Welwitschia). coside the C–OH is replaced by C–OR. The linkage –O– is a GLYCOSIDIC BOND, Golgi apparatus (Golgi body; Golgi com- joining monosaccharides in polysaccha- plex) An organelle of eukaryotic cells rides. Glycosides are formed by the reac- discovered by Camillo Golgi in 1898. It is associated with the endoplasmic reticulum

101 graft

Glyoxylate cycle but lacks ribosomes. It consists of stacks of or bud of the scion is grafted onto the flattened membrane-bounded sacs (cister- lower part of the stock. The cambia of nae) that bud off vesicles (Golgi vesicles) scion and stock merge, allowing the vascu- containing the products of the secretory ac- lar tissues of the two plants to combine. tivity in the Golgi apparatus. The Golgi Production of callus around the graft area apparatus is rich in secretory cells. The cis- helps to secure the union. Grafting is usu- ternae are spread randomly (as dic- ally successful only between closely related tyosomes) in plant cells but form a single species. It is often used to propagate vari- network in most animal cells. In the cister- eties derived from mutations or hybridiza- nae, materials (e.g. enzymes and polysac- tion that cannot be grown from seed, charides) are processed and leave in Golgi especially woody species such as apples, vesicles for transport, often to the plasma pears, and roses. membrane for secretion. If the vesicles re- lease their contents on the cell surface, their graft hybrid See chimera. membranes become part of the plasma membrane; thus the Golgi apparatus con- tributes to cell membrane synthesis. grain See caryopsis. graft The transplantation of an organ or Gram, Hans Christian Joachim (1853– tissue in plants and animals. In plants, 1938) Danish bacteriologist. He devised grafting is an important horticultural tech- the important method of staining bacteria nique in which an above-ground part (the that bears his name (see Gram’s stain) in scion) of one individual is united with the Berlin in 1884, building on the method of root of another (the stock) of the same or a Paul Erlich. The discovery that some bacte- different species. (Where more than two ria retain the stain (Gram positive), while parts are grafted together, the middle piece others do not (Gram negative), has been is called the interstock.) Usually the shoot important in identifying and classifying

102 ground layer bacteria and also in deciding upon the moves in response to a gravitational field (a treatment of bacterial diseases. STATOLITH). Starch-filled plastids (AMYLO- PLASTS) are thought to act as gravirecep- Gramineae See Poaceae. tors, moving in response to gravity and pressing on the endoplasmic reticulum Gram’s stain A stain containing the near the outer edge of specialized cells basic dye crystal violet or gentian violet called STATOCYTES located in the root cap that forms the basis for the division into or young root, and in a layer (termed the Gram positive and Gram negative bacteria. STARCH SHEATH) adjacent to the vascular The bacteria are first killed and stained tissues. Other possible gravireceptors are with the violet dye, then treated with an or- stretch-activated ion channels in mem- ganic solvent such as acetone or ethanol. branes, which may sense movement of the Gram positive bacteria (e.g. Actinobacte- protoplasm in response to gravity. Such ria) retain the deep purple color of crys- channels may be involved in the response tal violet; Gram negative bacteria (e.g. of the endoplasmic reticulum to moving Cyanobacteria) are decolorized, and may amyloplasts. See gravitropism. See also be counterstained with other dyes, such as photoreceptor. safranin, carbol fuchsin, or neutral red. A number of physiological differences are as- gravitropism (geotropism) A directional sociated with the difference in staining, in- growth movement of part of a plant in re- cluding nutrition, and susceptibility to sponse to gravity. Primary roots (tap roots) antibiotics. grow vertically toward gravity (positive gravitropism) whereas primary shoots granum (pl. grana)A stack of mem- grow vertically away from gravity (nega- branes (resembling a pile of coins) in a tive gravitropism), though the direction of CHLOROPLAST. With the light microscope shoot growth may also be modified by these stacks are just visible as grains light. leaves and some stem (grana). The photosynthetic pigments and structures (e.g. rhizomes and ) grow accessory pigments are bound to these horizontally (diatropism). Secondary (lat- membranes, which also contain the com- eral) roots and stem branches may grow at ponents of the electron-transport chain an intermediate angle with respect to grav- and enzymes needed for the light-depen- ity (plagiotropism). Gravitropic responses dent reactions of photosynthesis. are believed to involve hormones, maybe auxins or gibberellins. See also gravirecep- grape sugar See glucose. tor; klinostat; tropism. grassland A major world plant com- green algae See Chlorophyta; Gamo- munity (biome) dominated by grasses phyta. (Poaceae). Grasslands occur in many tem- perate regions where annual rainfall is greenhouse effect The rise in tempera- 250–500 mm and in tropical regions with ture of the atmosphere, analogous to that 750–1500 mm. They are widespread in the in a greenhouse. Solar (short-wave) radia- interiors of continents, where there is a dis- tion passes easily through the atmosphere tinct rainy season in spring and summer. (or glass in a greenhouse) and is absorbed Many grasslands, especially in the wetter by the earth’s surface. It is re-emitted in the areas, are naturally maintained by grazing form of infrared (long-wave) radiation, mammals and, to some extent, fire. which is absorbed by water vapor, carbon dioxide, methane, chlorofluorocarbons graticule See micrometer. (CFCs), and other so-called greenhouse gases in the atmosphere with a consequent gravireceptor A receptor that can sense increase in the atmospheric temperature. gravity. This is usually an object that Many scientists believe that increasing at-

103 ground meristem mospheric pollution by carbon dioxide, stops entirely, though it may become ex- mainly due to the burning of fossil fuels, tremely slow (indeterminate or indefinite and many other gases is leading to a rise in growth). This allows plants to continue global temperatures (global warming), growing and many plants grow laterally which will eventually affect other aspects showing clonal growth. The result is that of climate and have profoundly damaging some can be extremely long-lived, with effects on natural ecosystems and agricul- clones, such as those of the creosote bush ture, as well as raising sea level. () known to have lived for over 10 000 years. Grew, Nehemiah (1641–1712) British physician and botanist. Grew carried out growth factor Any substance that af- important work in and was fects the growth of a plant. This includes a contemporary of MALPIGHI. His work cul- GROWTH SUBSTANCES (HORMONES) and minated in his publication of The Anatomy GROWTH INHIBITORS, as well as food re- of Plants, in which he was among the first serves and minerals. to observe chloroplasts and stomata, de- scribing the latter as ‘breathing pores’. growth inhibitor A substance that slows the growth of a plant, for example ground layer The lowermost layer of the plant hormones ABSCISIC ACID and ETH- a plant community, usually comprising YLENE (ethene), which act at very low con- mosses, lichens, fungi, and low-growing centrations. herbs, especially plants and creep- ing, prostrate forms. Compare field layer. growth ring See annual ring. ground meristem The central region of growth substance See hormone. the apical meristem from which the ground tissues of pith, cortex, medullary rays, and GSC Gas–solid chromatography. See mesophyll differentiate. In root meristem it gas chromatography. is also called periblem. guanine A nitrogenous base found in ground tissues The unspecialized tis- DNA and RNA. Guanine has a purine ring sues that are found in any region of the structure. The nucleoside guanosine tri- plant not occupied by the specialized tissue phosphate (GTP) is derived from guanine, of vascular bundles, cambium, epidermis, and supplies energy for certain enzyme- etc. The pith and cortex of the root and catalyzed metabolic reactions. stem are , as are the meso- phyll layers of the leaf. guanosine (guanine nucleoside) A nu- cleoside present in DNA and RNA and growth An irreversible increase in size consisting of guanine linked to D-ribose via and/or dry weight. It excludes certain de- a b-glycosidic bond. velopmental processes that involve no size change, such as cleavage and uptake of A specialized kidney-shaped water by seeds (IMBIBITION). Growth in- epidermal cell, located to the side of a volves cell division and cell expansion . Two guard cells together encircle through synthesis of new materials, and is each stoma and control the opening and closely related to subsequent developmen- closing of the stomatal aperture. The con- tal processes. If some measure of growth of trol is effected through changes in turgid- an organism, such as height or weight, is ity. The wall of the guard cell bordering the plotted throughout its life, a characteristic pore is heavily thickened while the oppo- S-shaped (sigmoid) growth curve is ob- site wall is comparatively thin. Thus when tained for most organisms. In some organ- the guard cell is turgid the thin wall be- isms, including many plants growth never comes distended, bulging out away from

104 gynomonoecious the pore, and causes the thickened wall, dathodes as a result of root pressure. Drops which cannot distend, to be drawn out- of guttated water are often seen at the tips wards with it. This results in an aperture of grasses in the early morning, when the being formed between adjacent guard cells. low temperature reduces transpiration. It is When osmotic pressure of the guard cell also common in tropical plants living in en- drops the pore closes. Most plants exhibit vironments with high humidity that re- a stomatal rhythm whereby the stomata duces transpiration rates. open in light and close in darkness. A general term for any gum Any substance that swells in water seed plant except the angiosperms, charac- to form a gel or a sticky solution. Similar terized by bearing naked seeds. There are compounds that produce slimy solutions four widely different living groups: the cy- are called mucilages. Gums and mucilages cads, ginkgo, conifers, gnetophytes, and are not distinguishable chemically. Most several fossil groups. Gymnosperms lack are heterosaccharides, being large, com- endosperm and the precise double fertiliza- plex, flexible, and often highly branched tion characteristic of the angiosperms, al- molecules. though some show a primitive form of it. Cycads and ginkgo have motile sperm, un- guttation Loss of water as liquid from like gnetophytes and conifers, which pro- the surface of a plant. Water is normally duce pollen. The female gametophyte is lost as vapor during transpiration but, if reduced, but larger than in angiosperms; it the atmosphere is very humid, water may is not autotrophic. Most gymnosperms re- also be forced from the leaves through hy- tain archegonia except for the most ad-

105 H

habitat The place where a particular or- and are formed directly by the fusion of ga- ganism lives, described in terms of its cli- metes. Haplobiontic is thus a collective matic, vegetative, topographic, and other term for haplontic and diplontic. Compare relevant factors. diplobiontic. See alternation of genera- tions. hair () An outgrowth from a plant epidermal cell. Plant hairs may take a haploid (monoploid) Describing an or- wide variety of forms, often characteristic ganism, cell, or nucleus containing only of particular species or genera. They may one representative from each of the pairs of be unicellular or multicellular, simple, homologous chromosomes found in the forked, stellate (starlike), downy, silky, normal DIPLOID cell. Haploid chromosomes long, or short. Some, such as the hairs on are thus unpaired and the haploid chromo- the leaves of geraniums (Pelargonium) are some number (n) is half the diploid number glandular. See also . (2n). Meiosis, which usually precedes spore or, sometimes, gamete formation, halophyte A plant that grows in soils halves the chromosome number to produce with a high concentration of salt, as found haploid gametes. The diploid condition is in salt marshes or other littoral habitats. restored when the nuclei of two gametes Examples are species of Spartina. Halo- fuse to give the zygote. Gametes may de- phytes are adapted to obtain water from velop without fertilization, or meiosis may soil water with a higher osmotic pressure substantially precede gamete formation, than normal soil water, so they need to be leading to the formation of haploid organ- able to accumulate a high concentration of isms, or haploid phases in the life cycles of salts in their root cells. Many have a succu- organisms. lent growth form with swollen stems or leaves. haplont A haploid organism that repre- sents the vegetative stage in life cycles in halosere A plant community in a SUC- which diploidy is restricted to the zygote. CESSION that starts on land periodically in- Haplontic life cycles are typical of the fila- undated by the sea. Haloseres include salt- mentous green algae. Compare diplont. and flooding-tolerant pioneer communities on mudflats and saltmarshes. haplostele A type of protostele having a solid strand of stele with the xylem on the haplobiontic Describing life cycles in inside encircled by phloem, pericycle, and which only one type of somatic body is endodermis. This type of stele was present formed, which may be either haploid or in the earliest known vascular plants, such diploid, i.e. either the sporophyte or the ga- as . metophyte generation is missing. For ex- ample, in the unicellular Chlamydomonas hapteron (pl. haptera) 1. See holdfast. the vegetative cell is haploid, and the only 2. In species of (horsetails), the diploid cells are the zygospores, which ger- outer wall of the spore, which forms an X- minate to produce haploid zoospores. In shaped structure, the arms of which are diatoms the vegetative cells are diploid, wrapped around the spore. On drying out,

106 heath the arms, or haptera, act in similar manner equilibrium is described by the equation to the elaters of liverworts, and are thus p2 + 2pq + q2 = 1, where p2 and q2 are the termed elaters. See elater. frequencies of the double dominant and double recessive respectively, and 2pq is Haptomonada (Haptophyta) A phy- the frequency of the heterozygote. The lum of the PROTOCTISTA that contains Hardy–Weinberg equation p2 + 2pq + q2 = mostly unicellular algae, most of which 1 is obtained from the expansion of (p + have a threadlike structure called a hap- q)2, the total of the frequencies making up tonema between their two undulipodia. the gene pool being unity. The law was for- The haptonema is thought to be used for mulated in 1908 and disproved the then temporary anchorage rather than for loco- current theory that dominant genes always motion and may be coiled while the cell is tend to increase in a population at the ex- motile. Haptomonads are common in ma- pense of their equivalent recessive alleles. rine plankton and freshwater. The equilibrium holds only when the pop- ulation is sufficiently large to avoid chance haptonasty (haptonastic movements)A fluctuations of allele frequencies in the NASTIC MOVEMENT in response to contact. gene pool (GENETIC DRIFT) and providing An example is the sensitive plant (Mimosa there is no mutation, natural selection, or pudica), whose leaflets progressively col- migration into or out of the population. lapse, sometimes throughout the whole The fact that allele frequencies may be seen plant, once one leaflet is touched. See pul- to change fairly rapidly in large popula- vinus. tions that show minimal mutation and mi- gration emphasizes the important role haptophyta See Haptomonada. natural selection must play. Until the Hardy–Weinberg law was formulated, the haptotropism See thigmotropism; tro- extent of natural selection was not fully ap- pism. preciated. hardpan 1. A hardened soil horizon, Hartig net See ectomycorrhiza. usually in the middle or lower part of a soil profile, A hardpan is typical of PODSOLS, Hatch–Slack pathway See C4 plant. where iron compounds leached from the upper layers accumulate in the B horizon, haustorium (pl. haustoria) A special- staining it red-brown and forming an im- ized outgrowth of many parasitic fungi and permeable layer (iron pan) that can prevent certain parasitic plants such as dodder drainage, resulting the in waterlogging of (Cuscuta) and broomrape (Orobanche), higher horizons. that penetrates into and withdraws food 2. Also called calcrete, a hardened layer in material from the cells of the host plant. or on a soil formed on calcareous sub- strata, formed in arid and semiarid regions heartwood (duramen) The hard central as a result of fluctuating climatic condi- region of a tree trunk made up of xylem tions by calcite that precipitates out of so- vessels that are no longer involved in water lution as water evaporates at the surface. transport. Such vessels are often blocked by substances (e.g. resins and tannins) that Hardy–Weinberg equilibrium The sit- give the wood a darker color. Compare uation in a large, randomly mating, stable sapwood. population under specific conditions, in which the proportion of dominant to re- heath An area dominated by dwarf cessive genes remains constant from one shrubs of the heath family (Ericaceae). generation to the next. If a pair of alleles, Legumes (family Fabaceae) may also be A and a, have the frequencies p and q in common components of heath . a population, then the Hardy-Weinberg Heaths normally develop on poor, sandy,

107 heavy metal tolerance well-drained soils in temperate climates, hemicellulose One of a group of sub- and are usually subclimax communities stances that make up the amorphous ma- maintained by grazing, burning, and some- trix of plant cell walls together with pectic times, cutting. substances (and occasionally, in mature cells, with lignin, gums, and mucilages). heavy metal tolerance Biochemical They are heteropolysaccharides, i.e. POLY- and physiological adaptation to concentra- SACCHARIDES built from more than one type tions of heavy metals, such as copper, lead, of sugar, mainly the hexoses (mannose and and zinc, that would prevent the growth of galactose) and the pentoses (xylose and most plant species or genotypes. Such arabinose). Galacturonic and glucuronic adaptation usually involves converting the acids are also constituents. They vary metal to a harmless form; some species ex- greatly in composition between species. clude the metal from the plant altogether, The and pectic substances while others confine it to the roots, thus interact with the cellulose microfibrils of protecting the more sensitive shoots. Cer- the cell wall to give extra strength and at tain species or strains have evolved such the same time retain some plasticity. In tolerance, and some, for example certain some seeds (e.g. the endosperm of dates) strains of bent grass (Agrostis capillaris), hemicelluloses are a food reserve. See cell benefit from the reduced competition in wall. soil containing heavy metal ions. hemicryptophyte A perennial plant, helical thickening See spiral thicken- usually nonwoody, with its over-wintering ing. buds at or just below soil level. The buds are protected by the soil or surface litter. helicase Any of a group of enzymes that See also Raunkiaer’s plant classification. catalyze the unwinding of double-stranded nucleic acids such as DNA, using energy hemiparasite A that has from ATP. chlorophyll and can photosynthesize, but depends on another plant for its water and heliophyte A plant typical of sunny mineral salts, e.g. mistletoe. habitats. hemizygous Describing genetic material A form of PHOTOTROPISM that has no homologous counterpart and is in which flowers move to point toward the thus unpaired in the diploid state. Both sin- sun, tracking it as it moves across the sky. gle genes and chromosome segments may It is a daily movement, mainly in spring be hemizygous; for example, the X chro- flowers that are pale and cup-shaped like mosome in the heterogametic sex, and wood anemones and buttercups. whole chromosomes in aneuploids. helophyte A perennial marsh plant that Hepatophyta (; liver- has its overwintering buds under water. An worts) A phylum containing leafy and example is bulrush (Typha). prostrate thallose forms, commonly known as liverworts. Thallose liverworts are pros- hematoxylin A blue dye derived from trate and dichotomously branched with the logwood tree (Haemotoxylon cam- thalli several cells thick. There are five or- pechianum), which when oxidized to ders of liverworts, including the Metzgeri- hematin stains nuclei and cellulose cell ales, e.g. Pellia, and the Marchantiales, e.g. walls blue. See staining. Marchantia. heme (haem) An iron-containing por- herbaceous perennial See perennial. phyrin that is the prosthetic group in cy- tochromes. See porphyrins. The proportion of pheno-

108 heterosis typic variation due to genetic factors. It can be estimated from measurements of indi- viduals from different generations, and is used in plant and animal breeding to pre- dict how successful genetic selection will be in improving a particular trait. hermaphrodite (bisexual; monoclinous) A plant bearing stamens and carpels in the same flower. In many such plants, self- fertilization and inbreeding are prevented by specific self-incompatibility systems. Compare monoecious; dioecious. hesperidium (pl. hesperidia) A type of berry with a leathery epicarp, as in oranges and lemons. Each carpel forms a segment of the fruit, filled with fluid-filled tri- chomes. See berry. heteroecious Denoting rust fungi that require two host species to complete their life cycle. An example is Puccinia graminis, the stem rust of cereals and grasses, which overwinters on the barberry. Compare au- toecious. heterogametic sex The sex with dissim- ilar SEX CHROMOSOMES, one (in mammals the Y chromosome) being shorter than the other (the X chromosome). Heterophylly: examples of heterophylly in two different aquatic plants heterogamy See anisogamy. algae. Compare isomorphism; polymor- phism. See alternation of generations. heterokaryosis The presence of two or more nuclei with differing genotypes heterophylly Denoting plant species within a single cell (termed a hetero- that have more than one form of foliage karyon). Fusion of fungal hyphae of differ- leaf on the same individual. Examples in- ing genetic complements leads to hetero- clude certain species of Lycopodium and karyosis. Heterokaryons often grow better Selaginella, which have two rows of lateral than homokaryons, since any genetic defi- leaves and one or two rows of smaller ciencies, for example in the ability to syn- abaxial or adaxial leaves. thesize certain compounds, in one type of nucleus may be overcome by alleles present heterosis (hybrid vigor) The condition in the other nuclei. in which the expression of a characteristic is greater in the heterozygous offspring heteromorphism The existence of more than in either of the homozygous parents. than one form, used especially with refer- The effect arises from an accumulation of ence to life cycles in which the alternating dominant genes in the F1. Thus, if height is generations are markedly different mor- controlled by two genes, A and B, and tall phologically, as in ferns, bryophytes, and and short forms are determined by domi-

109 heterospory nant and recessive alleles respectively, then morphologically or in the size of the ga- the cross AAbb ´ aaBB would give an F1 metes they produce. When heterothallism AaBb, containing both dominant genes for is purely physiological, morphologically tallness. Usually the more unlike the par- identical strains exist (often designated ents are the more hybrid vigor is released, plus and minus strains). Compare homoth- but the effect diminishes in subsequent allism. generations as more recessive homozygotes reappear. For crop plants, hybrid vigor is heterotrophism A type of nutrition in particularly important for characteristics which the principal source of carbon is or- such as growth rate, yield, and disease re- ganic, i.e. the organism cannot make all its sistance. own organic requirements from inorganic starting materials. Most heterotrophic heterospory The production of two organisms are chemotrophic (i.e. show different sizes of spore: and chemoheterotrophism); these comprise all . The larger megaspores usu- animals and fungi, most bacteria, and ally contain more food reserves. Compare parasitic plants. A few heterotrophic or- homospory. ganisms are phototrophic (i.e. show photo- heterotrophism). The nonsulfur purple heterostyly A DIMORPHISM in which the bacteria, for instance, require organic mol- styles of flowers of the same species are of different lengths, thus dividing the species ecules such as ethanol and acetate. Com- into groups. An example is the primrose, pare autotrophism. See chemotrophism; which is divided into pin-eyed and thrum- phototrophism. eyed types. Pin-eyed plants have long styles and short stamens and thrum-eyed plants heterozygous Having two different al- conversely have short styles and long sta- leles at a given locus. Usually only one of mens. Such differences promote pollina- these, the dominant allele, is expressed tion between groups. in the phenotype. On selfing or crossing heterozygotes some double recessives may heterothallism A condition found in appear, giving viable offspring. Selfing het- algae and fungi in which sexual reproduc- erozygotes halves the heterozygosity, and tion occurs only between genetically differ- thus outbreeding maintains heterozygosity ent self-incompatible mating types (strains) and produces a more adaptable popula- of the same species. The strains may vary tion. Compare homozygous.

Heterostyly: example in Primula

110 Holocene hexose A SUGAR that has six carbon complex biosynthetic pathway. Glutamate atoms in its molecules. is an intermediate in its breakdown. hexose monophosphate shunt See pen- histochemistry The location of particu- tose phosphate pathway. lar chemical compounds within tissues by the use of specific staining techniques, for high-energy phosphate bond A phos- example phloroglucinol to stain lignin. phate linkage that hydrolyzes sponta- neously, releasing energy, for example the histogen theory A theory, proposed by last two phosphate bonds of ATP. ATP is Hanstein, in which the apical meristem is the main form in which chemical energy is considered to consist of three main zones, transported from one part of the cell to an- the dermatogen, periblem, and plerome, other. which differentiate into the epidermis, cor- tex, and stele respectively. This concept has Hill reaction The reaction, first demon- now been replaced, for stem apices, by the strated by Robert Hill in 1937, by which tunica–corpus theory. In roots, however, isolated illuminated chloroplasts bring the concept is still applied, and in some an- about the reduction of certain nonphysio- giosperm roots a fourth histogen zone is logical electron acceptors with accompany- recognized, the calpytrogen, which gives ing evolution of oxygen. For example, the rise to the root cap. Compare tunica– blue dye dichlorophenol indophenol corpus theory. (DCPIP) may be reduced to a colorless sub- The study of tissues and cells stance. The reaction involves part of the at microscopic level. normal light reaction of photosynthesis. Electrons from water involved in noncyclic histone One of a group of relatively photophosphorylation are used to reduce small proteins found in chromosomes of the added substance. It provided support eukaryotic cells, where they organize and for the idea that a light reaction preceded package the DNA. When hydrolyzed, they reduction of carbon dioxide in photosyn- yield a large proportion of basic amino thesis. See cyclic photophosphorylation. acids, as they are rich in the basic amino acids arginine and lysine. They dissolve hilum (pl. hila) 1. A scar on the testa of readily in water, dilute acids, and alkalis a seed marking the point at which it was at- but do not coagulate readily on heating. tached to the ovary wall by the funicle. It is a feature that distinguishes seeds from hnRNA (heterogeneous nuclear RNA) fruits. See messenger RNA. 2. The center of a starch grain around which the layers of starch are deposited. Hofmeister, Wilhelm Friedrich Bene- 3. A small projection at the base of a ba- dict (1824–77) German botanist who sidiospore near its attachment to the first demonstrated the ALTERNATION OF sterigma. GENERATIONS in lower plants. Hofmeister also showed that the gametophytes and hip A type of PSEUDOCARP (false fruit) of seed-bearing plants were consisting of a cup-shaped receptacle con- homologous with those of ferns and taining achenes, each of which bears small mosses and that the gymnosperms formed hooklike hairs. Hips are typical of the a plant group between the latter and the genus Rosa. angiosperms. histidine A basic AMINO ACID found only holdfast (hapteron) In algae, the cell or rarely in proteins. It is formed from ATP organ that attaches the plant to the sub- and 5-phosphoribosyl pyrophosphate in a strate. It is often disklike, sometimes di-

111 holoenzyme vided into many fingerlike processes, and is ent parts of the body during development. particularly prominent in the brown algae They encode proteins that regulate the ex- (e.g. Fucus). pression of other genes by binding to DNA. This binding capability can be pinpointed Holocene (Recent) The present epoch in to a characteristic base sequence known as the geological time scale, being the second a HOMEOBOX. Such genes often occur in epoch of the Quaternary period, dating complexes in which the physical order of from the end of the last glaciation, about the genes corresponds to the order in which 10 000 years ago, to the present day. they are expressed from anterior to poste- rior in the developing embryo. holoenzyme A catalytically active com- plex made up of an APOENZYME and a CO- homogametic sex The sex with ho- FACTOR (nonprotein prosthetic group). The mologous SEX CHROMOSOMES, in mammals, former is responsible for the specificity of designated XX. the holoenzyme whilst the latter deter- mines the nature of the reaction. Some homogamy The condition in flowers in holoenzymes, such as pyruvate dehydroge- which the anthers and stigmas ripen at the nase, are highly complex, with several co- same time, so encouraging self-pollination. factors. See coenzyme. Homogamy occurs in the closed cleistoga- mous flowers that appear late in the season holophytic The type of nutrition in in certain plants. Compare dichogamy. which complex organic molecules are syn- thesized from inorganic molecules using homologous 1. Describing structures light energy. It is another term for pho- that, though in different species, are be- totrophic. lieved to have the same origin in a common ancestor. Thus the stamens of angiosperms holozoic (heterotrophic) Designating and the MICROSPOROPHYLLS of lower vascu- organisms that feed on other organisms or lar plants are said to be homologous, being solid organic matter, i.e. insectivorous and derived from a leaflike structure bearing parasitic plants, fungi, many bacteria and the (anthers). Compare most animals. Compare holophytic. analogous. 2. See isomorphism. homeobox A segment of DNA found in many so-called homeotic genes concerned homologous chromosomes Chromo- with controlling the development of organ- somes that pair at meiosis. Each carries the isms. It consists of 180 base pairs, and the same genes as the other member of the pair sequence of bases is remarkably similar but not necessarily the same alleles for a across a wide range of eukaryotic species, given gene. One member of each pair of from yeasts to human beings. This suggests maternal origin, the other of paternal ori- it arose early in evolutionary time and has gin. With the exception of the sex chromo- been little changed since. The sequence en- somes (X and Y) they are morphologically codes the amino acids of a peptide se- similar, being of similar size and shape quence (the homeodomain) that enables with centromeres at the same location. the parent protein to bind to DNA. This is During the formation of the germ cells only consistent with the suggested role of one member of each pair of homologs is homeotic proteins as genetic switches, passed on to the gametes. At fertilization binding to genes to control their expres- each parent contributes one homolog of sion. See differentiation. each pair, thus restoring the diploid chro- mosome number in the zygote. homeotic gene (homeogene) Any of a class of genes that are crucial in determin- homospory The production of only one ing the DIFFERENTIATION of tissues in differ- kind of asexual spore, which then develops

112 humus into a hermaphrodite gametophyte. In the the Antarctic (1839–43) on HMS Erebus vascular plants the condition is seen in resulted in a six-volume flora of Antarc- most ferns, psilophytes, and Lycopodium tica, New Zealand, and Tasmania that at- species. Compare heterospory. tracted the attention of . Hooker was one of those in whom Darwin homostyly The usual condition found confided his theory and was instrumental in plants in which the styles of all flowers in encouraging Darwin to present his joint of the same species are about the same paper with Wallace on the theory of evolu- length. Compare heterostyly. tion by natural selection. In 1865 he succeeded his father as di- homothallism A condition found in rector of , which became an some algae and fungi in which each thal- international center for botanical research. lus is self-compatible. Homothallic species With , he produced the may produce distinctly different sizes of world flora . gametes that can fuse with each other and are thus effectively hermaphrodite. Com- hormone (growth substance; plant hor- pare heterothallism. mone; phytohormone) One of a group of essential organic substances produced in homozygous Having identical alleles plants. They are effective in very low con- for any specified gene or genes. A homozy- centrations and control growth, develop- gote breeds true for the character in ques- ment, and responses to external stimuli by tion if it is selfed or crossed with a similar their interactions. They affect physiologi- homozygote. An organism homozygous at cal processes, but are not actually part of every locus produces offspring identical to major metabolic pathways. The same hor- itself on selfing or when crossed with a mone may have many different effects, de- genetically identical organism. Homo- pending upon the balance between the is obtained by inbreeding, and concentrations of different hormones, the homozygous populations may be well particular tissues concerned, and the age of adapted to a certain environment, but slow the plant. See auxin; gibberellin; cytokinin; to adapt to changing environments. Com- abscisic acid; ethylene. pare heterozygous. See pure line. horsetails See Sphenophyta. honey guides Lines or spots on petals that direct a pollinating insect to the nec- host An organism used as a source of taries. nourishment by another organism, the par- asite, which lives in or on the body of the Hooke, Robert (1635–1703) English host. See parasitism. physicist. In 1665 he published his Micro- graphia, a beautifully illustrated book that humidity The moisture content of the described both his theories of the nature of atmosphere. The absolute humidity is the light and the investigations he had made amount of water vapor present in a unit with his improved microscope. While ob- volume of air (usually 1 m3 of air). The rel- serving cork, he described it as resembling ative humidity is the ratio of the amount of a honeycomb and was the first to refer to water vapor actually present in the air to the cavities as ‘cells’. the greatest amount that the air could hold at the same temperature, and is usually ex- Hooker, Sir Joseph Dalton (1817– pressed as a percentage. 1911) Plant taxonomist and explorer. Son of the British botanist William Jackson humus The nonliving finely divided or- Hooker (1785–1865), the first director of ganic matter in soil derived from the Kew Gardens, Hooker studied medicine at decomposition of animal and plant sub- Glasgow University. His famous voyage to stances by soil bacteria. Humus consists of

113 hyaline cells

60% carbon, 6% nitrogen, and small A specialized secretory struc- amounts of phosphorus and sulfur, and is ture involved in the removal of excess valued by horticulturalists and farmers as water from plants. It may be a modified it improves the fertility, water-holding ca- stoma with the guard cells permanently pacity, and workability of the soil. Humus open so that water is lost passively by has colloidal properties that enable it to re- hydrostatic pressure, as in sea lavenders tain water, so it can improve the moisture (Limonium), or a glandular hair from content of sandy soils. It aids the formation which water is actively secreted, as in run- of crumbs in the soil, and is often added to ner (Phaseolus). are clay soil to increase the particle size, pro- found at the leaf tips or along the leaf mar- moting drainage and aeration. gins. hyaline cells Any leaf cell with no hydrocarbon An organic compound chlorophyll that is normally used for stor- that contains carbon and hydrogen only, age of water or solutes. e.g. acetylene, butane. Many of the compo- nents of natural gas, petroleum, and coal hyaloplasm (cell matrix; ground sub- are hydrocarbons. stance) See cytoplasm. hydrogen An essential element in living hybrid An organism derived from cross- tissues. It enters plants, with oxygen, as ing genetically dissimilar parents. Thus water and is used in building up complex most individuals in an outbreeding popula- reduced compounds such as carbohydrates tion could be called hybrids, and geneti- and fats. Water itself is an important cists apply the term to the F generation 1 medium, making up 70–80% of the weight resulting from a monohybrid cross, where of organisms, in which chemical reactions the offspring differ in the alleles of a single of the cell can take place. Hydrogenated gene. However, the term is usually reserved compounds, particularly fats, are rich in for the product of a cross between individ- energy and on breakdown release energy uals that are markedly different. If two dif- for driving living processes. ferent species are crossed, the offspring is often sterile, owing to the nonpairing of the chromosomes necessary for gamete forma- hydrogen bond A type of bond occur- tion. In plants this is sometimes overcome ring between molecules. Hydrogen bond- by the doubling of the chromosome num- ing takes place between oxygen, nitrogen, ber, giving an allopolyploid. By contrast, or fluorine atoms on one molecule, and hy- hybrids derived from different varieties of drogen atoms joined to oxygen, nitrogen, the same species are often more vigorous or fluorine on the other molecule. The at- than their parents (hybrid vigor), and are traction is due to electrostatic forces. Hy- selected and propagated by vegetative drogen bonding is responsible for the means by and horticulturists. properties of water. It is important in many See chimera; heterosis. biological systems for holding together the structure of large molecules, such as pro- hybrid sterility The reduced ability of teins and DNA. some HYBRIDS to produce viable gametes. It is due to the lack of homologous pairs of hydrolase An enzyme that catalyzes a chromosomes, so chromosomes cannot HYDROLYSIS reaction. Glycosidases, pepti- pair in meiosis, and the resulting gametes dases, and phosphatases are examples. may have extra or missing chromosomes. Hydrolases play an important part in ren- Hybrid sterility is sometimes overcome by dering insoluble food material into a solu- polyploidy. Compare heterosis. ble form, which can then be transported in solution. They are important in the mobi- hybrid vigor See heterosis. lization of stored food, and in the uptake of

114 hypocotyl nutrients by fungi and other heterotrophic hydrosere Any plant community in a organisms. SUCCESSION that starts in fresh water. hydrolysis The breakdown of a com- hydrotropism (hydrotropic movement) pound due to a reaction with water (H+ or A TROPISM in which the stimulus is water. It OH– ions). is a special kind of chemotropism. Roots are positively hydrotropic and hypocotyls hydrophilic Describing a molecule or negatively hydrotropic. If water is in short surface that has an affinity for water. Such supply, the stimulus of water is stronger molecules are usually polar, for example than the stimulus of gravity in determining proteins. Compare hydrophobic. response. hydrophily Pollination in which water hygroscopic Able to absorb water from the surroundings, including moisture from carries the pollen from anther to stigma. the atmosphere. The pollen may be transported on the water surface or through the water. Pollen hymenium (pl. hymenia) A layer of the that floats is light and water repellent, and fruiting body of certain ascomycete and may have a coating of oil that alters the basidiomycete fungi in which the asci or surface tension of the water. basidia are borne. The hymenium may be directly exposed to the air, as in the gills of hydrophobic Describing a molecule or the mushroom (a basidiomycete) or may surface that has no affinity for water. Such open into a flask-shaped cavity, as in the molecules are nonpolar, e.g. benzene. perithecia of certain ascomycetes. Compare hydrophilic. hyperplasia Enlargement of a tissue due hydrophyte A plant found growing in to an increase in the number of its cells. In water or in extremely wet areas. Hy- plants this usually occurs in response to a drophytes show certain adaptations to disease-causing organism. such habitats, notably development of aerenchyma, reduction of cuticle, root sys- hypertonic Designating a solution with tem, and mechanical and vascular tissues, an osmotic pressure greater than that of a and divided leaves. Large intercellular air specified other solution, the latter being spaces in leaves, stems, and roots allow hypotonic. When separated by a selectively oxygen to diffuse through the plant. Hy- permeable membrane (e.g. a cell mem- drophytes with floating leaves can ex- brane) water moves by osmosis into the hy- change gases with the atmopshere; many pertonic solution from the hypotonic solution. Compare hypotonic; isotonic. submerged hydrophytes, such as spiked water milfoil (Myriophyllum spicata) have hypertrophy Enlargement of a tissue or no stomata, absorbing water and gases organ due to an increase in the size of its Compare over their entire surface. meso- cells. An example is the swelling of roots phyte; xerophyte. of plants from the Brassicaceae family infected with club root (Plasmodiophora (water culture) The growth brassicae). Compare hyperplasia. of plants in liquid culture solutions rather than soil. The solutions must contain the hypha (pl. hyphae) In fungi, a fine correct balance of all the essential mineral nonphotosynthetic tubular filament that requirements. The method is used commer- spreads to form a loose network termed a cially, especially for glasshouse crops, and mycelium or aggregates into fruiting bod- also in experimental work in determining ies (e.g. toadstools). Hyphae may be the effects of mineral deficiencies. branched or unbranched and may or may

115 hypodermis not have cross walls (septa) dividing them water-storing tissue as in certain succulent into cells. Many fungi have incomplete leaves and the aerial roots of epiphytes. septa. Hyphae are parasitic or saprophytic and the tips secrete enzymes to digest and Germination in penetrate the food supply. The hyphal seed plants during which the cotyledons re- walls of most species of fungi differ from main underground, as in broad bean (Vicia those of plants in being composed of mi- faba). The cotyledons thus act only as food crofibrils of a nitrogenous compound storage organs and not as photosynthetic called chitin or a form of fungal cellulose. organs and the hypocotyl does not elon- Hyphae also differ in lacking plastids, any gate. Compare epigeal germination. pigment being contained in the walls, cyto- plasm, or oil globules. hypogyny The simple arrangement of flower parts in which the receptacle is ex- hypocotyl The stem below the cotyle- panded at the top of the pedicel in such a dons, occupying the region between the way that the androecium and the perianth cotyledon stalks and the point where lat- arise from beneath the gynoecium giving a eral roots arise. Rapid elongation of the superior ovary. Compare epigyny; perig- hypocotyl after germination pushes the yny. cotyledons above ground in plants show- ing epigeal germination. Compare epicotyl. hypolimnion The lower noncirculating water in a thermally stratified lake. hypodermis One or more layers of cells that may be found immediately below the hyponasty (hyponastic movements) The epidermis of plants. It may be composed of curving of a plant organ upward and to- thin-walled colorless cells and functions as ward the axis, caused by greater growth on

116 I

IAA (indole acetic acid)A naturally oc- immunoelectrophoresis See electro- curring auxin. See auxin. phoresis.

ICBN See International Code of Botani- inbreeding Breeding between closely re- cal Nomenclature. lated individuals. The most extreme form of inbreeding is self-fertilization, which oc- Ice Age A period in the latter part of the curs in some plants and algae (e.g. Spir- Pleistocene characterized by successive ogyra). Inbreeding increases homozygosity coolings and warmings of the earth. In at so that deleterious recessive genes are ex- least four major glaciations (cold periods), pressed more often in the phenotype, and ice caps spread south from the Arctic and decreases heterozygosity and hence the po- north from the Antarctic. tential genetic variability of the popula- tion. There is also a general lowering of idioblast Any specialized plant cell that vigor in inbred stock (inbreeding depres- is dispersed among cells of a very different sion), which is especially pronounced kind. Idioblasts may contain a variety of amongst normally outbreeding popula- materials, e.g. tannins, oils, crystals, and tions. Compare outbreeding. See cleis- waste products. Isolated sclereids can be togamy. called idioblasts. incipient plasmolysis The condition of idiogram See karyogram. a cell that results when it is surrounded by a solution having the same osmotic pres- imbibition The phenomenon in which a sure as the cell contents. See plasmolysis. substance absorbs a liquid and swells, but does not necessarily dissolve in the liquid. incompatibility 1. The rejection of The process is reversible, the substance grafts, transfusions, or transplants between contracting on drying. Water is imbibed by plants or animals of different genetic com- many biological substances: cellulose, position. hemicelluloses, pectic substances, lignin 2. A mechanism in flowering plants that (all plant cell wall constituents); starch; prevents fertilization and development of certain proteins, especially in seeds; etc. an embryo following pollination by the Dry seeds absorb water by imbibition, ini- same or a genetically identical individual. tially via the testa, and as seed volume It results in self-sterility, thus preventing increases great imbibitional pressures de- inbreeding. It is due to interaction between velop. genes in the pollen grain and those in the stigma, in such a way that the pollen is ei- immunity The ability of plants and ani- ther unable to grow or grows more slowly mals to withstand harmful infective agents on the stigma. and toxins. In most plants this is achieved 3. A genetically determined mechanism in by physical barriers preventing entry of some fungi that prevents sexual fusion be- pathogens and by physiological reactions tween individuals of the same race or to isolate the pathogen and its effects. (e.g. Mucor). See also heterothallism.

117 incomplete dominance incomplete dominance (co-dominance) a range of color changes over a wide range The situation in which two different alleles from acid to alkaline, and can be used to are equally dominant. If they occur to- estimate the pH. gether the resulting phenotype is inter- mediate between the two respective indicator species An organism that can homozygotes. For example, if white antir- be used to measure the environmental con- rhinums (AA) are crossed with red antir- ditions that exist in a locality. For example, rhinums (A¢A¢) the progeny (AA¢) will lichen species are indicators of levels of be pink. Sometimes one allele may be pollution, as different species are sensitive slightly more dominant than the other to different levels and types of pollutants. (partial or incomplete dominance) in which case the offspring, though still inter- indigenous Describing an organism that mediate, will resemble one parent more is native to an area, rather than introduced. than the other. indole acetic acid (IAA) A naturally oc- indefinite growth (indeterminate curring auxin. See auxin. growth) A form of GROWTH that has no finite limit: the plant or organ continues to inducer A substance that activates a grow indefinitely. All plants are somewhat structural gene or block of genes by com- indeterminate, as they have meristems that bining with a REPRESSOR. See operon. can differentiate into almost any part given the right conditions. inducible enzyme (adaptive enzyme) An enzyme that is produced by a cell only in indehiscent Describing a fruit or fruit- the presence of high concentrations of its ing body that does not open at maturity to substrate or a structurally similar sub- release the seeds or spores. The fruit wall stance. either decays releasing the seeds in situ, or the fruit is eaten by an animal and the seeds indusium The flap of tissue, derived pass out intact in its droppings. Compare from the placenta, that encloses the devel- dehiscent. oping sporangia in the sorus of a fern. independent assortment The law, for- inferior Below. In botany, the term is mulated by MENDEL (Mendel’s Second generally applied to the position of the Law), that genes segregate independently OVARY of a flower in which the sepals, at meiosis so that any one combination of petals, and stamens arise above it. The alleles is as likely to appear in the offspring ovary appears to have sunk into and fused as any other combination. The work of with the cup-shaped receptacle. This con- T. H. Morgan later showed that genes are dition occurs only in epigynous flowers. In linked together on chromosomes and so a floral formula, an inferior ovary is de- tend to be inherited in groups. The law of noted by a line above the gynoecium sym- independent assortment therefore applies bol and number. Compare superior. See only to genes on different chromosomes. also epigyny. The term can also be applied to whole chromosomes. See linkage; Mendel’s laws. inflorescence A collection of flowers sharing a common stalk, the and indeterminate growth See indefinite usually subtended by a bract. Each flower growth. usually arises in the axil of a small bract, or bracteole, and may or may not be borne on indicator A substance used to test for its own individual stalk, called a pedicel. acidity or alkalinity of a solution by a color The development of an inflorescence may change. Examples are litmus and phe- represent the end of vegetative growth of nolphthalein. A universal indicator shows that apex (definite growth), or allow it to

118 inflorescence continue (indefinite growth). There are e.g. foxglove (Digitalis). In a compound many types of inflorescence, determined each branch of the inflorescence mainly by the method of branching. If the bears a smaller raceme of flowers, e.g. fes- stem ends in a flower and subsequent cues (Festuca). If the lateral branches of a growth is then from lateral buds below the raceme are themselves branched, as in apex, which themselves form flowers and many grasses, the inflorescence is called a more lateral shoots, the inflorescence is panicle. This term is often applied to any termed a cymose inflorescence or cyme, sort of branched racemose inflorescence, e.g. lime (Tilia). If one shoot develops be- for example the horse chestnut, in which hind each axis a monochasial cyme, or each branch is actually a cyme (Aesculus). monochasium, is formed, as in avens A spike is a type of racemose inflorescence (Geum). Variations in the inflorescence having sessile flowers (they have no arrangement occur, according to the direc- pedicels) borne on an elongated axis, as in tion of the lateral branches, for example in wheat (Triticum). The catkin and forget-me-not (Myosotis), all the branches are modifications of the spike. A catkin is a arise on the same side of the parent stem short, densely packed raceme bearing uni- while in buttercup (Ranunculus), the sexual flowers with highly reduced or ab- branches arise on alternate sides of the par- sent perianth, e.g. oak (Quercus). A spadix ent stem. If two shoots develop below each is a type of inflorescence found in the fam- axis this gives a dichasial cyme, or dicha- ily Araceae, e.g. cuckoopint (Arum macu- sium, as in the campions and catchflies (Si- latum). It is a modified spike with a large lene). Cymose inflorescences typically open fleshy axis on which are borne small her- from the apex downwards; in flat-topped maphrodite or, more usually, unisexual forms the oldest flowers are in the center. flowers. The inflorescence is enclosed by a In a raceme (racemose inflorescence) the large bract, the spathe, which may be fo- apex continues growing, and subsequent liose or petalloid and has been shown to at- flowers develop in sequence up the stem, tract insects in certain species. In a

Inflorescence: types of inflorescence

119 initial capitulum (pl. capitula), as in many Aster- e.g. intrafascicular CAMBIUM between xylem aceae, e.g. daisy (Bellis), the inflorescence and phloem. Apical initials are rarely single comprises many unstalked florets inserted cells except in certain lower plants, and ini- on the flattened disklike end of the pedun- tiate more than one structure, e.g. lateral cle and surrounded by a ring of sterile shoots, flowers, and leaves, as well as new bracts, the involucre. The internodes are stem tissues. Vascular cambium possesses not recognizable. Each floret may or may two distinct types of initials, ray initials not be borne in the axil of a bract on the forming medullary rays, and fusiform ini- disk. Many species of composite have two tials producing xylem and phloem el- distinct types of floret in the capitulum: ements. disk florets in the center are tubular florets ending in five short teeth, while ray florets initiation factor See translation. have a straplike extension to the tube and occur around the edge of the capitulum, inositol An optically active cyclic sugar rather like petals. Some species, such as alcohol, synthesized from glucose 6-phos- thistles, have only ray florets, others have phate. A derivative of iniositol, phytic acid, only disk florets, e.g. chicory (Cichorium), is an important storage compound in and many have both disk and ray florets, seeds, which is believed to be converted e.g. sunflower (Helianthus). In racemose during germination to inositol, and thence inflorescences the flowers typically open to glucuronic and uronic acids for the syn- from below upwards. An is a type of thesis of cell-wall components. inflorescence in which the stem axis is not elongated and individually stalked flowers insectivorous plant See carnivorous appear to arise from same point on the plant. stem. These flowers are massed on one plane, giving the appearance of an um- integument A layer surrounding the nu- brella, with the oldest flowers on the out- cellus in the ovules of gymnosperms and side and the youngest in the middle. The angiosperms. Most angiosperm ovules pos- umbel is typical of the carrot family (Api- sess two integuments, while gymnosperms aceae). Umbels may be grouped into a usually have only one. Enclosure of the nu- compound umbel, composed of umbels of cellus by the integuments is incomplete, the smaller umbels, as in onion (Allium). A micropyle remaining to allow access to the corymb is an inflorescence with flower embryo sac or, in gymnosperms, to the stalks of different lengths, the lowest being archegonium. the longest. This gives a flat-topped cluster of flowers at the same level that is charac- intercalary meristem A region of ac- teristic of many brassicas, e.g. candytuft tively growing primary tissue clearly sepa- (Iberis). Umbels, , and capitula rate from the apical meristem. Intercalary can in turn be grouped into cymose inflo- meristems occur at the internode and leaf- rescences, e.g. onion, Viburnum, and sheath bases (joints) of many monocotyle- scabious (Scabiosa) respectively. dons including grasses, and at the internodes of horsetails, where they serve initial A cell permanently in a MERISTEM, to increase longitudinal growth. actively dividing and adding new cells to the plant body. An initial never becomes intercellular Describing materials found differentiated. One of its daughters re- and processes occurring between cells. mains in the meristem as an initial, while Compare intracellular. the other differentiates to form a non- meristematic cell. Two basic groups exist: interfascicular cambium A single layer apical initials at root and shoot apices, and of actively dividing cells between the vas- lateral meristem initials whose position cular bundles in stems. It is formed when depends on the location of the meristem, parenchyma cells resume meristematic

120 inulin activity. The interfascicular cambium and intrafascicular cambium (fascicular INTRAFASCICULAR CAMBIUM (within the vas- cambium) The part of the VASCULAR cular bundles) link into a complete cam- CAMBIUM within the vascular between the bium cylinder that cuts off secondary xylem and phloem. xylem tissue to the inside, secondary phloem tissue to the outside, and paren- intraspecific selection Natural selec- chyma cells both sides (forming medullary tion acting on individuals of the same rays). See vascular cambium. species.

International Code of Botanical No- introgression (introgressive hybridiza- menclature (ICBN) A set of rules for tion) The introduction of genetic mater- the scientific naming of wild plants, algae, ial from one gene pool to another by fungi, and slime molds, which allocates a hybridization and subsequent back-cross- single unambiguous name to each taxon. ing to one or other of the parents. It forms Naming of families and lower taxa is based the basis of the evolution of most crop on the first valid name published, the start- plants, e.g. sunflower (Helianthus annuus). ing-point being taken as the publication of Where the ranges of two species overlap, and fertile hybrids can be produced, these , by LINNAEUS, in 1753. But if a plant is later assigned to different hybrids will tend to backcross with the taxa or rank, a different name may be most abundant species, so the offspring down many generations come to resemble given. Thus a plant may legitimately have the most abundant parents. two names, according to which genus dif- ferent authors place it in. If several taxa are introgressive hybridization See intro- combined into one, that TAXON takes on gression. the name of the former taxon with the old- est name. Different starting dates apply to intron A noncoding DNA sequence that fossil plants and bacteria. There is a sepa- occurs between coding sequences (exons) rate code for cultivated plants, the Interna- in many eukaryote genes. Messenger RNA tional Code of Nomenclature of Cultivated (mRNA) does not contain introns, these Plants (ICNCP). See classification. being removed during the transcription process. Intron removal is now thought to International Union for the Conserva- be an autocatalytic process in which the tion of Nature and Natural Resources RNA acts as its own enzyme (see ri- (IUCN) See World Conservation Union. bozyme). However, in the case of mRNA in the nucleus the process is regulated by a internode The region of the stem be- complex of proteins called a spliceosome. tween two NODES. See intercalary meris- Compare exon. tem. introrse Denoting anthers in which de- interphase The stage in the CELL CYCLE hiscence lines are toward the center of the when the nucleus is not in a state of divi- flower. Pollen is consequently shed toward sion. Interphase is divisible into various the carpels, which favors self-pollination. stages each characterized by a differing Compare extrorse. physiological activity. intussusception The incorporation of intine (endosporium) See pollen. cellulose molecules into the existing cell wall, giving an increase in wall area. The intracellular Describing the material process is usually preceded by water up- enclosed and processes occurring interior take, increasing the pressure on the cell to the cell membrane. Compare intercellu- wall, which stretches. The new wall is then lar. thickened by APPOSITION.

121 inversion inulin A polysaccharide food reserve of Lack of iron leads to CHLOROSIS, especially some higher plants, particularly the Aster- in young leaves. It is also found in cy- aceae, e.g. Dahlia root tubers. It is a poly- tochromes, which are important compo- mer of fructose. nents of the electron-transport chains of respiration and photosynthesis. inversion See chromosome mutation. isoelectric focusing A technique used invertase See sucrase. in ELECTROPHORESIS to separate amphoteric molecules (able to combine with either invert sugar See sucrose. acids or bases). in vitro Literally ‘in glass’; describing isoelectric point The value of pH in a experiments or techniques performed in medium at which a molecule has no elec- laboratory apparatus rather than in the liv- tric charge. Repulsion between molecules ing organism. Cell tissue culture is an ex- ceases, so they precipitate. This principle is ample. Compare in vivo. used to separate mixtures of amino acids or proteins. See isoelectric focusing. in vivo Literally ‘in life’; describing processes that occur within the living or- isoenzyme (isozyme) An enzyme that ganism. Compare in vitro. occurs in different structural forms within a single species. The isomeric forms all involucre 1. A protective structure con- have the same molecular weight but differ- sisting of a ring of bracts arising below the ing structural configurations and proper- INFLORESCENCE in angiosperms with con- ties, usually due to different combinations densed inflorescences (e.g. the capitulum of their component subunits. They are and umbel). formed by different alleles, and are a valu- 2. A sheathlike outgrowth of tissue in able tool for detecting genetic variation. bryophytes protecting the archegonia or antheridia in certain liverworts. Isoetales (quillworts) An order of the 3. A tubular extension of the thallus of LYCOPHYTA containing two genera – Iso- hornworts that rises up and surrounds the etes, found in most parts of the world, and base of the sporophyte. Stylites, found only in the Peruvian Andes. Quillworts are heterosporous, mainly iodine See staining. aquatic perennials, with a dense rosette of tubular quill-like leaves arising from a ion-exchange chromatography A type short, stout rootstock. of CHROMATOGRAPHY in which the com- pounds are separated according to their isogamy The sexual fusion of gametes acidity. of similar size and form. It occurs in fungi and some protoctists. Compare anisog- ionic bond A bond formed when an amy. electron passes from one atom to another. The atom that loses the electron becomes a isolating mechanism Structural, physi- positively charged ion, and the receiving ological, behavioral, genetic, geographical, atom becomes negatively charged, so the or other factors that restrict the interbreed- two ions become bonded by a strong elec- ing of one population with another, hence trostatic force. Compare covalent bond; restricting gene flow. The development of hydrogen bond. isolating mechanisms promotes the forma- tion of new varieties and species. iron A MICRONUTRIENT essential for plant growth. It is found in porphyrins, isoleucine A nonpolar AMINO ACID, syn- and is essential for chlorophyll synthesis. thesized from threonine.

122 Ivanovsky, Dmitri Iosifovich isomerase An enzyme that catalyzes the ther gains nor loses water by osmosis. conversion of a molecule from one iso- Compare hypertonic; hypotonic. meric form to another. For example, in gly- colysis, glucose phosphate isomerase isotopic dating See radiometric dating. catalyzes the conversion of glucose 6-phos- phate to fructose 6-phosphate. isozyme See isoenzyme. See World Conservation Union. isomorphism A condition seen in cer- IUCN tain algae (e.g. Ulva, the sea lettuce) in Ivanovsky, Dmitri Iosifovich (1864– which the alternating generations of the 1920) Russian botanist. In 1862, while life cycle are morphologically identical working on tobacco mosaic disease at (HOMOLOGOUS). Compare heteromorphism. the Technical Institute in St Petersburg, Ivanovsky noticed that healthy plants isotonic Designating a solution with an could be infected with the sap of diseased osmotic pressure or concentration equal to tobacco plants. He also noticed that the that of a specified other solution, usually sap filtrate contained tiny crystalline par- taken to be within a cell. It therefore nei- ticles that he believed were causative

123 J

Jacob–Monod model A model of regu- cotyledonous plants, mostly herbs, with lation of gene activity in bacteria proposed long, narrow grasslike or cylindrical leaves by F. Jacob and I. Monod in 1960. Using spirally arranged, but usually arising close the concept of the OPERON, it proposed that to the ground. They are found in temperate metabolism is regulated and coordinated and cold regions, and on tropical moun- by controlling messenger RNA synthesis tains, forming an important part of the (i.e. TRANSCRIPTION) and hence the produc- vegetation in poorly drained areas. tion of enzymes. A similar but more com- plex system is known to operate in junk DNA The noncoding part of eukaryotes. See inducible enzyme. DNA, i.e. the part that does not code for proteins. Its function is not yet understood, Johannsen, Wilhelm Ludwig (1857– but it appears that part of it contains in- 1927) Danish botanist and geneticist. Jo- tructions that help control where and when hannsen became interested in the Theory the different proteins are expressed. Com- of Heredity, a work by the British anthro- pare repetitive DNA; selfish DNA. pologist Sir , which stated that natural selection does not work on the Jurassic The middle period of the Meso- offspring of self-fertilizing plants. Jo- zoic era, 215–145 million years ago. The hannsen experimented with the Princess climate was warm and humid, and there bean and found that if the self-fertilizing was widespread expansion of forests, parent plants were from a mixed popula- which gave rise to thick deposits of coal. tion, then selection did indeed work. He The Jurassic has been called the ‘Age of Cy- described the offspring of a single parent as a ‘pure line’ and stated that they were ge- cads’. The seed-ferns declined, and there netically identical. In 1905 he coined the was a great increase in the number and di- terms ‘phenotype’ to describe the outward versity of cycads, palmlike gymnosperms, appearance of an individual and ‘genotype’ and conifers. Fossils of monocotyledonous to describe the genetic makeup of an indi- pollen and stems of palms suggest that the vidual. Following the rediscovery of angiosperms evolved during this period. MENDEL’S LAWS, he published his Elements See also geological time scale. of Heredity in 1905 and used the word ‘gene’ for Mendel’s factors of inheritance. Jussieu, Antoine-Laurent de (1748– His application of statistical methods to 1836) French plant taxonomist. Work- characteristics of populations proved to be ing at the Jardin du Roi in Paris, Jussieu a cornerstone of modern genetics. based his classification system on many plant characteristics rather than the small J-shaped growth curve See exponen- number used by LINNAEUS. Using the col- tial growth. lections of Banks, Linnaeus, and Philibert Commesson, he established 15 classes and Juncaceae (rushes)A family of mono- 100 families of plants in his classification.

124 K

karyogamy The fusion of two nuclei of NAD. In the Krebs cycle it undergoes de- that exist within a common cytoplasm, as carboxylation to form succinyl CoA. a- occurs in the formation of the zygote from ketoglutarate is also involved in amino two gametes. In the more advanced fungi, acid metabolism, especially TRANSAMINA- especially the Basidiomycota, nuclei may TION. fuse some considerable time after fusion of hyphae of different mating strains, forming ketohexose A ketose SUGAR with six a dikaryon. carbon atoms, e.g. fructose. karyogram (idiogram) The formalized ketopentose A ketose SUGAR with five layout of the KARYOTYPE of a species, often carbon atoms, e.g. ribulose. with the chromosomes arranged in a cer- tain numerical sequence. This usually takes ketose A monosaccharide SUGAR con- the form of drawings or photographs of the individual chromosomes, often arranged in homologous pairs in numerical sequence. karyokinesis During cell division, the process of nuclear division that precedes cytoplasmic division. There are two main types of nuclear division: MITOSIS, which results in daughter cells identical to their parents; and MEIOSIS (reduction division), which produces daughter cells which have half the number of chromosomes of their parent cells, and in which the genetic ma- terial has been recombined. Compare cy- tokinesis. karyotype The physical appearance of the chromosome complement of a given species. A species can be characterized by its karyotype since the number, size, and shape of chromosomes vary greatly be- tween species but are fairly constant within species. See karyogram. kelp See Laminariales. a-ketoglutaric acid A five-carbon di- carboxylic acid that is an intermediate in the KREBS CYCLE. It is synthesized by oxida- tion of isocitrate, coupled to the reduction Ketose: typical ketose sugars

125 kinase taining a ketone (=CO) or potential ketone rank of domain as being higher than that of group, e.g. dihydroxyacetone. kingdom. See Five Kingdoms classification. kinase (phosphotransferase; phosphory- klinostat (clinostat) An apparatus used lase) 1. Any enzyme that transfers a in tropism experiments to remove the uni- phosphate group, usually from ATP. directional influence of a stimulus on a 2. An enzyme that activates the inactive plant organ. form of other enzymes. For instance, when trypsinogen, the inactive form of trypsin, Kramer, Paul Jackson (1904– ) American plant physiologist. Working at comes in contact with enterokinase, active Duke University, Kramer made a signifi- trypsin is released. cant contribution to the study of water- uptake mechanisms in plants. Using radio- See cytokinin. kinetin actively labeled elements he demonstrated that the region of maximum absorption of kinetosome (basal body) A barrel- water is a few centimeters behind the root shaped body found at the base of all eu- tip and that quantities of minerals are ab- karyote UNDULIPODIA (cilic and flagella) sorbed passively in the transpiration stream. and identical in structure to the CENTRIOLE. It is essential for formation of undulipodia. Kranz anatomy The specific arrange- ment of the photosynthetic tissues in the kingdom Formerly, the highest ranking leaves of C4 PLANTS. The cells of the bundle category in most classification systems. sheath that surround the vascular tissues Most taxonomists today recognize the are large and contain specialized elongated

Krebs cycle

126 Krebs cycle chloroplasts that do not contain grana, but amounts of energy. It is the second stage of form starch grains. The chloroplasts of the AEROBIC RESPIRATION, requires oxygen, and mesophyll, by contrast, appear normal and occurs in the matrix of mitochondria. 2- contain grana, but do not form starch carbon acetate (acetyl coenzyme A), de- grains. In these chloroplasts carbon diox- rived from pyruvate by decarboxylation, ide combines with phosphoenolpyruvate to reacts with 4-carbon oxaloacetate to form form oxaloacetic acid, which is trans- 6-carbon citrate, which is then decarboxy- ported to the bundle sheath cells, where the lated in a series of steps to reconstitute ox- carbon dioxide is released, then fixed by aloacetate. Some ATP is produced by the enzyme ribulose bisphosphate carboxy- direct coupling with Krebs cycle reactions, lase to form glycerate 3-phosphate, the but most production is coupled to the ELEC- first step in the Calvin cycle. See photosyn- TRON-TRANSPORT CHAIN. The operation of thesis. this depends on the generation of reduced coenzymes, NADH and FADH2, by the Krebs, Sir Hans Adolf (1900–81) Ger- Krebs cycle. These reduced cofactors pro- man-born British biochemist best known vide the energy for ATP formation in the for his elucidation of the KREBS CYCLE. He respiratory electron-transport chain and shared the 1953 Nobel Prize for Physiol- OXIDATIVE PHOSPHORYLATION. Considering ogy or Medicine with Fritz Lipmann the Krebs cycle and electron-transport (1899–1986). chain together, each pyruvate molecule yields 15 ATP molecules. Since two pyru- Krebs cycle (citric acid cycle; TCA cycle; vate molecules enter the cycle from glycol- tricarboxylic acid cycle) A complex cycle ysis, 30 ATP are produced in all. The of reactions in which pyruvate, produced intermediates in the Krebs cycle may be by glycolysis, is oxidized to carbon dioxide used directly or indirectly in the synthesis and water, with the production of large of amino acids.

127 L

labeling The technique of using isotopes consists of two membranes lying close to- (usually radioactive isotopes) or other rec- gether with a cavity between them. An ex- ognizable chemical groups to investigate ample is the internal system of lamellae biochemical reactions. For instance, a com- (thylakoids) in CHLOROPLASTS. pound can be synthesized with one of the 2. A general term applied to any thin plate- atoms replaced by a radioactive isotope of like structure, e.g. the blade of a leaf. the element. The radioactivity can then be used to follow the course of reactions in- Lamiaceae (Labiatae) A dicotyledonous volving this compound. family of herbs and low-growing shrubs, most of which have stems of square cross- Labiatae See Lamiaceae. section and simple leaves with stipules arranged in opposite pairs. There are about lactic acid A 3-carbon hydroxy-acid 6700 species, distributed worldwide but formed as a result of fermentation by cer- especially numerous in the Mediterranean tain bacteria, e.g. Lactobacillus. region. The family includes many herbs, such as basil (Hyssopus officinalis), marjo- lactic acid bacteria A group of bacteria ram (Origanum), mint (Mentha), sage that ferment carbohydrates in the absence (Salvia officinalis), savory (Satureja), and of oxygen, with lactic acid always a major thyme (Thymus vulgaris). end product, e.g. Lactobacillus spp., Strep- tococcus lactis. They have a high tolerance lamina (pl. laminae) 1. (of angiosperms) of acid conditions. Lactic acid bacteria are A thin usually flat foliage leaf blade, com- involved in the formation of yoghurt, monly attached to the stem by a . In cheese, sauerkraut, and silage. They can most plants the leaf laminae are the main occur as spoilage organisms and some are photosynthetic organs. A lamina is termed pathogenic causing infections. simple if complete, and compound if di- vided into leaflets. Internal organization lacuna (pl. lacunae) 1. An intercellular shows a wide photosynthetic mesophyll cavity in plant tissues resulting from cell layer permeated by veins and bounded by breakdown, tissue splitting, or organized an epidermis. Leaf variation is mainly due formation. In hydrophytes a well-arranged to the structural diversity of the lamina. system of these air-filled cavities may exist 2. (of algae) The bladelike part of the thal- to provide buoyancy. A large lacuna occu- lus of certain algae, notably the Phaeo- pies the center of many stems. phyta (brown algae), e.g. kelps (Laminaria, 2. See leaf gap. Macrocystis, Nereocystis) and bladder- 3. A depression in a lichen thallus. wrack (Fucus vesiculosus). lag phase See exponential growth. Laminariales (kelps) An order of the PHAEOPHYTA (brown algae) that contains lamella (pl. lamellae) 1. A layer within the kelps (e.g. Macrocystis and Nereocys- the cytoplasm or inside an organelle, which tis) and is distinguished by its hetero- is formed from a flattened membrane- morphic ALTERNATION OF GENERATIONS in bounded vesicle or tube. The layer thus which a large sporophyte alternates with a

128 leaf curl microscopic gametophyte, and in which Law of Segregation See Mendel’s laws. growth takes place in a special meristem re- gion (see meristoderm) on the surface of layering A method of vegetative propa- the thallus. gation in which RUNNERS or STOLONS are pegged down to the soil surface. Where a laminarin The chief carbohydrate food node touches the soil, adventitious roots reserve of the Phaeophyta (brown algae). develop and a shoot arises from the LAT- ERAL MERISTEM. If the intervening runner or lateral meristem A MERISTEM that lies rots or is cut away, the daughter parallel to the sides of the organ in which it plants becomes independent. occurs and gives rise to secondary tissues that increase the girth of the organ. See leaching The removal of humus and soil cambium; secondary growth. Compare nutrients in solution by water moving apical meristem. down the soil profile. It makes soils more acid since cations (e.g. potassium and mag- lateral root Any ROOT that arises from nesium) are replaced by hydrogen ions. the PERICYCLE of another root. Leaching leads to the formation of pod- solized and lateritic soils. laterite A hard crust that may form on A flattened appendage of the stem the surface of the SOIL in tropical regions leaf with alternating wet and dry seasons. that arises as a superficial outgrowth from the apical meristem. Leaves are arranged in latex A liquid found in some flowering a definite pattern, have buds in their axils, and show limited growth. Most foliage plants contained in special cells or vessels leaves are photosynthetic, bilaterally sym- called laticifers (or laticiferous vessels). It is metrical, and externally differentiated into a complex variable substance that may LAMINA, MIDRIB, and PETIOLE. Many varia- contain terpenes (e.g. rubber), resins, tan- tions occur. Not all foliage leaves are pho- nins, waxes, alkaloids, sugar, starch, tosynthetic as they may be modified wholly enzymes, crystals, etc. in solution or sus- to bud scales and spines or partly to form pension. It is often milky in appearance, tendrils. See illustration overleaf. as in dandelion (Taraxacum spp.) and let- tuce (Lactuca sativa) but may be colorless, leaf buttress A leaf primordium, ap- orange, or brown. Commercial rubber pearing as a protuberance on the side of the comes from the latex of the rubber plants stem apex, produced by periclinal division Ficus elastica and Hevea brasiliensis. of the tunica and corpus, and associated Opium comes from alkaloids found in the with a procambium. It is the earliest stage latex of the opium poppy (Papaver som- in the development of a leaf, and forms the niferum). leaf base as the leaf grows out from it. The leaf axis arises from the leaf buttress and laticifers See latex. maintains procambial continuity in the central region. This differentiates into the Lauraceae A family of trees and shrubs vascular tissue of the developing leaf. (bays and laurels) usually with simple, leathery, evergreen alternate or opposite leaf curl Any disease of plants that leaves without stipules. There are about causes an increase in numbers of cells that 2500 species, mainly in the tropics and results in curling and puckering of the subtropics, especially in Amazonia and leaves. It may be caused by a fungus (e.g. Southeast Asia. Taphrinia deformans causes peach leaf curl), or by viruses (e.g. tobacco and cotton Law of Independent Assortment See leaf curl) transmitted by insects such as Mendel’s laws. whiteflies (Bemisia).

129 Leaf shapes

Leaf margins

130 lichens leaf fall The shedding of leaves, usually lemma See bract. associated with a fall in AUXIN concentra- tion. In both evergreen and deciduous trees lenticel See bark. old leaves are continually shed throughout the growing season, but the remaining leptosporangiate Describing the condi- leaves are also shed at the onset of winter tion, found in certain ferns (e.g. Filicales), or a dry season in deciduous species. See in which the sporangium develops from a abscission. single initial cell. The wall of a leptospo- rangium is usually only one cell thick, and leaf gap (lacuna) A region of paren- as it dries out, it flicks the spores away ex- chyma differentiated in the stem vascular plosively. Compare eusporangiate. cylinder immediately above a diverging LEAF TRACE. Lateral connections insure no leptotene See meiosis; prophase. break in the vascular system. leucine An amino acid synthesized from leaf mosaic 1. The nonoverlapping pat- pyruvate. It can be broken down to yield tern of leaves on a plant that results mainly acetoacetate. See amino acids. from the genetically programmed arrange- ment of leaves and branches, and in some leukoplast A colorless PLASTID, i.e. one species also by the twisting of the petioles not containing chlorophyll or any other due to EPINASTY. It ensures that the maxi- pigment. Leukoplasts are common in the mum amount of light is intercepted by the cells of roots and underground stems and leaves. See phyllotaxis. storage organs. 2. A virus disease of plants characterized by yellow and green or light- and dark- (liane) A long-stemmed woody green mottling of the leaves. climbing plant that grows from ground level to the canopy of trees, and often leaf trace A vascular bundle or group of hangs down freely from branches, e.g. cu- vascular bundles connecting the vascular rare (Strychnos toxifera). may have systems of leaf and stem. It stretches from stems up to 100 meters (330 feet) long, and the leaf base to the vascular ring in the stem may climb over the canopy to gain maxi- axis. mum sunlight. Many flower only when they reach the top of the canopy. Up to a lecithin (phosphatidyl choline) One of a quarter of all the woody growth in some group of phospholipids that contain glyc- rainforests may be lianas. erol, fatty acid, phosphoric acid, and choline that are found widely in higher lichens Symbiotic associations between plants and animals, particularly as a com- an alga or cyanobacterium (the photo- ponent of cell membranes. biont) and a fungus (the mycobiont). They are slow-growing but can colonize areas 1. (pod)A dry dehiscent single- too inhospitable for other plants. Usually or many-seeded fruit formed from a single the fungus is an ascomycete but occasion- carpel, that dehisces mechanically by split- ally it is a basidiomycete. Reproduction in ting, often explosively, along both sides. lichens may be asexual by soredia (algal The opposite tensions set up by oblique cells enclosed by fungal hyphae) or by sex- fibers in the drying pericarp cause dehis- ual fungal spores, which can survive only if cence. The valves may continue twisting some algal cells are also present. Fragments and remove any remaining seeds. It is the of the thallus containing both phycobiont typical fruit of the FABACEAE family, and is and mycobiont cells may also grow into found in certain other families. Compare new lichens, a form of asexual reproduc- lomentum. tion. Examples include Peltigesa and Xan- 2. Any plant of the family Fabaceae. thoria.

131 life cycle

Life cycle

132 limiting factor life cycle The sequence of developmen- in sclerenchyma and some xylem vessels, tal changes making up the span of an or- but values of 25–30% lignin and 50% cel- ganism’s life from the fertilization of lulose are average. It is a complex variable gametes to the same stage in the subse- polymer, derived from sugars via aromatic quent generation. See diplobiontic; haplo- alcohols. biontic. ligule 1. A scalelike outgrowth, varying ligase An enzyme that catalyzes the in shape and size, of certain angiosperm bond formation between two substrates at leaves. In grasses the ligule occurs at the the expense of the breakdown of ATP or junction of the leaf sheath and lamina. Al- some other nucleotide triphosphate. The though membranous in most species, it degree of bond formation by ligases is pro- may be only a fringe of hairs. Ligules are portional to the amount of ATP available important taxonomic features in grasses. in the cell at a particular instant. 2. A toothed, strap-shaped structure formed by the extension of one side of the light green A stain used in light mi- corolla tube in the florets of certain species croscopy to show up cytoplasm and cellu- of the ASTERACEAE. The teeth indicate the lose. It is commonly used as a counterstain number of fused petals. In some species, with safranin to highlight cytoplasm. See e.g. dandelion (Taraxacum spp.), all florets staining. of a capitulum are ligulate but in others e.g. daisy (Bellis perennis), only the ray florets light microscope See microscope. have ligules. 3. A very small tongue-shaped flap of tissue light reactions The light-dependent re- inserted on the upper surface of the leaves actions of PHOTOSYNTHESIS that convert (microsporophylls) and of cer- light energy into the chemical energy of tain clubmosses (e.g. Selaginella) and quill- NADPH and ATP, which are used in the worts (). dark reactions for carbon dioxide fixation. Light absorbed by two groups of pigments Liliaceae (lilies)A large family of mono- in the chloroplast – Photosystems I and II. cotyledonous plants. Their taxonomy is The absorbed light energy excites chloro- much in dispute, with groups that were for- phyll a molecules, causing them to emit merly sections now regarded with new high-energy electrons that pass down an evidence as being separate families. They ELECTRON-TRANSPORT CHAIN, releasing en- have long, narrow parallel-veined leaves, ergy to synthesize ATP (photophosphory- whorled or alternate, arising at the base of lation) and NADPH. The light reactions the stem or along it. Most are herbs, e.g. take place on the thylakoid membranes of lilies (Lilium) and tulips (Tulipa); some are chloroplasts. succulents or dwarf shrubs, e.g. butcher’s broom (Ruscus aculeatus), and a few are light saturation point See compensa- climbers (e.g. Smilax). Many species have tion point. perennating organs such as bulbs, corms, or rhizomes. lignin One of the main structural mate- rials of vascular plants. Together with cel- liming The addition of lime to a soil to lulose it is one of the main constituents of reduce its acidity, improve crumb struc- wood, where it imparts high tensile and ture, or increase its calcium content. See compressive strengths, making it ideal for flocculation. support and protection. Lignified tissues include SCLERENCHYMA and XYLEM. Lignin limiting factor Any factor in the envi- is deposited during secondary thickening ronment that governs the behavior or of cell walls. The degree of lignification metabolic activity of an organism or sys- varies from slight in protoxylem to heavy tem by being above or below a certain

133 limiting layer level. In general, a number of different fac- versity he began to investigate the sexuality tors (e.g. light intensity, temperature, car- of plants and by 1730 he had begun to set bon dioxide concentration) may be out a classification system based on the limiting, but at any one time the factor that number and arrangement of stamens and is closest to its critical minimum is the lim- pistils. After traveling through Lapland iting factor. For instance, the rate of pho- and discovering many new species, he set- tosynthesis rises with increasing light tled in Holland and published his Systema intensity as long as there is sufficient car- Naturae, in which he set out a classifica- bon dioxide available, but at high light in- tion of animals, plants, and minerals. He tensities carbon dioxide may become the placed flowering plants into classes based limiting factor instead. on the number of stamens and subdivided the classes into orders based on the number limiting layer See meristoderm. of their pistils. Linnaeus’s most famous contribution limnology The scientific study of fresh- to taxonomy was his system of BINOMIAL water and its flora and fauna. Limnology NOMENCLATURE, in which he gave plants also includes studying the chemical and two Latin names. The first name is the physical aspects of inland water. It may be generic name and the second is the specific divided into the study of standing water (or species) name. habitats, e.g. lakes and ponds, and running water habitats, e.g. rivers and streams. linoleic acid A common unsaturated 18-carbon fatty acid occurring in glyc- linear Describing leaves that are long, erides in linseed oil, cottonseed oil, and narrow, and parallel-sided for most of other oils. It is an essential nutri- their length. ent in the diet of mammals. linkage The occurrence of genes to- linolenic acid An unsaturated 18-car- gether on the same chromosome so that bon fatty acid occurring commonly in they tend to be inherited together and not plants as the glyceryl ester, for example in independently. Groups of linked genes are linseed oil and poppy-seed oil. Linoleic termed linkage groups and the number of acid is the predominant fatty acid in the linkage groups of a particular organism is chloroplast. It is an essential nutrient in the equal to its haploid chromosome number. diet of mammals. Linkage groups can be broken up by CROSSING OVER at meiosis to give new com- lipase Any of various enzymes that cat- binations of genes. Two genes close to- alyze the hydrolysis of fats to fatty acids gether on a chromosome are more strongly and glycerol. Lipases are important in ger- linked, i.e. there is less chance of a cross minating seeds, e.g. castor oil (Ricinus over between them, than two genes further communis) where they help to break down apart on the chromosome. Linked genes storage fats. are symbolized Ab...Y/aB...y, indicating that Ab...Y are on one homolog while lipid A collective term used to describe a aB...y are on the other homolog. Linkage is group of substances in cells characterized indicated when the associated inheritance by their solubility in organic solvents such of two or more nonallelic genes is greater as ether and benzene, and their absence of than would be expected from independent solubility in water. assortment. The genes on a single chromo- The group is rather heterogeneous in some form one linkage map. See chromo- terms of both function and structure. They some map; independent assortment. encompass the following broad bands of biological roles: (1) basic structural units Linnaeus, Carolus (1707–76) Swedish of cellular membranes and cytologically botanist. While working at Uppsala Uni- distinct subcellular bodies such as chloro-

134 loess plasts and mitochondria; (2) compartmen- species of organism. Compare benthic; talizing units for metabolically active pro- sublittoral. teins localized in membranes; (3) a store of 2. The zone between the water’s edge and chemical energy and carbon skeletons; and a depth of about six meters in a pond or (4) primary transport systems of nonpolar lake, where light reaches the bottom sedi- material through biological fluids. Plants ments. Rooted hydrophytes, both emer- synthesize a wide range of lipids. Long- gent and submergent, are found in this chain fatty acids (often 16–18 carbon zone. Compare profundal; sublittoral. atoms) are esterified to glycerol for use in membranes, and PHOSPHOGLYCERIDES are liverworts See Hepatophyta. particularly important components of mem- branes; of long-chain monohydric al- living fossil Modern organisms with cohols and fatty acids form the WAXES of anatomical or physiological features found the cuticle and the SUBERIN of the endoder- elsewhere only in extinct species. An exam- mis; and triacylglycerols are important ple is the maidenhair tree (Ginkgo biloba), storage lipids in seeds, especially in such discovered in Japan in the 17th century, species as the castor oil seed (Ricinis com- and later in China, but found only in culti- munis), where they are stored in oil bodies, vation. Fossil members of the Ginkgophyta as small lipid droplets coated with lipids are common from rocks of Mesozoic age. and protein. The fatty acid components of plant lipids come mainly from acetyl CoA, loam A medium-textured soil contain- produced in plastids (including chloro- ing a mixture of large and small mineral plasts) and mitochondria. GLYCOLIPIDS are particles. Loams are easy soils to work and synthesized in the endoplasmic reticulum, combine the good properties of sandy and mitochondria, and plastids. clay soils. lipopolysaccharide A conjugated poly- (loculus) An air-filled compart- saccharide in which the noncarbohydrate ment in an ovary in which the ovules de- part is a lipid. Lipopolysaccharides are a velop. The term locule can also be applied major component of the cell walls of to any other cavity in an organ in which Gram-negative bacteria. other structures develop, for example the of anthers in which pollen is lipoprotein Any conjugated protein formed. When all the carpel edges meet at formed by the combination of a protein the middle of a syncarpous ovary there are with a lipid. Most membranes, including as many locules as carpels, and the ovary is the plasma membrane, are composed designated bi-, tri-, quadri-, or multilocular mostly of lipoprotein. The lipid compo- according to number. Adjoining carpel nents are mainly phosphoglycerides and walls form septa that separate locules. A glycolipids. unilocular condition occurs in mono- carpellary and syncarpous ovaries litmus paper Red or blue acid–alkali in- lacking septa. dicator papers. In acids blue litmus paper turns red and in alkalis red litmus paper loculus (pl. loculi) See locule. turns blue. Litmus solution has a pH range from 4.5–8.3. locus (pl. loci) The position of a gene on a chromosome. Alleles of the same gene oc- littoral 1. The zone of the seashore be- cupy the equivalent locus on homologous tween the high and low tide mark. The chromosomes. term is also applied to organisms living in this zone. Since tidal ranges vary continu- loess A fine-textured, fertile, often cal- ally, the zone is often defined in terms of careous uniform soil comprising mainly the upper and lower limits of certain quartz particles about 0.015–0.005 mm in

135 log phase diameter. Widespread in central Europe, Most lycopods belong to the genera Ly- southern Russia, China, the central USA, copodium and Selaginella. There are five and Argentina, it is derived from wind- orders, three of which – the Lycopodiales, blown clay and silt particles originally de- Selaginellales, and Isoetales – contain both posited at the edge of the ice sheets at the living and fossil representatives. The re- end of the last ICE AGE. See chernozem. maining orders – Lepidodendrales and Pleuromeiales – are represented only by log phase (logarithmic phase) See expo- fossils. The extinct trees of the genus Lepi- nential growth. dodendron were once distributed widely and contributed largely to the coal seams lomasome An infolding of the plasma of the Carboniferous. membrane found particularly in fungal hy- phae and spores, and also in some algae Lycopodiales An order of the LYCO- and higher plants. PHYTA containing the homosporous club- mosses or ground pines. They are mainly lomentum (pl. lomenta) A dry dehiscent tropical, some of them epiphytic, but also fruit formed from a single carpel and bear- occur in temperate regions, especially in ing more than one seed, as in sainfoin montane habitats. Lycopodium and re- (Onobrychis viciifolia). It resembles a lated genera comprise the common club- legume or siliqua but is divided by false mosses or (North America) ground pines. septa into single-seeded compartments that rupture at maturity. Lycopodophyta See Lycophyta. long-day plant (LDP)A plant that Lycopsida See Lycophyta. flowers in response to a dark period shorter than a critical maximum, e.g. Lysenko, Trofim Denisovich (1898– spinach (Spinacea oleracea), which flowers 1976) Ukrainian agriculturalist. While in summer, when days are longer and working at the Kiev Agricultural Institute, nights shorter. See photoperiodism; critical Lysenko claimed to have invented VERNAL- day length. IZATION, the cold treatment of grain to pro- lumen The central space that remains, mote next-season flowering in species that surrounded by cell walls, in a cell that has would normally take two years to flower. lost its living contents (e.g. in xylem el- However, this had long been a common ements). practice in agriculture. Lysenko went fur- ther by stating that vernalized plants could lutein The commonest of the xantho- pass this ‘acquired characteristic’ to their phylls, an orange pigment found in green offspring, so that no further treatment was leaves and certain algae, e.g. the Rho- necessary – a return to the discredited ideas dophyta. See photosynthetic pigments. of Lamarck. In the 1930s he achieved great influence in the Soviet Union under Stalin lyase An enzyme that catalyzes the sepa- and used this to enforce his unorthodox ration of two parts of a molecule with the ideas. In particular, he rejected the chro- formation of a double bond in one of them. mosome theory of inheritance, acting to For example, fumarase catalyzes the inter- discredit those scientists who followed this conversion of malic acid and fumaric acid. view and even changing school textbooks The reaction does not involve hydrolysis. and courses to suit his own beliefs. He fi- nally fell from power in 1964. Lycophyta (Lycopodophyta; Lycopsida) A phylum of spore-bearing vascular plants lysine A basic amino acid synthesized containing about 1000 living species and 6 from aspartic and pyruvic acids, and bro- genera, mostly found in tropical regions. ken down through acetyl CoA and the

136 lysosome

Krebs cycle. Lysine is a precursor of certain daughter cells. In a process called induc- alkaloids. See amino acids. tion, certain environmental factors can cause the phage to leave the host DNA and lysis (degeneration) The death and sub- resume the lytic cycle. sequent breakdown of a cell. See also au- tolysis; lysogeny. lysosome An organelle of plant and an- imal cells that contains a range of digestive lysogeny 1. The formation of an inter- enzymes whose destructive potential neces- cellular space in plants by dissolution of sitates their separation from the rest of the cells. Compare schizogeny. cytoplasm. Lysosomes are bounded by a 2. A stable phage–bacteria relationship in single membrane and have homogeneous which lysing of the bacteria does not occur. contents that often appear uniformly gray The phage (known as a temperate phage) with the electron microscope. They are penetrates the host cell and its nucleic acid usually spherical and about 0.5 m m in di- becomes integrated into the bacterial ameter, although lysosomal compartments DNA. In this state the phage is termed a may range from small Golgi vesicles to prophage; most of the viral genes are large plant vacuoles. Lysosomes may be repressed and both bacteria and phage formed directly from endoplasmic reticu- reproduce together, producing infected lum or by budding off of Golgi vesicles

137 M

macromolecule A very large molecule, pollen production is prevented by the mu- usually a polymer, having a very high mol- tation of one or more of the genes involved. ecular weight (10 000 or more atoms). Male sterility is used by plant breeders to Proteins and nucleic acids are examples. ensure cross-pollination takes place. It is also used in genetic engineering to ensure macronutrient A nutrient required in that genetically modified varieties do not more than trace amounts by an organism. release pollen containing modified genes. It may be an organic or inorganic com- pound. See essential element. Compare mi- malic acid (2-hydroxybutanedioic acid) cronutrient. A colorless crystalline 4-carbon carboxylic acid, which occurs in acid fruits such as macrosclereids Elongated rod-shaped grapes and gooseberries. In biological SCLEREIDS that form a close outer protective processes malate ion is an important inter- layer in the seed testas and fruit walls of mediate in the KREBS CYCLE. some plants. Malpighi, Marcello (1628–1694) Ital- magnesium An element essential for ian histologist. Often described as the plant and animal growth. It is contained in father of microscopy, Malpighi, a philoso- the chlorophyll molecule and is thus essen- pher and professor of medicine, initiated tial for photosynthesis. It is an important the detailed study of plant and animal tis- component of membranes, and an essential sues. He is best known for his investiga- cofactor for certain phosphate-transferring tions into mammalian blood and also enzymes, e.g. phosphohydrolase and phos- studied many body organs, including the photransferase. High concentrations of skin. He described the stomata on the magnesium ions, Mg2+, are needed to main- abaxial surfaces of leaves, the annual rings tain ribosome structure. Deficiency of of woody plants, and also made a distinc- magnesium may result in CHLOROSIS ,, NE- tion between monocotyledonous and di- CROSIS, stunted growth, and in some cotyledonous plants. species, whitening and puckering of the leaf edges. It may be remedied by adding maltose A sugar found in germinating magnesium sulfate or magnesium oxide to cereal seeds. It is a disaccharide composed the soils. of two glucose units linked by an a- 1,4–GLYCOSIDIC BOND. Maltose is an im- Magnoliaceae A family of primitive di- portant intermediate in the enzyme cotyledons that includes and the hydrolysis of starch. It is further hy- tulip trees (Liriodendron spp.). They are drolyzed to glucose. mostly trees and shrubs of warm temperate to tropical parts of east Asia and tropical manganese A metallic MICRONUTRIENT North America. needed by plants for growth. Manganese ions are cofactors in various enzymes, e.g. maidenhair ferns See Adiantaceae. kinases, IAA oxidase, and certain enzymes involved in the biosynthetic pathway for male sterility A condition in which chlorophyll. Manganese is required for the

138 mass flow light reactions of Photosystem II (see pho- caceae (heath family), interspersed with tosynthesis). Deficiency of manganese may aromatic herbs and smaller shrubs, such as cause dwarfing, mottling of the upper thyme (Thymus), Cistus, and myrtle (Myr- leaves, CHLOROSIS between the leaf veins, tus), and scattered small trees such as olive ‘blight’ of sugar cane, and ‘gray speck’ of (Olea europaea) and figs (Ficus). Similar oats. vegetation in California is called CHAPAR- RAL. mangrove Any of a range of trees and shrubs that form forests and dense thickets Marattiaceae (giant ferns) A family of along muddy coasts and estuaries and in large tropical EUSPORANGIATE ferns, some- salt marshes, some of which have prop times called the giant ferns, that dates back roots. Adventitious roots arch downward to the Carboniferous period. They have to the mud and send up new trunks. The stout erect stems and large compound prop roots trap sediment, building up fronds, in some species, e.g. Angiopteris the land and aiding SUCCESSION. Many evecta, up to 4.5 m (15 ft) long. mangrove species have PNEUMATOPHORES, or ‘breathing knees’, that rise up out of the Marchantiales An order of thallose liv- mud. They have abundant lenticels, erworts with flat, ribbonlike, dichoto- through which air diffuses to supply the mously branching thalli that lie close to the waterlogged roots below the mud. ground. Marchantia, a common species of gardens, riverbanks, and wet moorland, mannitol A soluble sugar alcohol (car- has umbrellalike, separate male and female bohydrate) found widely in plants and reproductive structures on long upright forming a characteristic food reserve of the stalks. Phaeophyta (brown algae). It is also the main soluble sugar in fungi and lichens. marker gene A gene of known location Mannitol is a hexahydric alcohol, i.e. each and function that can is used to establish of the six carbon atoms has an alcohol (hy- the relative positions and functions of droxyl) group attached. It may be derived other genes. During gene transfer, a from mannose or fructose. marker gene may be linked to the trans- ferred gene to determine whether or not the mannose A simple sugar found in many transfer has been successful. See chromo- polysaccharides. It is an aldohexose, iso- some map; genetic engineering. meric with glucose. In some plants, such as members of the family Fabaceae (legumes). marsh See swamp. It is the main unit of storage polysaccha- rides, forming polymers called mannans. It Marsileaceae A family of highly spe- is also a component of some hemicellu- cialized heterosporous water ferns. The loses. Reduction of mannose yields manni- leaves are borne on creeping rhizomes, and tol, a major sugar in fungi, lichens, and may be cylindrical and pointed, as in Pilu- some algae. laria, or divided into two or four leaflets, as in Marsilea and Regnelidium. mantle See mycorrhiza. mass flow A hypothesis put forward by maquis A kind of Mediterranean scrub Münch (1930) to explain the mechanism found on poor soils in regions with a pro- of phloem transport. The movement of nounced dry season. It is made up of thick- substances is believed to be the result of ets of evergreen sclerophyllous (having changes in osmotic pressure. Thus in an ac- thick leathery leaves) or spiny bushes and tively photosynthesizing region (source) shrubs up to about 3 meters (9 ft) tall, e.g. where sugars are being produced the os- gorse (Ulex), broom (Cytisus, Genista), motic pressure is high and water is taken laurels (Laurus), and members of the Eri- in. Conversely, in regions (sinks) where

139 medulla photosynthetic products are being used up megaspore The larger of the two types or converted to storage compounds there is of spores in heterosporous pteridophytes a lowering of osmotic pressure and water is and seed plants that produces the female lost. A system is then set up in which there gametophyte. In some pteridophytes the is mass flow of water and dissolved sugars megaspores are released from the sporo- from source to sink. phyte, in others they are retained and fer- tilized on the sporophyte; in the seed plants medulla 1. See pith. they are retained. The megaspores of pteri- 2. In fungal fruiting bodies, the layer of dophytes and gymnosperms produce a fe- longitudinal hyphae. male gametophyte called a PROTHALLUS 3. In lichens, the layer of loosely organized that forms two or more archegonia each hyphae lying below the cortex and algal containing a haploid female gamete. In an- layer. giosperms the megaspore becomes the em- 4. In multicellular algal thalli, the inner lay- bryo sac, lacking an obvious prothallus but ers of the thallus, comprising nonpig- containing, most commonly, eight nuclei, mented cells that often form a storage one of which is organized as the female ga- tissue. mete. Other angiosperms have four or six- teen or more nuclei in the embryo sac. medullary ray A linear group of living Compare . parenchyma cells in the secondary xylem of a . They extend from the megasporophyll A leaf, modified leaf, vascular cambium for various distances or leaflike structure that bears the mega- sporangium. Simple megasporophylls in- into the wood. clude the fertile ligulate leaves of hetero- sporous lycopods (e.g. Selaginella). They megaphyll (macrophyll) A foliage LEAF are usually grouped in a STROBILUS that with a branched system of veins in the may also contain MICROSPOROPHYLLS or blade and with the leaf trace leaving a leaf vegetative leaves. The carpel of angio- gap in the stele. It is typical of ferns and sperms and the ovuliferous scale of gym- seed-bearing plants. The large often pin- nosperms are modified megasporophylls. nately divided megaphyll of ferns, often frond termed a , contrasts with the gener- meiosis The process of cell division ally much smaller leaves (MICROPHYLLS) of leading to the production of daughter nu- clubmosses and horsetails. The scale- or clei with half the genetic complement of the needlelike leaves of conifers have charac- parent cell. Cells formed by meiosis give teristics of both types since they have a sin- rise to gametes and fertilization restores gle vein like a microphyll but leave a leaf the correct chromosome complement. trace like a megaphyll. Meiosis consists of two divisions during which the chromosomes replicate only megasporangium (pl. megasporangia)A once. Like mitosis the stages PROPHASE, sporangium that produces megaspores. In METAPHASE, and ANAPHASE can be recog- Selaginella the megasporangium is borne in nized. However during prophase homolo- the axis of a located in a stro- gous chromosomes attract each other and bilus. Usually all the spore mother cells de- become paired forming bivalents. At the generate except one, which forms a tetrad end of prophase genetic material may be of cells. One or more of these may develop exchanged between the chromatids of ho- into MEGASPORES, which, when shed, de- mologous chromosomes. Meiosis also dif- velop into the female gametophyte. The fers from mitosis in that after anaphase, ovule of seed plants may be considered instead of nuclear membranes forming, equivalent to the megasporangium of there is a second division, which may be di- the pteridophytes. Compare microsporan- vided into metaphase II and anaphase II. gium. See heterospory. The second division ends with the forma-

140 Mendel, Gregor Johann tion of four haploid nuclei, which develop membrane, affecting, for example, cell–cell into gametes. Compare mitosis. See alter- recognition (as in embryonic development nation of generations; telophase. and immune mechanisms), permeability, and hormone recognition. Membranes membrane A structure consisting mainly may contain efficient arrangements of mol- of lipid and protein (lipoprotein) surround- ecules involved in certain metabolic ing all living cells as the plasma membrane, processes, e.g. electron transport and phos- or plasmalemma, and also found sur- phorylation (ATP production) in mito- rounding organelles within cells. Mem- chondria and chloroplasts. See osmosis; branes function as selectively permeable freeze fracturing; plasma membrane. barriers, controlling passage of substances between the cell and its organelles, and membrane potential The potential dif- their internal environment, either actively ference that exists across a membrane as a or passively. Membranes are typically result of the action of PROTON PUMPS, which 7.5–10 nm in thickness with two regular maintain an imbalance of positive and neg- layers of lipid molecules (a bilayer) con- ative ions on either side of the membrane. taining various types of protein molecules. Such pumps are usually coupled to ATP: Some proteins penetrate through the mem- energy is required to maintain the potential brane, while others are associated with one difference. side; some float freely over the surface while others remain stationary. Some are Mendel, Gregor Johann (1822–84) enzyme controlling, for instance, in the ac- Austrian . Having devel- tive transport of molecules or ions through oped an interest in as a child the membrane. Larger molecules or parti- growing up on a peasant farm, Mendel was cles can pass from one side of a membrane able to carry out many plant breeding ex- by ENDOCYTOSIS or EXOCYTOSIS. periments after entering an Augustinian The lipids are mostly phospholipids. monastery at Brünn (now Brno in the These are polar molecules: one end (the Czech Republic). He used the pea plant in phosphate end) is hydrophilic (water- his experiments and selected seven charac- loving) and faces outward, while the other teristics of the plant to observe. First, he end (consisting of two fatty acid tails) is isolated his selected pure-bred parent hydrophobic (water-hating) and faces in- plants and carried out his crosses by trans- ward. Short chains of sugars may be asso- ferring pollen from one to another. He then ciated with the proteins or lipids forming collected and counted the seeds and grew glycoproteins and glycolipids. The particu- them on, patiently observing and carefully lar types of carbohydrates, lipids, and pro- recording the characteristics and their fre- teins determine the characteristics of the quency in the next and often in successive

Meiosis: the differences in chromosome behavior in meiosis and mitosis

141 Mendelian ratio generations. Mendel concluded from his contains two alleles of a given gene, each of results that each character is controlled by which is located on one of a pair of ho- two ‘factors’ in the somatic cells, but each mologous chromosomes. Only one ho- gamete can pass on only one of these fac- molog of each pair is passed on to a tors to the next generation. He also discov- gamete. Thus the Law of Segregation still ered that the factors were distributed at holds true. Mendel envisaged his factors as random in the gametes and that each pair discrete particles but it is now known that of factors segregates independently of one they are grouped together on chromo- another (see Mendelism; Mendel’s laws). somes. The Law of Independent Assort- Mendel’s ‘factors’ of inheritance are, in ment therefore applies only to unlinked fact, genes and he is recognized as being the genes. See linkage. first person to provide genetics with a mathematical basis. meristem A distinct region of actively or Mendel published his results in a local potentially actively dividing cells primarily scientific journal and his paper was widely concerned with growth. Numerous meris- circulated, but unfortunately ignored. tems occur in plants. In active meristems When Mendel became abbot of the separation occurs between the cell that re- monastery, he had to abandon his scientific mains meristematic (initial) and the cell ul- studies. It was not until 1900, some 16 timately being differentiated. Two basic years after his death, that his work was re- meristematic groups are the primary apical discovered by DE VRIES and others and its meristems at root and shoot apices, and the accuracy and ingenuity appreciated. secondary lateral meristems, which include vascular and cork cambia. See also inter- Mendelian ratio The ratio of contrast- calary meristem; secondary growth. ing alleles present in the offspring of a cross involving genes that behave according to meristoderm (limiting layer) The outer- MENDEL’S LAWS. most cellular layer of the thallus of certain Phaeophyta (brown algae). It consists of Mendelism The theory of inheritance small densely packed rectangular cells con- according to which characteristics are de- taining brown pigmented plastids and cov- termined by particulate ‘factors’, or genes, ered by a mucilaginous layer to prevent that are transmitted by the germ cells. It is desiccation. The meristoderm maintains its the basis of classical genetics, and is meristematic activity and assists the outer founded on the work of MENDEL in the cortical layers in adding to the thickness of 1860s. See Mendel’s laws. the thallus and replacing tissues worn away by tidal action. Mendel’s laws Two laws formulated by MENDEL to explain the pattern of inheri- mesarch Denoting a stele or part of a tance he observed in plant crosses. The first stele in which the protoxylem is sur- law, the Law of Segregation, states that any rounded by metaxylem. Compare cen- character exists as two factors, both of trarch; endarch; exarch. which are found in the somatic cells but only one of which is passed on to any one mesocarp See pericarp. gamete. The second law, the Law of Inde- pendent Assortment, states that the distrib- mesophilic Designating microorganisms ution of such factors to the gametes is with an optimum temperature for growth random; if a number of pairs of factors is between 25–45° C. Compare psychro- considered, each pair segregates indepen- philic; thermophilic. dently. Today Mendel’s ‘characters’ are termed mesophyll Specialized tissue located be- genes and their different forms (factors) are tween the epidermal layers of the leaf and called alleles. It is known that a diploid cell composed mainly of photosynthetic cells

142 metaxylem

(CHLORENCHYMA). Veins, supported by plasm. One strand of the double helix sclerenchyma and collenchyma, are em- of DNA acts as a template along which bedded in the mesophyll. Palisade meso- complementary RNA nucleotides become phyll consists of cylindrical cells, at right aligned and joined together by the enzyme angles to the upper epidermis, with many mRNA polymerase. These form a polynu- chloroplasts and small intercellular spaces. cleotide identical to the other DNA strand, It is the main photosynthesizing layer in the except that the thymine bases are replaced plant. Spongy mesophyll, adjacent to the by uracil. This polynucleotide is called het- lower epidermis, comprises interconnect- erogeneous nuclear RNA (hnRNA) and it ing irregularly shaped cells with few contains both coding and noncoding se- chloroplasts and large intercellular spaces quences (see exon; intron). In the nucleolus that communicate with the atmosphere POST-TRANSCRIPTIONAL PROCESSING occurs: through stomata allowing adenine polynucleotide (ply A) is added to between the cells and the atmosphere. The one end and a guanine derivative to the distribution of mesophyll tissue varies in other, and (in eukaryotic cells) the introns different leaves depending on the environ- are then removed to produce mRNA ment in which the plant lives. (sometimes called mature mRNA). The whole process is termed transcription. The mesophyte A plant that is adapted to new mRNA molecule thus has a copy of grow under adequate conditions of water the genetic code, which directs the forma- supply and has no particular adaptations tion of proteins in the ribosomes. Compare to withstand environmental extremes. In transfer RNA. drought conditions wilting is soon ap- parent as the plants have no special metabolism The chemical reactions mechanisms to conserve water. Most an- that take place in cells. The molecules tak- giosperms are mesophytes. Compare hy- ing part in these reactions are termed drophyte; xerophyte. metabolites. It is metabolic reactions, par- ticularly those producing energy, that keep Mesozoic The middle era in the most re- cells alive. Metabolic reactions characteris- cent (Phanerozoic) eon of the geological tically occur in small steps, comprising a time scale, dating from about 230–66 mil- metabolic pathway. Metabolic reactions lion years ago. It is divided into three main involve the breaking down of molecules periods: the Triassic, Jurassic, and Creta- to provide energy (CATABOLISM) and the ceous. During the Triassic period seed- building up of more complex molecules ferns predominated, but as the climate and structures from simpler molecules (AN- warmed the cycads came to dominate, and ABOLISM). the Mesozoic is sometimes called ‘The Age of Cycads’. In the Jurassic period, tropical metabolite See metabolism. and temperate regions were dominated by forests of palmlike cycads and conifers, metaphase The stage in mitosis and and the dawn redwoods made their first meiosis when the chromosomes become appearance. Ginkgoes were also wide- aligned along the equator of the nuclear spread. The first angiosperms arose in the spindle. Metaphase follows prophase and Cretaceous period, and rapidly diversified, precedes anaphase. becoming the dominant vegetation by the end of the Mesozoic as the climate cooled metaphloem The primary PHLOEM formed and dried. See also geological time scale from the PROCAMBIUM after the PRO- TOPHLOEM. It is found behind the zone of messenger RNA (mRNA) The form of elongation below the meristem, and is RNA that transfers the information neces- more durable than the protophloem. In sary for protein synthesis from the DNA in plants showing no secondary thickening the nucleus to the ribosomes in the cyto- the metaphloem is responsible for the

143 methionine transport of most of the organic materials photorespiration and contain high levels of in the plant, but this function is taken over glycolate oxidase and other associated en- by the secondary phloem in regions where zymes. Glycolate comes from chloroplasts secondary tissues have differentiated. and products such as glycine are passed to mitochondria. Hence these three organelles metaxylem The primary XYLEM el- often appear close together. See photores- ements that are differentiated from the piration. PROCAMBIUM after the PROTOXYLEM. They are found some distance behind the apical EVOLUTION on a scale meristem beyond the zone of elongation. within a species, which results from the ac- The secondary cell walls show reticulate tion of natural selection on the genetic vari- and scalariform thickening and are thus in- ation between individuals of a given extensible. population. methionine A nonpolar sulfur-contain- microfibril A ribbonlike structure at ing AMINO ACID synthesized from aspartate. least 60 nm long and about 45 nm in di- Methionine can be broken down to suc- ameter, made of parallel chains of cellulose cinyl CoA, which is further oxidized in the molecules linked by hydrogen bonds. Mi- Krebs cycle. The ATP-activated derivative crofibrils are found in the CELL WALLS of of methionine, S-adenosyl methionine, do- plants and some algae. In secondary cell nates its methyl group to many different walls they are packed more closely, leaving molecules in various metabolic pathways. less room for water between them. methylene blue A blue dye used in light microfilament A minute filament, about microscopy to stain bacterial protoplasm. 0.4–0.7 nm wide, found in eukaryotic cells It is also used to stain nuclei blue. See stain- and having roles in cell motion and shape. ing. Microfilaments are made of two helically twisted strands of globular subunits of the The study of microscopic protein ACTIN, almost identical to the actin organisms (e.g. bacteria and viruses), in- of muscle. They can undergo rapid exten- cluding their interactions with other organ- sion or shortening by subunit assembly or isms and with the environment. disassembly, or form complex three- dimensional networks. Their occasional microbody A common organelle of association with myosinlike protein (as in plant and animal cells, bounded by a single muscle) suggests they may also be contrac- membrane, spherical, and usually about tile. They often occur in sheets or bundles 0.0–1.0 m m in diameter. Microbodies just below the plasma membrane and at the originate from the ENDOPLASMIC RETICULUM interface of moving and stationary cyto- and contain oxidative enzymes such as plasm. They are involved in CYTOKINESIS, catalase, which decomposes hydrogen per- CYCLOSIS, cell movements, e.g. ameboid oxide, a toxic waste product of the activi- movement, and movement of subcellular ties of other enzymes in the microbody. components, e.g. pinocytotic vesicles. GLYOXYSOMES contain enzymes of the glyoxylate cycle, transaminases, and en- micrograph A photograph taken with zymes associated with b-oxidation of fatty the aid of a microscope. Photomicrographs acids. They play a major role in conversion and electron micrographs are produced of lipids to sucrose in fatty or oily seedling using optical microscopes and electron mi- tissues, e.g. the endosperm of castor-oil croscopes respectively. seeds. PEROXISOMES occur in certain animal tissues, and are numerous in photosyn- micrometer1 In microscopy, a device thetic cells of plant leaves, where they are for measuring the size of an object under concerned with glycolate metabolism in the microscope. An eyepiece micrometer

144 microscope

(graticule) of glass or transparent film, growth around the apex of the ovule. with a scale etched or printed on it, is Pollen tubes usually pass through it prior placed in the eyepiece so that both the ob- to fertilization. In most seeds the micropyle ject to be measured and the scale are in forms a small hole in the testa through focus. The scale of the graticule changes at which water is absorbed, but in some seeds different magnifications and it must there- it is closed. fore be calibrated against a stage microme- ter, which is a glass slide with a scale microscope An instrument designed to etched into it, and placed on the micro- magnify objects and thus increase the reso- scope stage. lution with which one can view them. Res- olution is the ability to distinguish between micrometer2 (micron) Symbol: m mA two separate adjacent objects. Radiation unit of length equal to 10–6 meter (one mil- (light or electrons) is focused either lionth of a meter). It is often used in mea- through the specimen by a condenser lens surements of cell diameter, sizes of or onto it. The resulting image is magnified bacteria, etc. Formerly, it was called the by further lenses. Since radiation must pass micron. through the specimen in transmission mi- croscopy, it is usual to cut larger specimens micron See micrometer. into thin slices of material (sections) with a microtome. Biological material has little micronutrient (trace element) A nutri- contrast and is therefore often stained. If ent required in trace amounts by an organ- very thin sections are required the material ism. For example, a plant can obtain is preserved and embedded in a supporting sufficient of the essential trace element medium. manganese from a solution containing 0.5 The light microscope (optical micro- parts per million of manganese. Many act scope) uses light as a source of radiation. as enzyme cofactors or are components of With a compound microscope the image is pigments. They include boron, cobalt, cop- magnified by two lenses, an objective lens per, manganese, molybdenum, and zinc. near the specimen, and an eyepiece, where See deficiency disease. the image is viewed, at the opposite end of a tube. Its maximum magnification is lim- microphyll A foliage leaf that has a sin- ited by the wavelength of light: it can dis- gle unbranched vein running from base to tinguish between two points only 0.3 m m apex and no leaf gap associated with the apart. Its main use is the observation of leaf trace: the stele remains entire when a thin sections of tissues and organs, and the leaf trace branches off to a leaf. Micro- study of fine detail of microorganisms. For phylls are the typical leaves of clubmosses viewing relatively large specimens and for and horsetails. Compare megaphyll. , the lower resolution stereo- scopic binocular microscope is used: it uses The production of incident light and has two eyepieces, giving large numbers of plants under laboratory a three-dimensional image. Various modi- conditions. It usually involves the use of fications of the light source and back- TISSUE CULTURE to produce large numbers ground illumination allow different aspects of genetically identical plants (clones), but of cell structure to be observed, e.g. bright- can involve seeds, e.g. of orchids. It is used field illumination, DARK-GROUND ILLUMI- to produce disease-free plants, and also to NATION, PHASE CONTRAST MICROSCOPY, multiply genetically engineered plants, and interference microscopy, and CONFOCAL to increase rare plant populations for con- MICROSCOPY. Much greater resolution be- servation purposes. came possible with the introduction of the electron microscope, which uses electrons micropyle A pore leading to the nucel- as a source of radiation, because electrons lus formed by incomplete integument have much shorter wavelengths than light.

145 microsomes

However, only dead material can be ob- ecules such as myosin and DNA. In non- served because the specimen must be in a touching mode it provides a tool for the vacuum and electrons eventually heat and study of real-time changes in the surface destroy the material. The electron micro- without interfering with processes causing scope uses electromagnetic lenses to focus the changes, e.g. the change in structure of a parallel beam of electrons onto an object starch as it is digested by an enzyme. Mod- and produce an image of the resulting elec- ifications of the technique can give further tron scattering projected onto a photo- information; it is used, for example, to graphic plate or fluorescent screen. The measure the volume of chromosomes. process takes place in a vacuum to mini- mize unwanted electron scattering. Since microsomes Fragments of endoplasmic the wavelength of an electron beam is reticulum and Golgi apparatus in the form 0.005 nm, an electron microscope has of vesicles formed during homogenization about 300 times greater resolution than a of cells and isolated by high-speed centrifu- light microscope. In theory, this micro- gation. Microsomes from rough endoplas- scope can distinguish between objects mic reticulum are coated with ribosomes 0.0025 nm apart, but in practice, resolu- and can carry out protein synthesis in the tions of less than about 1 nm are rarely at- test tube. tained. Electron microscopes are of two main types, the transmission electron mi- microsporangium (pl. microsporangia) croscope (TEM) and the scanning electron In heterosporous plants, the sporangium microscope (SEM). The former produces that produces the MICROSPORES, located an image by passing electrons through the on the microsporophyll. The microspo- specimen. With the scanning microscope rangium wall splits to disperse the mature electrons scan the surfaces of specimens microspores. Microsporangia are found in rather as a screen is scanned in a TV tube, some pteridophytes (e.g. Selaginella, water allowing surfaces of objects to be seen with ferns) and are represented by the pollen greater depth of field and giving a three- sacs in gymnosperms and angiosperms. dimensional appearance to the image. The Compare surface of entire objects can be scanned, megasporangium. and material to be viewed is often given an electron-reflecting coating. Scanning mi- microspore The smaller of the two croscopes cannot operate at such high types of spores produced in large numbers magnifications as transmission micro- by seed plants and heterosporous pterido- scopes, but can still resolve to at least 2 nm. phytes. In pteridophytes, microspores Of growing importance is the scanning develop into the male gametophyte genera- probe microscope (atomic force micro- tion, but in gymnosperms and angio- scope), in which a highly sensitive probe sperms, the microspores (POLLEN grains) mounted on a cantilever is moved to and develop into a very reduced gametophyte fro over a surface, and responds to the represented by the pollen tube and the veg- minute intermolecular distance between etative nucleus (pollen-tube nucleus) and the probe and the surface, which causes the the generative nuclei. The latter are the probe to be deflected from the surface. The male gametic nuclei. Compare megaspore. deflection of the probe is measured with a laser beam, providing a topographical map microsporophyll A leaf or modified of the surface without penetrating it. The leaf on which the microsporangium is probe may be dragged over the surface of borne. Simple microsporophylls include the object, tap the object at intervals, or the scales found in the male cones of gym- travel just above the surface. The scanning nosperms and the fertile photosynthetic probe microscope can achieve resolutions leaves of clubmosses, usually grouped in a of less than 1 nm in x-, y-, and z-directions, strobilus. In gymnosperms the male scale is and can produce images of individual mol- the microsporophyll, while in angiosperms

146 mitochondrion the stamen is a highly modified micro- 2. Any thickened linear structure running sporophyll. Compare megasporophyll. down the center of an algal thallus, such as a seaweed, e.g. Fucus spp. microtome An instrument for cutting thin sections (slices a few micrometers mildew A fungal disease of plants in thick) of biological material for micro- which the hyphae appear on the surface of scopic examination. The specimen is usu- the plant. ally embedded in wax for support and cut by a steel knife. Alternatively it is frozen Mississippian In North America, the and a freezing microtome, which keeps the Lower Carboniferous period, dating from specimen frozen while cutting, is used 345 to 310 million years ago. See Car- (cryo-microtomy). For electron micros- boniferous; Pennsylvanian. copy, extremely thin (20–100 nm) sections can be cut by an ultramicrotome. Here the mitochondrial DNA (mt-DNA) The specimen is embedded in resin or plastic for DNA found in mitochondria. It is a circu- support and mounted in an arm that ad- lar molecule not associated with histones vances slowly, moving up and down, to- or other proteins and independent of nu- ward a glass or diamond knife. As sections clear DNA. It codes for specific RNA com- are cut they float off on to the surface of ponents of ribosomes, and also for certain water contained in a trough behind the respiratory enzymes that occur in the mito- knife. chondria and are synthesized on mitochon- drial ribosomes. Plant mt-DNA evolves microtubule A thin cylindrical un- very slowly (unlike animal mt-DNA), and branched tube of variable length found in can break apart and recombine. It is usu- eukaryotic cells, either singly or in groups. ally transmitted from females to their off- Its walls are made of the globular protein spring in the cytoplasm of the ova, but tubulin. Microtubules have a skeletal role, mitochondria do not usually pass into male helping cells to maintain their shape. They gametes. occur freely in the cell, and also form part of the structure of the SPINDLE during cell mitochondrion (pl. mitochondria) An division, bringing about chromosome organelle of all plant and animal cells movement. Microtubules also help to ori- chiefly associated with aerobic respiration. entate materials and structures in the cell, It is surrounded by two membranes sepa- e.g. cellulose fibrils during the formation of rated by an intermembrane space; the inner plant cell walls, and may be involved in the membrane forms fingerlike processes transport of materials with and between called cristae, which project into the gel- cells, e.g. phloem transport. They are also like matrix. Mitochondria are typically part of the centrioles found in many fungal sausage-shaped, but may assume a variety and algal cells, and the basal bodies and of forms, including irregular branching undulipodia of many motile unicells. shapes. The diameter is always about 0.5–1.0 m m, and the length averages 2 middle lamella In mature plant tissues, m m. Mitochondria contain the enzymes the material cementing the walls of adja- and cofactors of AEROBIC RESPIRATION and cent cells. It is derived from the first- therefore are most numerous in active cells formed layer of the primary cell wall, (up to several thousand per cell). The reac- which is laid down by the tions of the KREBS CYCLE take place in the (cell plate), and is composed mainly of cal- matrix and those of electron transport cou- cium and magnesium pectates. pled to oxidative phosphorylation (i.e. the respiratory chain) on the inner membrane. midrib 1. The thick vein running down Within the membrane the components of the middle of the leaf from the petiole to the respiratory chain are highly organized. the leaf tip of a plant. The matrix is also involved in amino acid

147 mitosis metabolism via Krebs cycle acids and fixation. Deficiency of molybdenum may transaminase enzymes, and in fatty acid lead to CHLOROSIS between the leaf veins. oxidation. Like chloroplasts, mitochondria contain their own ribosomes and DNA, Monera An alternative name for the which is circular in form, like that of bac- kingdom BACTERIA (Prokaryotae). teria, and reproduce by binary fission. It is believed that mitochondria and chloroplas- monochasial cyme (monochasium) See tsmay be the descendants of once indepen- inflorescence. dent organisms that early in evolution invaded eukaryotic cells, leading to an ex- Monocotyledonae A class of the flow- treme form of symbiosis. See endosym- ering plants (phylum Anthophyta) deriving biont theory. See also electron-transport their name from single cotyledon in the em- chain. bryo (though some dicotyledons have this, too). They are usually herbaceous plants mitosis (karyokinesis) The ordered pro- and most do not show secondary growth. cess by which the and cyto- Examples of monocotyledons are the plasm divide in two during the division of grasses (Poaceae) and lilies (Liliaceae). body (i.e. nongermline) cells. The chromo- somes replicate prior to mitosis to form The growing of a single two sister chromatids, which are then sep- species or variety of crop over a large area. arated during mitosis in such a way that The economic reasons for this are the abil- ity to grow crops of uniform height for har- each daughter cell inherits a genetic com- vesting, and economies of scale in the plement identical to that of the parent cell. harvesting process. The major disadvan- Although mitosis is a continuous process it tage is increased susceptibility to pests and is divided into four phases; prophase, pathogens, especially if a single variety is metaphase, anaphase, and telophase. Telo- grown. phase may be followed by cytokinesis. Compare meiosis; amitosis; endomitosis. monoecious Denoting plants in which the male and female reproductive organs See mitosporic fungi Fungi Anamor- are borne on separate structures on the phici. same individual plant. Monoecious flower- ing plants bear separate unisexual male molecular systematics A branch of bi- and female flowers, e.g. maize and many ology that compares functionally equiva- temperate trees such as birch, hazel, and lent macromolecules from different oak. Many monoecious plants are wind- organisms as a basis for classification. Se- pollinated. Compare dioecious; hermaph- quences of amino acids in proteins (e.g. en- rodite. zymes) or of nucleotides in nucleic acids (e.g. ribosomal RNA) are determined using monohybrid An indiviudal heterozy- automated techniques, and compared sta- gous at one locus (i.e. for a single gene), ob- tistically using sophisticated computer tained by crossing homozygous parents programs. Essentially, how closely two or- with different alleles at a given locus; for ganisms are related in evolutionary terms is example, Mendel’s cross between tall (TT) reflected in the degree of similarity of their and dwarf (tt) garden peas to give a tall macromolecules. monohybrid (Tt). When a monohybrid is selfed, dominant and recessive phenotypes molybdenum A MICRONUTRIENT (trace appear in the offspring in the ratio of 3:1 element) needed for plant growth. It forms (the monohybrid ratio). Compare dihy- part of the enzymes nitrate reductase, brid. which is involved in nitrate reduction, and nitrogenase, the key enzyme in nitrogen monoploid See haploid.

148 mutagen monopodial Describing the system of ferns See Azollaceae. branching in plants in which the main axis of the stem (the monopodium) continues to mosses In older classifications, a class of grow indefinitely by the terminal bud. bryophytes containing leafy plants with Monopodial growth is also called indefi- multicellular . In the FIVE KING- nite or racemose branching and is typical DOMS CLASSIFICATION, mosses comprise the of the formation of a racemose inflores- phylum BRYOPHYTA. They differ from liver- cence. Compare sympodial. worts in their greater differentiation of the gametophyte and also their complex mech- monosaccharide A SUGAR that cannot anisms of capsule dehiscence, with no for- be hydrolyzed to simpler carbohydrates of mation of elaters in the capsule. Orders smaller carbon content. Glucose and fruc- include the Bryales (e.g. Funaria, Poly- tose are examples. Monosaccharides may trichum, Mnium) and the Sphagnales (e.g. be classified according to the number of Sphagnum). carbon atoms they contain: 5-carbon pen- toses (e.g. ribose); 6-carbon hexoses (e.g. mucilage Any of various gumlike carbo- glucose, fructose). The general formula of hydrates that swell when wet, becoming a monosaccharide is (CH2O)n. slimy and jellylike. See gum. monosomy See aneuploidy. mucopolysaccharide See polysaccha- ride. monotrichous Describing BACTERIA that possess one flagellum, e.g. Vibrio. Mucorales (pin molds) An order of the ZYGOMYCOTA, containing fungi that form a Moraceae A family of trees, shrubs, dense mycelium bearing sporangia or coni- lianas, and herbs, found mainly in tropical dia. The tips of the spore-bearing hyphae and subtropical regions. There are about often become pigmented, hence the name 12 000 species. The Moraceae contains pin molds (e.g. Mucor mucedo). Most are many important food plants, such as edible saprobes in soil, dung, and other organic figs (Ficus carica), breadfruit (Artocarpus debris. Many cause spoilage of stored artilis), jackfruit (A. heterophyllus), and food. A few are parasites on other fungi, mulberry (Morus). plants, or animals. morel See Pezizales. multicellular Consisting of many cells. The development of multifactorial inheritance See contin- form and structure. uous variation. morphology The study of the form of multiple allelism The existence of a se- organisms. The term may be used synony- ries of alleles (three or more) for one gene. mously with ‘anatomy’ although generally Only two alleles of the series can be present the study of external form is termed ‘mor- in a diploid cell. Dominance relationships phology’ while the study of internal struc- within an allelic series are often compli- tures is termed ‘anatomy’. cated. Multiple alleles are common in the incompatibility systems of plants, such as mosaic 1. A hybrid organism whose Sinapis, poppy (Papaver), and clover (Tri- cells differ genetically, although they have folium). The numbers of such alleles are all arisen from a single zygote. often considerable, e.g. Brussels sprouts 2. A pattern of light and dark green patches (Brassica oleracea var. bullata) has up to on the leaves of a plant, usually due to viral 40 incompatibility alleles, or S alleles. infection. 3. See leaf mosaic. multiple fruit (composite fruit) A type

149 mutation of pseudocarpic (‘false’) fruit that incorpo- bacterium can live independently of the rates parts of the inflorescence. Examples plant; in lichens often the alga can live in- are figs (Ficus) and (Ananus co- dependently of the fungus. In many mutu- mosus). alistic associations, the relationship can change from a parasitism to a mutualism mutagen Any physical or chemical or vice versa. Compare symbiosis; com- agent that induces MUTATION or increases mensalism; amensalism. the rate of spontaneous mutation. Chemi- cal mutagens include compounds that react mycelium (pl. mycelia) A filamentous with nucleotides, e.g. nitrous acid, pro- mass comprising the vegetative body of a flavin; base analogues, which are incorpo- fungus, each filament being called a hypha. rated in place of normal bases during DNA The mycelium often forms a loose mesh as replication; and acridines, which cause in Mucor, or hyphae may anastomose to base-pair deletions or additions. Physical form a netlike structure, as in Ascomycota mutagens include short-wave radiation and Basidiomycota. See hypha. such as ultraviolet light, x-rays, and cosmic rays; and ionizing radiation, such as a and mycoplasmas (PPLO; pleuropneumonia- b particles. like organisms) A group of extremely small bacteria that naturally lack a rigid mutation Mutations are the ultimate cell wall. They often measure less than 200 source of all genetic variation. They are in- nm in diameter and their cells are delicate herited only if they occur in the cells that and plastic. Mycoplasmas are implicated in give rise to the gametes; somatic mutations certain plant diseases, such as witches may give rise to chimaeras and cancers. broom of alfalfa, maize stunt, and yellows GENE MUTATIONS alter only a single gene, diseases. They can kill tissue cultures and resulting in new allelic forms of the gene cause serious diseases in humans and ani- and hence new variations upon which nat- mals, e.g. pleuropneumonia. ural selection can act. Most mutations are deleterious but are often retained in the mycorrhiza (pl. mycorrhizae) The asso- population because they also tend to be re- ciation between the hyphae of a fungus and cessive and can thus be carried in the geno- the roots of a higher plant. Two main types type without affecting the viability of the of mycorrhiza exist, ectomycorrhizae (ec- organism. The natural rate of mutation is totrophic mycorrhizae), in which the fun- low, and varies with different gene loci. gus forms a mantle around the smaller The mutation frequency can be increased roots, as in many temperate and boreal by mutagens. See also chromosome muta- trees, and endomycorrhizae (endotrophic tion; mutagen; polyploid. mycorrhizae), in which the fungus grows around and within the cortex cells of the mutualism The close relationship be- roots, as in many herbaceous plants. Or- tween two or more species of organisms in chids and heathers have further specialized which all benefit from the association. types of mycorrhizae, in which there is There are two types of mutualism: obliga- often a mantle and cell penetration. In ec- tory mutualism, in which one cannot sur- tomycorrhizae the fungus, which is usually vive without the other, for example the a member of the Agaricales, benefits by ob- algal/fungal partnership found in lichens; taining carbohydrates and possibly B- and facultative or nonobligatory mutual- group vitamins from the roots. The trees ism, in which one or both species can sur- benefit in that mycorrhizal roots absorb vive independently, for example, in the nutrients more efficiently than uninfected association between the bacterium Rhizo- roots, and it is common forestry practice to bium and members of the family Fabaceae, insure the appropriate fungus is applied which leads to the formation of root nod- when planting seedling trees. In endomyc- ules in which nitrogen fixation occurs, the orrhizae the fungus is generally a species of

150 Myxomycota

Rhizoctonia and again both partners bene- gametes, and the movement of slime mold fit nutritionally from the relationship. In plasmodia. Myosin is thought to interact a special form of endomycorrhiza, the with the protein ACTIN to bring about vesiculo-arbuscular mycorrhiza (arbuscu- movement. lar mycorrhiza), the fungus lives between the cells of the cortex and projections of Myrtaceae A family of trees, shrubs and its hyphae actually penetrate the cortical creepers found in most parts of the tropics cells. Sometimes the projections are finely and warm temperate regions, especially branched hyphae called arbuscules. Australia. The family includes many im- portant forest trees such as myosin A contractile protein found in species, especially in Australia, sources of cells, which is involved in the movement of timber, tannin and oils. Some species pro- cytoplasm, for example in the cytoplasmic duce edible fruits, e.g. guava (Psidium gua- streaming of plant cells, the ameboid java), or spices, e.g. allspice (Pimenta movement of some protoctists and fungus dioica) and cloves ( caryophyllus).

151 N

NAD (nicotinamide adenine dinucleotide) thousand-millionth, or 10–9. For example, A pyridine-based nucleotide that acts as a 1 nanometer (nm) = 10–9 meter. See SI units. coenzyme to oxido-reductase enzymes in electron-transfer reactions, such as the ox- nastic movements (nasties) Movements idation of citrate to a-ketoglutarate in the of plant parts in which direction of move- KREBS CYCLE. The reduced form is written ment is independent of the direction of the as NADH, and may act as a reducing agent stimulus that induces them. With pho- or be reoxidized in the respiratory chain tonasty the stimulus is light. For instance, coupled to ATP formation. Its main impor- at constant temperature Crocus and tulip tance is as an electron acceptor in oxida- (Tulipa) flowers open in the light and close tion reactions. in the dark because of slight growth move- ments. Opening is caused by epinasty – i.e. NADP (nicotinamide adenine dinucleo- greater growth of the upper surface of a tide phosphate)A pyridine-based nu- plant organ – and closed by hyponasty – cleotide that acts as a COENZYME to oxido- i.e. greater growth of the lower surface of a reductase enzymes. Unlike NAD, it usually plant organ. Similarly, at constant light in- acts as an electron donor in enzyme- tensity crocus and tulip flowers show ther- catalyzed reduction reactions. monasty; i.e. they open in warm air and close in cool air. Such day–night rhythms Naegeli, Karl Wilhelm von (1817–91) are examples of nyctinasty and may also Naegeli was educated at Zurich, Geneva, occur in leaves, as in Oxalis. Nongrowth and Berlin, where he studied philosophy nastic movements also occur and are more before resuming his botanical work. He rapid. The ‘sensitive plant’ (Mimosa pu- produced an accurate account of cell divi- dica) rapidly closes its leaflets upwards and sion (1842) while studying pollen forma- tion and even observed chromosomes in the petioles droop in response to touch, seismonasty dividing nuclei, calling them ‘transitory shock ( ), or injury. This plant, cytoblasts’. His observations disproved and many other legumes, shows non- SCHLEIDEN’S theory that new cells were growth nyctinastic movements. Movement formed by budding off the nuclei of exist- is the result of osmotic changes in special ing cells. Working on apical growth in swollen groups of cells (pulvini) at the plants in 1845, he concluded that there bases of the moving structures. When the were distinct regions of cell division (meris- stimulus is contact, the movement is hap- tems). Naegeli also laid the foundations of tonastic as in the closure movements of in- cell ultrastructure when he studied starch sectivorous plants. The closure of the two grains in cells and formulated his micellar halves of the Venus fly-trap leaf (Dionaea theory. Sadly, his own entrenched views on muscipula) is a nongrowth haptonastic heredity and evolution, which arose from movement caused by a loss of turgor in the his studies on the hawkweed (Hieracium), cells along the midrib following stimula- led him to reject MENDEL’s very important tion of the sensitive hairs on the leaf. See work on pea plants. also taxis; tropism. nano- Symbol: n A prefix denoting one nasties See nastic movements.

152 neutral theory natural selection The process, which negative staining A method of prepara- DARWIN called the ‘struggle for survival’, by tion of material for electron microscopy which organisms less adapted to their envi- used for studying three-dimensional and ronment tend to perish, and better-adapted surface features, notably of viruses, macro- organisms tend to survive. There has been molecules (e.g. enzyme complexes), and much debate about the level at which nat- the cristae of mitochondria. A stain is used ural selection acts: traditionally it is re- that is not taken up by certain components garded as the action of the environment of the specimen. Usually the stain covers on individual organisms that determines the background and penetrates surface fea- which genotypes survive and reproduce, tures of the specimen, but leaves the speci- but there are strong arguments for natural men itself unstained. For example, nigrosin selection acting at the level of the gene (the or Indian ink is used to make bacteria vis- ‘selfish gene’ hypothesis) and some authors ible. In electron microscopy a similar tech- argue that it happens in the long term at nique mixes the specimen material with higher levels such as population or species. electron-dense negative stains, such as In the end the less successful types will die phosphotungstic acid (PTA) or potassium molybdate, which is not taken up by pro- out. According to DARWINISM, natural se- lection acting on a varied population re- teinaceous material in the specimen. The sults in evolution. electron-transparent proteinaceous mater- ial shows up as dark areas against a light necrosis The death of a cell or group of background on the resulting electron mi- crograph. See staining. cells in a living plant, especially where the dead area is discolored. Necrosis is often neo-Darwinism DARWIN’s theory of caused by fungal infections. evolution through NATURAL SELECTION, modified and expanded by genetic studies necrotrophic Describing a parasitic or- arising from the work of MENDEL and his ganism that obtains its nutrients from dead successors. This fusion of Darwin’s theory cells and tissues of its host. of natural selection with Mendel’s genetics was called the neo-Darwinian synthesis in A sugar-containing fluid secreted nectar the 1930s. The inclusion of genetics in evo- by the nectaries in plants. It is mainly pro- lutionary studies answered many questions duced in flowers and attracts insects or that Darwin’s theory raised but could not other animals, who feed on it and may un- adequately explain because of lack of intentionally transfer pollen from one knowledge at the time it was formulated. flower to another, thus facilitating cross- Notably, genetics has revealed the source pollination. Floral nectar contains variable of variation on which natural selection op- proportions of different monosaccharide erates, namely mutations of genes and and disaccharide sugars and, frequently, chromosomes, and provided mathematical other substances such as amino acids and models of how alleles fluctuate in natural volatile compounds as well. Some plants, populations, thereby quantifying the pro- particularly trees, produce nectar on stems cess of evolution. More recent discoveries and leaves. This may be collected by insects in have added to our un- such as ants that can protect the plant from derstanding of the causes of variation and insect herbivores. the nature of evolution at the molecular level. nectary A patch of glandular epidermal cells usually on the receptacle or the petals, neritic See sublittoral. but sometimes in other parts of the flowers or in specialized organs. They produce a neutral theory The theory that most sugary liquid (nectar) that attracts insects. evolutionary changes are caused by ran- dom GENETIC DRIFT (random changes in the negative feedback See feedback loop. frequencies of alleles in a population)

153 niacin rather than by NATURAL SELECTION. Genetic nitrification The oxidation of ammonia MUTATIONS arise at random. Many are neu- to nitrite, and/or nitrite to nitrate, carried tral – they do not affect the fitness or sur- out by certain nitrifying bacteria in the soil. vival of the carrier. They therefore survive The chemosynthetic bacteria Nitrosomo- in the genome and changes in their fre- nas and Nitrobacter carry out the first and quencies are due to the breeding system second stages respectively of this conver- and chance rather than to natural selec- sion. The process is important in the NI- tion. See gene frequency; variation. TROGEN CYCLE since many plants assimilate nitrate as their source of nitrogen. Com- niacin See nicotinic acid. pare denitrification. niche See ecological niche. nitrifying bacteria See nitrification. nicotinic acid (niacin) A carboxylic nitrogen An essential element found in acid that is one of the water-soluble B- all amino acids and therefore in all pro- group of VITAMINS. It also forms part of the teins, and in various other important alkaloid nicotine, hence its name. It is organic compounds, e.g. nucleic acids. sythesized from various precursors, but es- Gaseous nitrogen forms about 80% of the pecially from tryptophan. As nicotinamide, atmosphere but is unavailable in this form it is a constituent of two coenzymes, NAD except to a few nitrogen-fixing bacteria. and NADP, which operate as hydrogen Nitrogen is incorporated into plants as the – and electron-transfer agents and play a nitrate ion, NO3 or, especially in acid + vital role in metabolism. In humans, defi- soils, the ammonium anion NH4 , ab- ciency of nicotinic acid causes pellagra. sorbed in solution from the soil by roots. Diets based on maize, which is particularly Plants suffering from nitrogen deficiency poor in nicotinic acid, tend to lead to an in- tend to develop CHLOROSIS and become eti- crease in pellagra in the population. olated, the effects being seen first in the oldest parts. See also nitrogen cycle. Nidulariales The bird’s nest fungi – an order of the BASIDIOMYCOTA in which the nitrogenase See dinitrogenase. basidioma is nestlike, usually cup-shaped (e.g. splash cups, Cyathus striatus) or fun- nitrogen cycle The circulation of nitro- nel-shaped (e.g. white-egg bird’s nest, Cru- gen between organisms and the environ- cibulum laeve), containing several separate ment. Atmospheric gaseous nitrogen can spore cases (the ‘eggs’) that are exposed be used directly only by certain nitrogen- when the covering membrane dies back. fixing bacteria (e.g. Clostridium, Nostoc, The spores are dispersed by rain splash. Rhizobium). They convert nitrogen to am- Bird’s nest fungi may be found on the monia, nitrites, and nitrates, which are re- ground, or on wood or dung. leased into the soil by excretion and decay. Some are free-living, while others form ninhydrin A reagent used to test for the symbiotic associations with plants (see ni- presence of proteins and amino acids. A trogen fixation). Another method by which colorless aqueous solution turns blue in the atmospheric nitrogen is fixed is by light- presence of a-amino acids in solution. ning, which causes nitrogen and oxygen to When dissolved in an organic solvent it combine. The oxides so produced dissolve is used as a developer to color amino acids in rain to form nitrous and nitric acids; in on chromatograms. If a chromatogram the soil these acids combine with mineral treated with ninhydrin is heated strongly salts to form nitrites and nitrates. When the amino acids appear as purple spots that plants and animals die, the organic nitro- can be identified by measuring the Rf gen they contain is converted back into ni- value. Ninhydrin is carcinogenic. See also trate in the process termed nitrification. . Apart from uptake by plants, nitrate may

154 nucleic acids also be lost from the soil by denitrification formed at telophase have an extra chromo- and by leaching. The use of nitrogen fertil- some (i.e. are n + 1) and give a trisomic zy- izers in agriculture and the emission of ni- gote (i.e. 2n + 1) on fusion with a normal trous oxides in car exhaust fumes have gamete. If two gametes deficient for the influenced the nitrogen cycle and con- same chromosome fuse then nullisomy (2n tributed to urban air pollution and acid – 2) will result, which is almost always precipitation. See acid rain. lethal, and if two gametes with the same extra chromosomes fuse, tetrasomy (2n + nitrogen fixation The formation of ni- 2) results. All these abnormal chromosome trogenous compounds from atmospheric conditions are collectively referred to as nitrogen. In nature this may be achieved by aneuploidy. electric discharge in the atmosphere or by the activities of certain microorganisms. nonsense triplet A triplet of bases For example, symbiotic bacteria of the (CODON) in DNA or RNA that does not genus Rhizobium are associated with legu- code for an amino acid. Some nonsense minous plants (FABACEAE), causing the root codons (stop or termination codons) spec- cortex to form root nodules that house the ify the termination of polypeptide synthesis bacteria. These bacteria contain the nitro- during TRANSLATION. See genetic code. genase enzyme that catalyzes the fixation of molecular nitrogen to ammonium ions, nucellus (pl. nucelli) The parenchyma- which the plant can assimilate. In return tous tissue core of an ovule, enclosing the the legume supplies the bacteria with car- megaspore or egg cell (or in angiosperms bohydrate. the EMBRYO SAC). The pollen tube gains entry to the nucellus through a gap in the node The point of leaf insertion on a surrounding integuments called the MI- stem. At the apex of the stem the nodes are CROPYLE. In some angiosperm species the very close together but become separated nucellus persists as the perisperm, provid- in older regions of the stem by intercalary ing nourishment for the developing em- growth, which forms the internodes. In bryo. certain monocotyledons such as those forming bulbs, the nodes are very closely nuclear membrane (nuclear envelope) spaced on a condensed stem. See nucleus. noncompetitive inhibition A form of nuclease See endonuclease; exonucle- enzyme inhibition in which the inhibitor ase. binds to the enzyme at a site other than the active site, altering the conformation of the nucleic acid hybridization (DNA hy- enzyme and affecting its ability to bind bridization) The pairing of a single- with the substrate. stranded DNA or RNA molecule with another such strand, forming a DNA– noncyclic photophosphorylation See DNA or RNA–DNA hybrid. In order to photosynthesis. achieve hybridization the base sequences of the strands must be complementary. This nondisjunction The failure of homolo- phenomenon is exploited in many tech- gous chromosomes to move to separate niques, notably in GENE PROBES, which are poles during anaphase I of meiosis, both designed to bind to particular complemen- homologs going to a single pole. This re- tary base sequences among a mass of DNA sults in two of the four gametes formed at fragments. telophase missing a chromosome (i.e. being n – 1). If these fuse with normal haploid (n) nucleic acids Organic acids whose mol- gametes then the resulting zygote is mono- ecules consist of chains of alternating sugar somic (i.e. 2n – 1). The other two gametes and phosphate units, with nitrogenous

155 nucleolar organizer bases attached to the sugar units. Essen- sine, cytidine, guanosine, thymidine, and tially they are polymers of NUCLEOTIDES uridine are common nucleosides. formed by condensation reactions between the component nucleotides, which are nucleosome See chromosome. linked by phosphodiester bonds. There are two forms of nucleic acids, DNA (deoxyri- nucleotide The compound formed by bonucleic acid) and RNA (ribonucleic acid). condensation of a nitrogenous base (a In DNA the sugar is deoxyribose; in RNA PURINE or PYRIMIDINE) with a sugar (ribose it is ribose. Nucleic acids occur in the cells or deoxyribose) and phosphoric acid. ATP of all organisms, and may be single- or is a mononucleotide (consisting of a single double-stranded. nucleotide), the coenzymes NAD and FAD are dinucleotides (consisting of two linked nucleolar organizer See nucleolus. nucleotides), and the nucleic acids are polynucleotides (consisting of chains of nucleolus (pl. nucleoli) A more or less many linked nucleotides). spherical structure found in nuclei of eu- karyote cells, and easily visible with a light nucleus (pl. nuclei) An organelle of eu- microscope, staining densely with basic karyote cells containing the genetic infor- dyes. There may be one to several per nu- mation (DNA) and hence controlling the cleus. The nucleolus is the site of manufac- cell’s activities. It is the largest organelle, ture of ribosomal subunits and is thus most typically spherical and bounded by a dou- ble membrane, the nuclear envelope or nu- conspicuous in cells making large quanti- clear membrane, which is perforated by ties of protein. Nucleoli disappear during many pores (nuclear pores) that allow ex- cell division. The nucleolus synthesizes ri- change of materials with the cytoplasm. bosomal RNA (rRNA) and is made of The outer nuclear membrane is an exten- RNA (about 10%) and protein. It forms sion of the endoplasmic reticulum. In the around particular loci of one or more chro- nondividing (interphase) nucleus the ge- mosomes called nucleolar organizers. netic material is irregularly dispersed as These loci contain numerous tandem re- CHROMATIN; during nuclear division (mito- peats of the genes coding for ribosomal sis or meiosis) this condenses into densely RNA. TRANSCRIPTION of this code requires staining CHROMOSOMES, and the nuclear a specific RNA polymerase found only in envelope disappears, as do the nucleoli that the nucleolus. are normally present. See nucleolus. nucleoplasm The material of the NU- nullisomy See aneuploidy. CLEUS, especially the material inside the nu- clear envelope. nut A dry indehiscent fruit resembling an achene but derived from more than one nucleoprotein A compound consisting carpel. It has a hard woody pericarp and of a protein associated with a nucleic acid. characteristically a single seed. Cupules, Examples of nucleoproteins are the chro- supporting the nuts, may be distinctive of mosomes, made up of DNA, some RNA, the species. Examples are the fruits of the and histones (proteins); and the ribosomes beech (Fagus), hazel (Corylus), oak (Quer- (ribonucleoproteins), consisting of riboso- cus), and sweet chestnut (). See mal RNA and proteins. achene. nucleoside A molecule consisting of a nutation See circumnutation. PURINE or PYRIMIDINE base linked to a sugar, either ribose or deoxyribose. Adeno- nyctinasty (nyctinastic movements) The

156 O

obligate Describing an organism that re- penoids or derivatives of benzene. Some quires specific environmental conditions flowers, such as the Dalechampia (Eu- for its survival and cannot adopt an alter- phorbiaceae) and some orchids, produce native mode of living. For example, an ob- oils that are collected by pollinating bees. ligate aerobe is an organism that can grow Some repel insects or grazing animals, only under aerobic conditions and cannot while others (allelochemicals) deter en- survive in anaerobic conditions. Compare croaching neighboring plants. Resinous facultative. oils that appear at the site of wounds help to prevent loss of sap and protect against octadecanoic acid See stearic acid. the entry of pathogens and parasites. A short runner arising from an ax- oil immersion A microscopic technique illary bud near the base of a stem and using special high-powered objective growing horizontally above ground away lenses. A drop of immersion oil (e.g. cedar- from the parent, seen in daisies (Bellis) and wood oil) is placed on the coverslip of a mi- houseleeks (Sempervivum). It stores no croscope slide and the objective lens food but turns up at the end and produces carefully lowered into it. The oil has the a new plant from the apical bud. Like same refractive index as the lens glass and stolons and suckers, offsets are a means of increases the resolving power obtainable vegetative propagation in angiosperms. by letting a wider angle of rays enter the objective lens. oil 1. A TRIACYLGLYCEROL that is liquid at room temperature. The commonest fatty oleic acid An 18-carbon unsaturated acids in oils are oleic and linolenic acids, fatty acid occurring as the glyceride in oils which are unsaturated. Storage oils, syn- and fats. Oleic acid occurs naturally in thesized in the endoplasmic reticulum, larger quantities than any other fatty acid make up as much as 60% of the dry weight in plant cells except in chloroplasts, where of certain seeds, such as the castor bean linolenic acid is the dominant fatty acid. In (Ricinus communis), corn (Zea mays), many organisms oleic acid can be synthe- (Linum usitatissimum), (Glycine sized directly from stearic acid and further max), sunflower (Helianthus), safflower enzymatic paths exist for conversion to (Carthamus tinctorius), and mustards linoleic acid and linolenic acid. This path- (family Brassicaceae). Oils can be stored in way does not occur in humans and the oil bodies (elaiosomes) – oil-secreting higher animals so plant sources are an es- structures that attract ants to disperse the sential dietary element. seeds. Oil may also make up part of the fleshy part of a fruit, as in olive, the oil Oligocene The epoch of the Tertiary pe- palm, and palm. riod 38–25 million years ago. The world’s 2. Essential oils. Any of a number of climate ranged from temperate to subtrop- volatile oils secreted by aromatic plants ical, and the epoch was marked by an ex- that are the source of characteristic odors pansion of grasslands at the expense of or tastes. They are stored as droplets in forests, a change that promoted the evolu- plant glands. Most essential oils are ter- tion of large herbivores. Around the great

157 oligosaccharide

Tethys Sea there were tropical swamps, oogonium (pl. oogonia) The female re- which gave rise to extensive deposits of lig- productive organ of certain algae and nite in Germany and neighboring coun- fungi, often distinctly different in shape tries. The angiosperms were evolving and and size from the male reproductive organ diversifying rapidly, and overtook the (the antheridium). This initially unicellular gymnosperms in their abundance. Flowers gametangium contains one or more large and insects were evolving together, and nonmotile haploid eggs called oospheres. complex coevolution was taking place as These may be liberated prior to fertiliza- pollination mechanisms diversified. See tion, e.g. Fucus, or remain within the oogo- also geological time scale. nium, e.g. Pythium. oligosaccharide See polysaccharide. Oomycete See Oomycota. oligotrophic Describing lakes and ponds Oomycota (Oomycete) A phylum of that are low in nutrients and consequently funguslike protoctists that includes the low in . With little decaying water molds, downy mildews, and white vegetation, the oxygen content of the water rusts. Most oomycetes live in fresh water is adequate for fish and other animal life. or soil. They are mainly parasites or sapro- Compare eutrophic. phytes, and feed by extending hyphae into the tissues of their host, releasing digestive one gene–one enzyme hypothesis The enzymes and absorbing nutrients through theory that each gene controls the synthe- the hyphal walls. Some are important crop sis of one enzyme, which was advanced fol- pests, such as potato blight (Phytophthora lowing studies of nutritional mutants of infestans), damping off fungus (Pythium), fungi. Thus by regulating the production of mildew of grapes (Plasmopara viticola), enzymes, genes control the biosynthetic re- and Saprolegnia parasitica, a parasite of actions catalyzed by enzymes and ulti- aquarium fish. mately the character of the organism. Genes also code for proteins, or polypep- oosphere A large nonmotile female ga- tides that form proteins, other than en- mete or egg cell. In lower plants oospheres zymes, so the idea is perhaps more may be produced inside an oogonium; they accurately expressed as the one gene–one may be retained inside the oogonium and polypeptide hypothesis. fertilized there, or released for fertilization in the water. In angiosperms the oogonium ontogeny The course of development of is enclosed in the EMBRYO SAC and pro- an organism from fertilized egg to sexual tected by the integuments of the ovule. maturity and the production of gametes and the next generation. Occasionally on- oospore A diploid zygote of certain pro- togeny is used to describe the development toctists, produced by fertilization of the fe- of an individual structure. male gamete (the oosphere), by the male gamete, the antherozoid. The zygote may oogamy Sexual reproduction involving form a thick coat and go through a resting the fusion of two dissimilar gametes. The period before germination. The term male gamete is usually motile and smaller ‘oospore’ distinguishes a zygote produced than the female gamete, which is usually oogamously from a zygote produced isog- nonmotile, contains a food store, and may amously or anisogamously and called a zy- be retained by the parent. The term is gen- gospore. See zygote. erally restricted to descriptions of plants, particularly those that produce female ga- OP See osmotic pressure. metes in oogonia. It is an extreme form of anisogamy. Compare isogamy. See anisog- operator gene A bacterial regulatory amy. gene that contains the genetic code for

158 starting TRANSCRIPTION of one or more But if lactose binds to the regulator pro- structural genes. The structural genes to- tein, the regulator can no longer bind to the gether with the relevant operator genes are operator, so transcription can proceed and linked together in a functional unit called enzyme synthesis is initiated. See Jacob– an OPERON. The operator is under the con- Monod model; repressor. trol of a regulator gene, which codes for a small protein called a repressor molecule, Ophioglossales (moonworts and which binds to the operator gene and pre- adder’s-tongue ferns) An order of euspo- vents it from initiating transcription. If rangiate ferns (see Filicinophyta) charac- production of the repressor ceases or if an- terized by fronds that are divided into a other molecule, such as the substrate of an distinct sterile green blade and a fertile enzyme, binds with it, preventing it from spike bearing sporangia sunken into its binding to the operator gene, TRANSCRIP- sides. Unlike all other ferns, the fronds are TION can proceed. not rolled up as they push up through the ground. They are found in temperate and operculum (pl. opercula) 1. The circu- tropical regions. lar lid of the capsule of many mosses. It covers the peristome and may be forcibly An ALKALOID derived from the blown off the capsule by pressure develop- opium poppy () in a ing in the lower portion of the capsule, or latex that oozes from cut seed capsules. by the swelling of cells of the annulus. are highly addictive drugs that re- 2. In the pollen grains of many species, a lid semble endorphins, chemicals naturally covering the aperture, which is displaced produced in the mammalian body that sup- by the emerging pollen tube. press pain and enhance mood by occupy- 3. A cap covering the ostiole of the perithe- ing certain receptor sites on nerve cells that cium in certain ascomycetes. affect transmission of impulses. Opiates occupy the same receptor sites. They are operon A genetic unit found in prokary- the source of codeine, morphine, , otes and comprising a group of closely and other drugs, many of which have linked genes acting together and coding for medicinal uses in the treatment of severe the various enzymes of a particular bio- pain. chemical pathway. They function as a unit, because either all are transcribed or none. Orchidaceae The orchid family com- At one end is an operator, which contains prising monocotyledonous perennial herbs. a site called the PROMOTER. For transcrip- The family includes terrestrial forms, epi- tion of the structural genes (the genes that phytes, and full saprophytes with no green actually code for the enzymes) to proceed, parts (e.g bird’s-nest orchid (Neottia nidus- mRNA polymerase must first bind with the avis) and coralroot, (Corallorhiza spp.)). It promoter. Whether or not this happens is is the largest family of flowering plants controlled by another gene outside the containing over 25,000 species. See illus- operon, the regulator gene. The regulator tration overleaf. gene produces a regulator protein that binds with the operator, renders it inoper- order A collection of similar families. ative, and so prevents enzyme production. The Latin names of plant orders generally The presence of a suitable substrate pre- end in ales (e.g. Liliales). Orders may be di- vents this binding, and so enzyme produc- vided into suborders. Similar orders consti- tion can commence. An example is the lac tute a class. See International Code of operon in the bacterium Escherichia coli, Botanical Nomenclature. which is involved in the metabolism of lac- tose. In the absence of an inducer molecule Ordovician The second oldest period of (in this case, the substrate lactose or one of the Paleozoic era, some 510–440 million its derivatives), no transcription occurs. years ago. Algae were abundant in the

159 organ

Orchidaceae: front view of an orchid flower oceans, especially mat-forming algae and the nucleus; other examples are CHLORO- algae in symbiotic relationships in the PLASTS, MITOCHONDRIA, VACUOLES, and RI- corals of the reefs dominating the many BOSOMES. Organelles allow division of shallow, warm seas of the time. Some algae labor within the cell. may also have been terrestrial, forming mats on wet ground. See also geological organochlorines Organic compounds time scale. containing chlorides, used in such as DDT, and in other chemicals used organ A part of an organism that is to control plant pests, which are not read- made up of a number of different tissues ily broken down and persist in tissues of specialized to carry out a particular func- animals, affecting breeding success and tion. Examples include the stem, leaf, causing other problems. They are banned flower, and root. from use in most countries, but are still used in some, especially where insect-borne organ culture The maintenance or diseases such as malaria are prevalent. growth of living organs, usually embry- onic, in vitro. Mature plant organs, no- origin of life The development of living tably roots, may be cultured indefinitely organisms from inorganic matter. Geolog- in suitable media. Leaves, embryos, meris- ical evidence strongly suggests that life tems, and ovules have also been success- originated on earth about 4600 million fully cultured. years ago. The earliest evidence of life is bacterialike fossils in rocks over 3500 mil- organelle A discrete membrane- lion years old. The basic components of or- enclosed subcellular structure with a par- ganic matter – water, methane, ammonia, ticular function. The largest organelle is and related compounds – were abundant in

160 osmotic pressure the atmosphere, which had much greater tainer of pure solvent. Solvent entering the reducing properties than today’s atmos- porous pot by osmosis lifts the piston until phere. Until recently, it was widely ac- the force of the water entering the pot cepted that life evolved in warm lagoons or equals the hydrostatic pressure created by hot springs, where energy from the sun the weight of the piston. (cosmic rays) and lightning storms caused simple molecules to recombine into in- osmosis The movement of solvent from creasingly complex organic molecules that a dilute solution to a more concentrated so- eventually showed the characteristics of lution through a semipermeable membrane living organisms. However, recent discov- (selectively permeable membrane) – one eries suggest that life may have evolved that allows the passage of some kinds of deep underground in fissures in hot rocks – molecule and not others. For example, if a a habitat in which today vast numbers of concentrated sugar solution (in water) is thermophilic bacteria live. Life probably separated from a dilute sugar solution by a had its origins in organic self-replicating selectively permeable membrane, water molecules that consumed the chemicals molecules can pass through from the dilute around them to duplicate themselves. solution to the concentrated one by diffu- These molecules were probably a form of sion. RNA. Osmosis between two solutions will continue until they have the same concen- ornithine A nonprotein AMINO ACID that tration. If a certain solution is separated occurs in the cell walls of certain bacteria. from pure water by a membrane, osmosis It is an important intermediate in the syn- also occurs. The pressure necessary to stop thesis of the amino acid arginine, and the this osmosis is called the OSMOTIC PRESSURE synthesis of many alkaloids. (OP) of the solution. The more concen- trated a solution, the higher its osmotic orthogenesis An early theory of evolu- pressure. Osmosis is a very important fea- tion in which evolutionary change was en- ture of both plant and animal biology. Cell visaged as occurring in a definite direction membranes act as differentially permeable and along a predetermined route, irrespec- membranes and osmosis can occur into or tive of natural selection. out of the cell. It is necessary for an animal to have a mechanism of osmoregulation to orthotropism See tropism. stop the cells bursting or shrinking. In the case of plants, the cell walls are slightly orthotropous (atropous) See ovule. ‘elastic’ – the concentration in the cell can be higher than that of the surroundings, osmium tetroxide A stain used in elec- and osmosis is prevented by the pressure tron microscopy because it contains the exerted by the cell walls (wall pressure). heavy metal osmium. It is fat-soluble, and Where the solvent is water, physiologists unsatured fats will reduce it to black os- now describe the tendency for water to mium dioxide. It therefore stains lipids, in- move in and out of cells in terms of WATER cluding membranes, particularly intensely. POTENTIAL. It also acts as a fixative. osmotic potential See water potential. osmometer An instrument that is used to measure OSMOTIC PRESSURE (WATER PO- osmotic pressure (OP) The force (pres- TENTIAL). In its simplest form, a porous pot sure) that must be applied to a solution to is made selectively peremeable by impreg- prevent the passage of water or another nating its pores with copper ferrocyanide. pure solvent through a selectively perme- The pot is filled with an osmotically active able membrane separating the solvent from solution, and a piston placed on the solu- the solution (i.e. to prevent OSMOSIS occur- tion. The pot is then immersed in a con- ring). Osmotic pressure rises as the concen-

161 Osmundales tration of the solution rises. The osmotic tercup (Ranunculus). In a floral formula a pressure is equal to the diffusion pressure superior ovary is denoted by a line below deficit plus the turgor pressure. This term is the carpel number. After fertilization, the becoming obsolete, as it ignores capillary ovary wall becomes the pericarp of the and imbibitional forces that are significant fruit enclosing seeds in its central hollow. in certain situations. A better concept is that of WATER POTENTIAL. ovate Up to four times as long as broad, tapering at both ends, with the broadest Osmundales (royal ferns) An order of part below the middle. ferns often large, retaining some primitive features, whose whose sporangia combine ovule Part of the female reproductive eusporangiate and leptosporangiate char- organs in seed plants. It consists of the nu- acters (see Filicinophyta). cellus, which contains the embryo sac, sur- rounded by the integuments. The ovule is ostiole A small pore through which surrounded by a cuticle, and is thought to spores or gametes are released from the re- be derived from the megasporangium of its productive bodies of certain fungi and nonseed-bearing ancestors. After fertiliza- algae. Examples include the CONCEPTACLES tion the ovule develops into the seed. In an- of wracks (Fucus spp.). giosperms the ovule is contained within an ovary, and attached to the placental tissue outbreeding Breeding between individ- of the ovary by a stalk, the funiculus. It uals that are not closely related. In plants may be orientated in different ways being the term is often used to mean cross- fertilization, and various methods exist to promote it, e.g. stamens maturing before pistils, SELF-INCOMPATIBILITY. The most ex- treme form – crossing between species – usually results in sterile offspring and there are various mechanisms to discourage it. Outbreeding increases heterozygosity, giv- ing more adaptable and more vigorous populations. Compare inbreeding. See het- erostyly; protandry; protogyny. ovary The swollen base of the CARPEL in the gynoecium of plants, containing one or more ovules, each attached by a funiculus. The gynoecium of angiosperms may con- sist of more than one carpel that fuses in certain species forming a complex ovary. Where the fused carpel walls remain intact, this is a multilocular ovary; where they break down it becomes a unilocular ovary. If the ovary lies below the other floral or- gans in a flower, it is said to be an inferior ovary, as in Rosa. In such cases, the ovary usually appears sunk into and fused with a cup-shaped receptacle. In a floral formula, an inferior ovary is denoted by a line above the carpel (gynoecium) symbol and num- ber. If the petals, sepals, etc. are inserted below it, it is a superior ovary, as in but- Ovule: types of orientation

162 ozone layer upright, inverted, or sometimes horizontal. toward the formation of citric acid in a re- Where the developing ovule has turned action involving pyruvate and coenzyme A, through 180°, so that the micropylar end part of a series of reactions that are used to has folded over and lies close to the base of replace Krebs cycle intermediates removed the funiculus, it is termed anatropous. This for use in biosynthesis. When these reac- is the most common position of the ovule tions run in reverse order, they form a in flowering plants. Where the ovule has pathway for glucose synthesis. developed vertically so that the micropylar end is directly over the funiculus, it is oxidase An enzyme that catalyzes a re- termed orthotropous, e.g. Polygonum. action involving the oxidation of a sub- Where the funiculus appears to be attached strate using molecular oxygen as electron half way between the chalaza and the mi- acceptor, e.g. cytochrome c oxidase in the cropyle, and the micropylar end is turned respiratory ELECTRON-TRANSPORT CHAIN. through 90° relative to the orthotropous Many oxidases are flavin-linked dehydro- condition so that the ovule is horizontal, it genases in which the reduced COENZYME is termed campylotropous, e.g. mallow can be reoxidized by molecular oxygen to (Malva sylvestris). form water and hydrogen peroxide. See de- In gymnosperms ovules are larger but hydrogenase; oxidative phosphorylation. are not contained within an ovary. They are borne on ovuliferous scales. Gym- b-oxidation The main pathway for the nosperm seeds are thus naked while an- breakdown of fatty acids to acetyl CoA. b- giosperm seeds are contained within a oxidation involves the removal of carbon fruit, which develops from the ovary wall. acids from fatty acid chains two at a time, forming one molecule of acetyl CoA each ovuliferous scale The megasporophyll time. The acetyl CoA may enter the Krebs found in the axis of the bract scale in the fe- cycle and be further oxidized, or it may male strobili of the Coniferophyta. It is a enter the glyoxylate cycle and be converted large woody structure and bears ovules to sugar. b-oxidation is a major energy- and later seeds, on its upper surface. It is liberating process in germinating oil-rich thus homologous with the carpel of the An- thophyta. seeds, where the associated enzymes are located in the glyoxysome together with ovum (pl. ova)(egg cell) An unfertilized those of the glyoxylate cycle. egg cell. oxidative phosphorylation The pro- oxalic acid (ethanedioic acid) A dicar- duction of ATP from phosphate and ADP boxylic acid, that occurs in various plants, in aerobic respiration. Oxidative phospho- e.g. in the leaves of rhubarb (Rheum), rylation occurs in mitochondria, the energy dumbcanes (Dieffenbachia), wood sorrel being provided by steps in the electron- (Oxalis acetosella), and the garden oxalis transport chain. (Oxalis spp.). It plays a role in the removal of excess cations, such as sodium, potas- oxygen An element essential to living sium, and calcium, which are incorporated organisms both as a constituent of carbo- into oxalates. hydrates, fats, proteins, and their deriva- tives, and in aerobic respiration. It enters oxaloacetic acid (OAA) A water-solu- plants in both carbon dioxide and water, ble carboxylic acid, structurally related to the oxygen from water being released in fumaric acid and maleic acid. Oxaloacetic gaseous form as a byproduct of photosyn- acid is an intermediate of the KREBS CYCLE, thesis. Plants are the main, if not the only, and in the Hatch–Slack pathway of C4 source of gaseous oxygen and as such are PLANTS. It is produced from L-malate in an essential in maintaining oxygen levels in NAD-requiring reaction and itself is a step the air for aerobic organisms.

163 ozone layer

b-oxidation ozone layer A layer consisting of ozone stabilizes the stratosphere, preventing it (O3) molecules scattered through the from mixing with other layers, so reducing stratosphere roughly 15–50 km above the winds in the lower atmosphere. earth’s surface. The ozone is formed by the In the early 1980s it was discovered action of solar radiation on oxygen mol- that the ozone layer was becoming thinner, ecules. The ozone layer absorbs about with seasonal ‘holes’ appearing over 99% of the harmful ultraviolet radiation Antarctica and elsewhere. This depletion is entering the earth’s atmosphere, and hence believed to be caused by increasing atmos- provides a shield for living organisms with- pheric concentrations of chlorine com- out which life on earth would be impos- pounds, especially chlorofluorocarbons sible. This absorption of ultraviolet (CFCs) – very stable chemicals used as re- radiation has two consequences: it warms frigerants and aerosol propellants – which the stratosphere, helping to maintain the disrupt the delicate balance between ozone temperature of the Earth’s surface; and it production and breakdown in the atmos-

164 P

P680 See photosynthesis. impressions left by them. Sometimes the subject is divided into , the P700 See photosynthesis. study of fossil plants, paleozoology, the study of fossil animals, and PALEOECOLOGY. pachytene In MEIOSIS, the stage in mid- prophase I that is characterized by the Paleozoic The first and oldest era in contraction of paired homologous chro- which multicellular life became abundant, mosomes. At this point each chromosome about 590–230 million years ago. It is di- consists of a pair of chromatids and the vided into six main periods: the Cambrian, two associated chromosomes are termed a Ordovician, Silurian, Devonian, Carbon- tetrad. iferous, and Permian. Beginning with aquatic invertebrates and algae, the era pairing See synapsis. ended with the invasion of land by tree ferns and reptiles. See also geological time palea See bract. scale. paleobotany See . palisade mesophyll See mesophyll.

Paleocene The oldest epoch of the Ter- palmella (pl. palmellae) A stage formed tiary, 65–55 million years ago. It was a pe- under certain conditions in various unicel- riod of warm humid climate with large lular algae in which, after division, the areas of temperate and subtropical forests, daughter cells remain within the envelope but only limited grasslands, as the grasses of the parent cell and are thus rendered im- were of very recent origin. During the Pa- mobile. The cells may continue dividing leocene there was a great deal of coevolu- tion of plants and animals, the plants giving a multicellular mass, which is con- tained in a gelatinous matrix. Palmelloid evolving adaptations to animal pollination and fruit and seed dispersal. Bilateral flow- forms may develop undulipodia and revert ers evolved, e.g. those of the pea family to normal mobile cells at any time. (Fabaceae) and orchids (Orchidaceae). In the latter part of the Paleocene, large an- palmitic acid (hexadecanoic acid) A 16- giosperm fruits and seeds evolved as fruit- carbon saturated fatty acid occurring and seed-eating mammals also evolved and widely in fats and oils of animal and veg- diversified. See also geological time scale. etable origin. It is the commonest saturated fatty acid in plants, and an intermediate in paleoecology The investigation of pre- the synthesis of oleic and stearic acids. See historic ecology as revealed by studying also carboxylic acid; oleic acid. fossils and ancient human artefacts, pollen (see palynology) samples, and the mineral palynology The study of living and fos- deposits in which such structures occur. sil pollen. It is mainly used as a means of obtaining information on the composition paleontology The study of extinct or- and extent of past floras, and may give re- ganisms, including their fossil remains, and liable quantitative information on the veg-

165 panicle etative cover many thousands of years ago. cose, which is the assimilation product of See also pollen analysis. euglenas (Eustigmatophyta). panicle See inflorescence. paraphysis (pl. paraphyses) A sterile unbranched (usually) multicellular hair pantothenic acid (vitamin B5) The pre- found in large numbers between the re- cursor of coenzyme A, pantothenic acid productive organs of certain algae and is essential for several fundamental reac- bryophytes. Club-shaped hyphae in the hy- tions in metabolism. It is one of the water- menial layer of certain basidiomycete fungi soluble B group of VITAMINS, and is synthe- are also termed paraphyses. sized by plants and bacteria but not by ver- tebrates, so is an essential vitamin in their parasitism An association between two diet. organisms in which one, the parasite, ben- efits at the expense of the other, the host. paper chromatography A chromato- The tolerance of the host varies from being graphic method by which minute amounts almost unaffected to serious illness and of material can be analyzed. An absorbent often death. An obligate parasite such as paper strip with a drop of test material at potato blight (Phytophthora infestans) can the bottom is dipped into the solvent live only in association with a host, (which rises up the paper by capillary ac- whereas a facultative parasite such as the tion) and removed when the solvent front damping-off fungus (Pythium) can exist in almost reaches the top of the strip. other ways, for example as a saprophyte. papilla (pl. papillae)A short sometimes paratonic movements Movements of cone-shaped projection from a cell, usually plants or plant parts in response to external an extension of the outer wall of an epi- stimuli. They may be divided into mechan- dermal cell. Papillae are often found on ical movements (e.g. the hygroscopic petals, giving them a velvety appearance. movements of dead cells) and movements See hair. caused by the stimulation of sensitive cells (e.g. tropisms and nastic movements). pappus (pl. pappi) The ring of hairs, Taxes are also paratonic movements. scales, or teeth that makes up the calyx in Compare autonomic movements. flowers of the ASTERACEAE. It persists on the fruit and serves to aid the wind dispersal of parenchyma Tissue made up of living seeds. An example is the cypsela of the dan- thin-walled cells that are not differentiated delion (Taraxacum) in which the pappus for any specific function, but in which im- remains attached by a long thin stalk and portant metabolic processes are carried acts as a parachute. out. The leaf mesophyll and the stem medulla and cortex consist of parenchyma. parallel evolution (parallelism) The de- The vascular tissue is also interspersed velopment of similar features in closely with parenchyma; for instance the related organisms as a result of strong se- medullary rays of secondary vascular tis- lection in the same direction. This may sue. Parenchyma often forms a basic occur between species of the same genus ground tissue in which other tissues are that are widely separated geographically embedded. It is generally considered to be but which live in similar environments. the tissue from which other types of tissues There are few examples of this phenome- evolved. non and some authorities deny its exis- tence. Compare convergent evolution. parthenocarpy The development of fruit in unfertilized flowers, resulting in paramylum A starchlike polysaccharide seedless fruits. It may occur naturally, as in composed of b(1-3)-linked units of glu- the banana (), which is triploid. It

166 pepo may also be induced artificially by the ap- pelagic Inhabiting the open upper wa- plication of auxin, as in commercial ters rather than the bed of a sea or ocean. tomato growing. Compare benthic. See also photic zone; plankton. parthenogenesis Development of un- fertilized ovules to form new individuals. It pellicle (periplast) A thin flexible trans- occurs regularly in certain plants, e.g. dan- parent outer protective covering of some delion (Taraxacum), in which meiosis is unicellular algae that lack a cellulose cell defective, resulting in a diploid egg. In wall, e.g. Euglena. It is made of protein, hawkweeds (Hieracium) a cell of the nucel- and maintains the shape of the body. lus takes the place of the megaspore. See also apomixis. Pennsylvanian The US name for the late Carboniferous period in the geological partial dominance See incomplete time scale, 320 to 286 million years ago. dominance. Rocks that originated during this period are widespread in the state of Pennsylva- passage cells Endodermal cells that nia, USA. have CASPARIAN STRIPS but remain other- wise unthickened after deposits of lignin pentose A SUGAR that has five carbon and cellulose have been laid down else- atoms in its molecules. Examples include where in the endodermis. Passage cells are ribose, deoxyribose, and arabinose. Pen- found opposite the protoxylem and allow toses are synthesized in the Calvin cycle, transport of water and solutes from the derived from hexoses in the PENTOSE PHOS- cortex to the stele. PHATE PATHWAY, and also made by decar- boxylation of nucleoside diphosphate pathogen Any organism that is capable sugars. Polymers of pentoses are called of causing disease or a toxic response in an- pentosans. other organism. pentose phosphate pathway (hexose PCR See polymerase chain reaction. monophosphate shunt)A pathway of glucose breakdown in which pentoses are peat Partially decomposed plant mater- produced, in addition to reducing power ial that accumulates in waterlogged anaer- (NADPH) for many synthetic reactions. It obic conditions in temperate humid is an alternative to GLYCOLYSIS, and pro- climates, often forming a layer several me- duces sugars containing 3 to 7 carbon ters deep. Peat varies from a light spongy atoms. Glucose 6-phosphate is decar- material (Sphagnum moss) to a dense boyxlated and converted to a 5-carbon brown humidified material in the lower sugar, with the formation of NADPH. This layers. 5-C sugar than enters a sequence of reac- tions, until eventually glucose 6-phosphate pectic substances Polysaccharides that, is reformed. together with hemicelluloses, form the ma- trix of plant cell walls. They serve to ce- PEP See phosphoenolpyruvate. ment the cellulose fibers together. Fruits are a rich source, especially apples (Malus), pepo A succulent fruit resembling a pears (Pyrus communis), and citrus fruits. berry. The hard outer ring orginates either See also middle lamella. from the epicarp or from noncarpellary tis- sues of the plant. In members of the Cu- pedicel See inflorescence. curbitaceae, such as Cucurbita species (e.g. cucumber, squash, and pumpkin), the rind peduncle See inflorescence. is formed from the receptacle, while the

167 peptidase

Peptide bond flesh enclosing the seeds is derived from the ride molecules cross-linked by short pep- ovary wall. tides chains to form a rigid framework. peptidase An enzyme that is responsible perennating organ In biennial or peren- for catalyzing the hydrolysis of certain pep- nial plants, a that enables the tide bonds. The peptidases help break plant to survive an adverse season, such as down peptides into amino acids. winter or a dry season (perennation). Most such organs are underground, e.g. rhi- peptide A molecule consisting of a num- zomes, corms, bulbs, swollen taproots, and ber of amino acid molecules linked to- tubers. Perennating organs are also used by gether. Peptides can be regarded as formed many plants in asexual reproduction (veg- etative growth). Buds developing in the by a CONDENSATION REACTION in which the carbonyl group of one amino acid reacts axils of scale leaves on CORMS, BULBS, and stem TUBERS may become detached, put with the amino group of another amino down ADVENTITIOUS roots, and grow into acid with the elimination of water. This independent plants. Similarly, new shoots link between amino acids is called a pep- arise from buds on RHIZOMES, and the in- tide bond . According to the number of tervening segments of may break amino acids linked together they are called or rot away. di-, tri-, oligo-, or polypeptides. In general, peptides have an amino group at one end perennation See perennating organ. of the chain and a carboxyl group at the other. They can be produced by the partial perennial A plant that may live for sev- hydrolysis of proteins. eral years. Perennials may reproduce in their first growing season or may have peptide bond See peptide. to attain a certain age before seed produc- tion commences. Herbaceous perennials peptidoglycan The principal compo- die back each year and survive until the nent of the cell walls of most BACTERIA (but next growing season as tubers (e.g. Dahlia), not of Archaea), consisting of polysaccha- bulbs (e.g. daffodil, Narcissus), rhizomes

168 peristome

(e.g. Iris), etc. Woody perennials, such as periclinal Describing a line of cell divi- trees and shrubs, persist above ground sion parallel to the surface of the organ, re- throughout the year but may show adapta- sulting in the formation of periclinal walls tions (e.g. leaf fall) to survive unfavorable between daughter cells and an increase in seasons. girth of the organ. Compare anticlinal. perforation plates The remains of the pericycle The parenchymatous layer of cross walls between the vessel elements in cells that lies within the endodermis, form- the xylem vessels of angiosperms, the Gne- ing the outermost part of the stele. It re- tophyta, and some ferns, which leave an mains meristematic in most roots and gives opening allowing water to move freely rise to the lateral roots. In most dicotyle- through the vessel. The cross wall may donous roots showing secondary growth, have disintegrated completely so that the the pericycle is also involved in the origina- vessel is effectively one long cylinder (e.g. tion of the vascular cambium and the phel- oak) or parts of the cross wall may remain logen. as bars across the cavity of the vessel, e.g. alder (Alnus). Compare sieve plate. periderm The secondary tissue that arises from the activity of the cork cam- perianth The part of the flower that en- bium. It forms the outer layer of the stem circles the stamens and carpels. It usually and root in plants with secondary growth, consists of two whorls of leaflike structures having a protective role. It is made up of that are differentiated into the sepals (calyx) the PHELLOGEN (cork cambium), which and petals (corolla). In many monocoty- gives rise to the PHELLEM (cork) and PHEL- ledons there is no such distinction, and the LODERM (secondary cortex). Periderm may individual perianth units are called , also develop at the site of a wound, form- e.g. tulip (Tulipa). In some flowers, espe- ing a barrier to pathogens. cially those that are wind pollinated, the perianth is reduced (e.g. grasses) or absent perigyny The arrangement in flowers in (e.g. ). which the perianth and androecium are in- serted on the receptacle around the gynoe- periblem See ground meristem; histogen cium rather than above or below it. The theory. receptacle is extended to a flat or saucer- shaped organ and carries the gynoecium in pericarp The ovary wall that becomes the middle and the other floral parts the wall of the FRUIT as the fruit develops. around the edge. The OVARY is technically Depending on how the pericarp tissues superior even though in extreme forms of differentiate, the resulting fruit may be perigyny in which the receptacle is cup- dry (papery or leathery) or succulent. The shaped, e.g. the wild rose (Rosa sp.), the outer layer is the exocarp, often a tough floral parts may be inserted at a level above skin. The middle layer is the mesocarp, the gynoecium. In perigynous flowers the whose texture varies with different fruits, ovary is not fused with the receptacle. e.g. it is juicy in drupes like the plum Compare epigyny; hypogyny. (Prunus domestica), and hard in almonds (Prunus dulcis). The mesocarp may be pro- periplast See pellicle. tective or aid in dispersal. The endocarp is the innermost layer and forms the stony perisperm The nutritive tissue in the covering of the seed in a drupe, but in other seeds of many Caryophyllaceae (the carna- fruits (e.g. the berry), it is indistinguishable tion family). It is a diploid tissue derived from the mesocarp. If the pericarp opens from the nucellus rather than the em- when ripe, the fruit is said to be DEHISCENT; bryosac. Such seeds therefore differ from if it remains closed, it is INDEHISCENT. other angiosperm seeds that have either

169 perithecium cotyledons or endosperm tissue acting as restrial habitats; they include downy the food store. mildews (Peronospora spp.), late blight of potatoes (Phytophtherainfestans), and peristome A single or double ring of damping-off fungus (Pythium spp.). teeth around the opening of the capsule in mosses that is involved in spore dispersal. peroxidase An enzyme that catalyzes It is revealed when the operculum falls off the oxidation of certain organic molecules or is removed. The teeth absorb moisture using hydrogen peroxide as electron accep- from the atmosphere, and the coiled fibers tor. of which they are made change length and twist and bend when subjected to humidity peroxisome See microbody. changes, either opening and closing the capsule to release the spores or scattering petal One of the usually brightly colored them through jerky movements. parts of the flower, which together make up the corolla. The petals are pigmented perithecium (pl. perithecia) See as- and often scented to attract insects. They coma. are reduced or absent in most wind-polli- nated flowers. Petals are thought to be permanent stain See staining. modified leaves, with a much simplified in- ternal structure and vascular system, hav- permanent wilting point The point at ing only one vascular bundle. They may which soil has dried to the extent that derive from sterile stamens. Compare plants can no longer remove the remaining sepal. water held on the soil particles, and begin to wilt and will not recover, even if moved petiole The stalk that attaches the leaf to a cool, dark place, unless more water is blade to the stem. It is similar to the stem added to the soil. At permanent wilting except that it is asymmetrical in cross- point the water potential of the soil is equal section with the vascular and strengthening to or lower than the water potential of the tissues arranged in a V shape rather than a plant. circle. Leaves that lack a petiole are termed sessile. Permian The most recent period of the Paleozoic era, some 280–250 million years Pezizales (cup fungi; morels) An order ago. As the climate warmed and dried of ascomycete fungi in which the asci (see during the Permian, the ferns, seed ferns, ascus) are borne in a HYMENIUM that lines and that dominated Permian the surface of a cup-shaped fruiting body marshes and swamps were joined by the (apothecium). The spores are discharged evolving coniferophytes and some other forcibly into the air. Most are saprobes liv- gymnosperms, whose evolution was prob- ing in soil, dung, or rotting wood. A few ably driven by the need to cope with drier species are involved in mycorrhizae. environments. These groups gradually began to replace the older floras domi- pH A measure of the acidity or alkalinity nated by nonseed-bearing plants. See also of a solution on a scale 0–14. A neutral so- geological time scale. lution (such as pure water) has a pH of 7. Acid solutions have a pH below 7; alkaline Peronosporales An order of the OOMY- solutions have a pH above 7. The lower the COTA characterized by sexual reproduction pH value, the higher the concentration of + + (usually) by zoospores and sexual repro- H . The pH is given by log10 (1/[H ]), duction by zoospores produced from where [H+] is the hydrogen ion concentra- oospores. The Peronosporales contains tion in moles per liter. Thus one integer on saprobes and many harmful plant para- the pH scale represents a tenfold difference sites. Most are parasites found in damp ter- in the concentration of H+.

170 phenolphthalein

Phaeophyta (brown algae)A phylum of condenser. Light that is not diffracted protoctists comprising mainly marine passes through the holes in the plate. Light algae, notably the macroscopic thallose that has been diffracted by the specimen seaweeds that inhabit the intertidal zones. passes through the thicker part of the plate, They contain the pigments chlorophyll a so is slowed down even further. This tech- and c, b-carotene, and the xanthophylls nique can be used to study real-time events (including fucoxanthin), which give the such as cell division. algae their characteristic brown color. phellem (cork) A protective layer of rad- phage See bacteriophage. ically arranged cells produced to the out- side of the PHELLOGEN (cork cambium). phagocytosis The taking up of food The cork replaces the epidermis in certain particles by a cell using protrusions of the woody plants, forming an impervious layer outer part of the cell, generated by cyto- broken only by lenticels. The older cork plasmic flow. These protrusions surround cells are dead, suberized, and frequently the particle, enclosing it in a membrane- only air-filled, although lignin, fatty acids, bound vesicle and so introducing it into the and tannins often accumulate. The cork cell as a food vacuole. Lysosomes then con- oak, , of Southern Europe, gregate around the food vacuole, fusing forms a very thick layer and is the main with it and releasing hydrolytic enzymes to source of commercial corks. digest the particle. See endocytosis. phelloderm The inner layer of the PERI- Phallales (stinkhorns) An order of ba- DERM. During secondary growth, the sidiomycetes in which the BASIDIOMA has a meristematic PHELLOGEN (cork cambium) two-layered wall enclosing a slimy, gelati- gives rise to parenchyma tissue, the perid- nous layer. See Basidiomycota. erm, which forms the secondary cortex. The cells of the phelloderm arise in radial phanerogam In early classifications, columns from the phellogen, and this dis- any plant whose reproductive organs are tinguishes them from the primary cortex. flowers or cones. Compare cryptogam. phellogen (cork cambium; periderm cam- phanerophyte A perennial plant with bium) A subepidermal layer of cells persistent shoots and buds on upright forming a lateral meristem that arises fol- stems well above soil level. Phanerophytes lowing the onset of secondary growth. The include herbs, shrubs, trees, and climbing cells of the phellogen (cork cambium) give plants. See Raunkiaer’s plant classification. rise externally to the PHELLEM (cork) and internally to the PHELLODERM. Phanerozoic See Precambrian. phelloid A thin-walled unsuberized cork phase contrast microscopy A form of cell. light microscopy that amplifies the differ- ence in the phase of light after passing phenolics (phenols) Compounds that through a specimen to give greater contrast contain a benzene ring or other aromatic in the image. It is based on the fact that ring substituted with one or more hydroxyl light is diffracted (changes phase) accord- groups, e.g. phenol (C6H5OH). Phenolics ing to the refractive index or thickness of are similar to alcohols but are more soluble the material through which it passes. The in water. Many have sharp, spicy odors. It phase contrast microscope uses a special is thought that most phenolics are secreted condenser, which produces a particular by plants to deter herbivores as many are pattern of light, usually a hollow cone. enzyme inhibitors and disrupt digestion. This pattern is matched by a groove in a Plant phenolics include the FLAVONOIDS, glass plate (the retardation plate) in the LIGNIN, and TANNINS.

171 phenotype phenolphthalein An acid–alkali indica- linking a sugar and a phosphate group by tor that is colorless in acids and red in al- means of an oxygen bridge, for example, kalis. It has a pH range from 8.4–10.0 and the bonds linking sugars and phosphate is a frequently used indicator for the detec- groups in the backbone of NUCLEIC ACIDS. tion of pH change in acid–alkali titrations. Phosphodiester bonds are formed by con- Phenolphthalein, together with borax, is densation reactions and may be broken by used to test for saccharide derivatives, e.g. hydrolysis. glycerol. Such derivatives turn the solution from red to colorless but on boiling the red phosphoenolpyruvate (PEP) A 4-car- color returns. bon phosphorylated organic acid that is the main substrate for carbon dioxide fixa- phenotype The observable characteris- tion in C4 PLANTS, being carboxylated to tics of an organism, which are determined oxaloacetic acid in a reaction mediated by by the interaction of the GENOTYPE with the the enzyme phosphoenolpyruvate carboxy- environment. Many genes present in the lase (PEP carboxylase). It is also an impor- genotype do not show their effects in the tant intermediate of GLYCOLYSIS, being the phenotype because they are masked by immediate precursor of pyruvic acid. In the dominant alleles. Genotypically identical GLYOXYLATE CYCLE for the synthesis of glu- organisms may have very different pheno- cose it is generated from oxaloacetate. types in different environments, an effect particularly noticeable in plants grown in phosphoglyceric acid See glycerate 3- various habitats. phosphate. phenylalanine An aromatic AMINO ACID phosphoglyceride (phospholipid)A com- derived from phosphoenolpyruvate and plex lipid similar to acylglycerides (see erythrose 4-phosphate. It is a precursor of acylglycerol), but with a backbone of glyc- many aromatic compounds and of alka- erol 3-phosphate rather than glycerol. loids such as curare and morphine. Phenyl- Phosphoglycerides are major consitituents alanine can be deaminated to cinnamic of the membranes of cells and organelles, acid, required in the synthesis of lignin. with the exception of chloroplasts, in which GLYCOLIPIDS are more important. phloem Plant vascular tissue in which They have both hydrophilic and hydropho- food is transported from areas where it is bic groups, so are reasonably soluble in made to where it is needed or stored. It both water and lipids. consists of SIEVE ELEMENTS and COMPANION CELLS. The PROTOPHLOEM and METAPHLOEM phospholipid See lipid. are primary tissues and derived from the procambium, while the secondary phloem phosphorescence In general usage the is formed from the VASCULAR CAMBIUM. term is applied to the emission of ‘cold There are also fibers (sclerenchyma), and light’ – light produced without a high tem- parenchymatous packing tissue in the perature. The light comes from excited phloem. See also mass flow. atoms produced directly in the reaction – not from the heat produced. It is thus an phloroglucinol A temporary stain that example of chemiluminescence. In biologi- dyes lignin magenta red in thin sections of cal systems, the light emitted is often light plant tissue. It is usually acidified with hy- that has been previously absorbed, but drochloric acid before use. See staining. which is re-emitted at a different wave- length. Phosphorescent seas are due to the phosphatide A glycerophospholipid. See bioluminescence of dinoflagllates (Dino- lipid. mastigota), in particular Noctiluca species. phosphodiester bond A covalent bond phosphorus One of the essential el-

172 photoperiodism ements in living organisms, a macronutri- depth is termed the euphotic zone, the part ent needed for plant growth. Phospholipids below the compensation depth is the dys- are important in cell membrane structure, photic zone. The depth of the euphotic and phosphates are necessary for the for- zone depends on the turbidity of the water. mation of the sugar–phosphate backbone See plankton. Compare aphotic zone. of nucleic acids. Phosphates are also neces- sary for the formation of high-energy photoautotrophism See autotrophism; bonds in compounds such as ATP. Phos- phototrophism. phorus has other important roles in living tissues, being a component of certain coen- photoheterotrophism See heterotro- 3– zymes. The phosphate ion, PO4 , is an im- phism; phototrophism. portant buffer in cell solutions. Plants take up phosphorus from the soil water as the photolysis Chemical breakdown caused 3- PO4 ion. Phosphorus deficiency results in by light. In PHOTOSYNTHESIS the process is stunted roots, bluish, bronzed, or purple important in providing hydrogen donors leaves, and poor germination, seed set, and by the splitting of water, as follows: + – fruit ripening. 2H2O ® O2 + 4H + 4e In the CHLOROPLAST photolysis of water oc- phosphorylase See kinase; phosphory- curs in the oxygen- releasing complex lation. linked to Photosystem II. The protons are released into the lumen of the thylakoid. phosphorylation The addition of a The high-energy electrons go to replace phosphate group to a compound. Phos- electrons displaced from the reaction cen- phorylation reactions are catalyzed by ter of Photosystem II by the absorption of phosphorylase enzymes. Phosphorylation light energy. The electrons are the ener- is the first step in many metabolic path- gized by light energy and pass to the ways, the phosphate bond serving as an en- electron-transport chain of noncyclic pho- ergy source in a later stage of the pathway. tophosphorylation, which generates ATP For example, glycolysis starts with the and reduced NADP. phosphorylation of glucose and fructose 1,6-bisphosphate. Phosphorylation is also photomicrograph See micrograph. needed for the activation or deactivation of some enzymes. See also oxidative phospho- photonasty (photonastic movements)A rylation; photophosphorylation. nastic movement in response to light. Flowers such as the marigold (Calendula phosphotransferase See kinase. officinalis) open on exposure to light. Care must be taken not to confuse photonasty photic zone The surface layer of an with nyctinasty, in which day–night open- ocean or lake that is penetrated by sunlight ing/closing of flowers is governed by an in- and in which the flourish. ternal CIRCADIAN RHYTHM. See nastic Red and yellow wavelengths of light pene- movements. trate to about 50 m while blue and violet light may reach 200 m. The diatoms, photo-oxidation The oxidation of a which are the main components of phyto- substance as a result of the absorption of plankton, may be found down to 80 m. light energy. Very high light intensity can Most of the plankton lives in the top 10 m. cause oxidation (bleaching) of photosyn- Beyond 200 m the water is perpetually thetic pigments such as chlorophylls, inac- dark. The depth at which the rate of fixa- tivating them. tion of carbon dioxide by photosynthesis is equal to the rate at which it is lost by res- photoperiodism The response of an or- piration is called the compensation depth. ganism to changes in day length (photope- The part of the photic zone above this riod). In plants, leaf fall and flowering are

173 photophosphorylation common responses to seasonal changes in interaction between light and dark periods day length. that in some way affects flowering through Plants are classified as short-day plants the mediation of phytochrome. Phy- (SDPs), e.g. Chrysanthemum, or long-day tochrome is a protein that can absorb red plants (LDPs), e.g. cucumber (Cucumis light, which converts it to a form (PFR) that sativus), according to whether they flower absorbs far-red light. Absorption of far-red in response to short or long days. Day- light causes the phytochrome to revert to neutral plants, e.g. pea (Pisum sativum), the red light-absorbing form (P ). The P have no photoperiodic requirement. In R FR fact, the critical factor is not the length of form of phytochrome inhibits flowering in the day, but the length of the dark period, SDPs (PFR slowly disappears during long since flowering of SDPs is inhibited by even nights) and promotes flowering in LDPs a brief flash of red light in the dark period (PFR remains at high levels in short nights). (a PHYTOCHROME response), and an artifi- The light stimulus is perceived by the leaves cial cycle of long days and long nights in- and in some unknown way transmitted to hibits flowering in LDPs. Thus, it is the the floral apices. See circadian rhythm;

Photorespiration

174 photosynthesis critical day length; thermoperiodism; ver- photosynthesize from inorganic materials. nalization. All green plants photosynthesize, as well as algae and certain bacteria. In green plants, photophosphorylation (photosynthetic photosynthesis takes place in chloroplasts, phosphorylation) The conversion of mainly in leaves and stems. Directly or in- ADP to ATP using light-induced electron directly, photosynthesis is the source of transport. See photosynthesis. carbon and energy for all except chemoau- totrophic organisms. The mechanism is photoreceptor Any light-sensitive organ complex and involves two sets of stages: or organelle. The paraflagellar body, a light reactions followed by dark reactions. swelling on the undulipodium of certain The overall reaction in green plants can be species of Euglena, is an example. summarized by the equation: CO2 + 4H2O ® [CH2O] + 3H2O + O2 photorespiration (C2 cycle) A light- The light reactions of photosynthesis dependent metabolic process of most green involve reaction centers in the chloroplasts. plants that resembles true (or ‘dark’) respi- Each reaction center lies in a flat sheet of ration only in that it uses oxygen and 200-400 pigment molecules in the thy- produces carbon dioxide. Carbon dioxide lakoid membrane, the antenna complex, production during photorespiration may and contains a pair of chlorophyll a mol- be up to five times greater than in dark res- ecules. The whole unit is called a photosys- piration. Photorespiration takes place in tem. There are two kinds of photosystems PEROXISOMES rather than mitochondria, in green plants, Photosystems I and II. and it is not coupled to oxidative phospho- Both are usually involved in the light reac- rylation. It wastes carbon dioxide and en- tions of photosynthesis. Light energy ab- ergy, using more ATP than it produces. Its sorbed by the pigments of the antenna substrate is glycolate produced in C3 plants complex is passed to the reaction-center by the oxidation of ribulose bisphosphate chlorophyll molecules, from which it to glycerate 3-phosphate and phosphogly- passes along an electron-transport chain. colic acid. During photorespiration the en- Photosystem II contains a type of chloro- zyme glycolate oxidase catalyzes the phyll a (P680) that shows maximum light oxidation, by molecular oxygen, of gly- absorption at a wavelength of 684 nm. colic acid to glyoxylic acid and hydrogen When activated by light, a pair of electrons peroxide in the peroxisomes. The glyoxy- are excited and leave Photosystem II, being late is converted to glycine, and carbon replaced by electrons from the photolysis dioxide is released when the glycine mol- of water: + – ecules are converted to serine in the mito- 2H2O ® O2 + 4H + 4e chondria. The term photorespiration is Molecular oxygen is released, and the being replaced by the C2 cycle (oxidative protons pass into the lumen of the thy- photosynthetic carbon cycle), to better re- lakoid. The electrons pass through an flect the fact that unlike respiration it uses ELECTRON-TRANSPORT CHAIN in the thyla- rather than releases energy. koid membrane, being accepted first by It is estimated that in C3 plants 50% of plastoquinone, then by the cytochrome b/f the potential yield of photosynthesis can be complex (a proton pump that pumps H+ lost through photorespiration in warm into the thylakoid lumen), and on via plas- weather. It is therefore economically im- tocyanin to Photosystem I, which contains portant and ways of inhibiting the process chlorophyll a (P700) with a maximum are being investigated. light absorption at 700 nm. Here the elec- trons are boosted by more absorbed light photosynthesis The synthesis of or- energy to an even higher energy level and ganic compounds using light energy ab- pass through a second electron-transport sorbed by chlorophyll. With the exception chain involving the protein ferredoxin, re- of a small group of bacteria, organisms ducing NADP to NADPH + H+ for use in

175 photosynthesis

Photosynthesis: the Calvin cycle

176 photosynthetic pigments

Light reaction of photosynthesis other reactions, such as various syntheses. with the 5-carbon sugar ribulose bisphos- This sequence, which involves both photo- phate (RuBP), forming two molecules of systems, is termed noncyclic electron flow. phosphoglyceric acid (PGA). This reaction However, it is possible for electron flow to is catalyzed by the enzyme ribulose bispho- involve only Photosystem I, and the ferre- sphate carboxylase (rubisco). In a series of doxin/cytochrome b/f/ plastocyain elec- reactions using NADPH and ATP from the tron- transport chain, electrons returning light reactions, PGA is converted to a suc- to Photosystem I to be reenergized. This cession of 3-, 4-, 5-, 6-, and 7- carbon sugar cyclic electron flow does not produce phosphates in a series of reactions collec- NADPH. It simply generates a proton gra- tively termed the Calvin cycle. These prod- dient across the thylakoid membrane. This ucts are then used in the synthesis of is used to drive ATP synthesis (cyclic pho- carbohydrates, fats, proteins, and other tophosphorylation). ATP synthesis associ- compounds, and RuBP is regenerated. See ated with the noncyclic electron-transport C3 plant; C4 plant. chain is termed noncyclic photophos- phorylation. The dark reactions of photo- photosynthetic bacteria A group of synthesis take place in solution in the bacteria able to photosynthesize through chloroplast stroma. Here, ATP and possession of chlorophyll pigments. They NADPH from the light reactions are used include the Cyanobacteria, green sulfur to reduce carbon dioxide to carbohydrate. bacteria (Chlorobia), green nonsulfur bac- Carbon dioxide is ‘fixed’ by combination teria (Chloroflexa), purple sulfur bacteria,

177 photosystem and purple nonsulfur bacteria (both in the phototropism (phototropic movement) phylum ). The photosyn- A directional growth movement of part of thetic bacteria fix carbon using the Calvin a plant in response to light. The phenome- cycle, but use a variety of hydrogen non is clearly shown by the growth of sources. shoots and coleoptiles toward light (posi- tive phototropism). The stimulus is per- photosynthetic pigments Pigments that ceived in the region just behind the shoot absorb the light energy required in photo- tip. If light falls on only one side of the synthesis. They are located in the chloro- apex then auxins produced in the apex plasts of plants and algae, whereas in most tend to diffuse toward the shaded side. photosynthetic bacteria they are located in Thus more auxin diffuses down the stem thylakoid membranes, typically distributed from the shaded side of the tip. This results around the cell periphery. All photosyn- in greater elongation of cells on the shaded thetic organisms contain chlorophylls and side thus causing the stem to bend toward carotenoids; some also contain phyco- the light source. Most roots are apho- bilins. Chlorophyll a is the primary pig- totropic (light-insensitive) but some (e.g. ment in plants, algae, and cyanobacteria, the adventitious roots of climbers such as and bacteriochlorophyll is the pigment ivy) grow away from light (negative pho- used by many photosynthetic bacteria, totropism). See also tropism. since energy absorbed by this is used directly to drive the light reactions of pho- phragmoplast A barrel-shaped body tosynthesis. The other pigments (chloro- appearing in dividing plant cells during late phylls b, c, and d, and the carotenoids and anaphase and telophase between the two phycobilins) are accessory pigments that separating groups of chromosomes. It con- pass the energy they absorb on to chloro- sists of MICROTUBULES associated with the phyll a. They broaden the spectrum of light SPINDLE, together with Golgi apparatus, en- used in photosynthesis. See absorption doplasmic reticulum, and ribosomes, and spectrum. transports vesicles that coalesce to form the early cell plate. photosystem See photosynthesis. phycobilins (phycobiliproteins) A group phototaxis (phototactic movement)A of accessory photosynthetic pigments TAXIS in response to light. Many motile found in Cyanobacteria and Rhodophyta algae are positively phototactic, e.g. (). They comprise the blue phyco- Volvox. Chlamydomonas cells are also cyanins, which absorb extra orange and positively phototactic, swimming toward red light, and the red phycoerythrins, light to increase their photosynthesis rate, which absorb green light, enabling red but if the light intensity becomes too great, algae to grow at depth in the sea. they show negative phototaxis, swimming away to protect their pigments. Chloro- See phycobilins. plasts also show phototaxis, and take up specific orientations in relation to the di- phycoerythrin See phycobilins. rection of the incident light. phylloclade See cladode. phototrophism A type of nutrition in which the source of energy for synthesis of phyllode An expanded flattened petiole organic requirements is light. Most pho- that acts as the photosynthetic organ if the totrophic organisms are autotrophic (i.e. lamina is missing or very reduced. Phyl- show photoautotrophism); these comprise lodes are seen in various Australian species the green plants, Cyanobacteria, and some of Acacia. Compare cladode. photosynthetic bacteria (the purple and green sulfur-bacteria). phyllotaxis (phyllotaxy) The arrange-

178 physiological race

Phyllotaxis ment of leaves on a stem. There may be phylogeny The evolutionary history of one, two, or several leaves at each node. groups of organisms. See phylogenetic. When there are three or more leaves form- ing a circle around the node the arrange- phylum (pl. phyla) One of the major ment is said to be whorled, e.g. bedstraws groups into which a kingdom of organisms (Galium spp.). When leaves arise singly the is classified. Phyla may be divided into sub- arrangement may be spiral, e.g. poplar phyla. The names of plant phyla end in (Populus spp.), or alternate, e.g. hazel -phyta, e.g. Anthophyta, Lycophyta. In (Corylus spp.), and when they arise in pairs some plant classifications (especially older the arrangement is termed opposite, e.g. ones) the term ‘division’ is used instead of lilac (Syringa spp.). Opposite phyllotaxis phylum. may be either distichous or decussate de- pending on whether the leaf pairs are in the physiological drought A situation in same plane up the stem or arise alternately which plants are unable to take up suffi- at right angles to each other. cient water even though there is water in the soil: they are effectively in a drought phylogenetic Relating to the study of situation. It occurs when the concentration evolutionary relationships between and of solutes in the soil water is equal to or within taxonomic levels, in particular the higher than that in the root cells, so water relationships between groups of organisms is unable to enter the root by OSMOSIS. based on their past evolutionary history. Physiological drought occurs in salt

179 physiology marshes and other brackish coastal habi- pico- Symbol: p A prefix denoting one tats, and in deserts where there are high million-millionth, or 10–12. For example, 1 concentrations of salts in the upper layers picogram (pg) = 10–12 gram. See SI units. of the soil. It also occurs in cold weather because the permeability of the endoder- pileus (pl. pilei) The cap of the mature mal cells decreases rapidly below 5oC, so mushroom or toadstool (sporophore) in the root resists the entry of water (root re- certain basidiomycete fungi (e.g. Agaricus). sistance). Its undersurface comprises gills or pores lined with basidia. The upper surface may physiological race The existence of be fleshy or leathery, and may retain the physiologically distinct but morphologi- flaky remains of the universal veil. cally identical races within a species. Such physiological races are important in pili (sing. pilus)(fimbriae sing. fimbria) host–pathogen studies, particularly in Fine, straight, hairlike protein structures planning programs to breed for crop resis- emerging from the walls of certain bacte- tance. ria. They confer the property of ‘stickiness’ whereby bacteria tend to adhere to one an- physiology The way in which organ- other. They are hollow tubes and may isms or parts of organisms function. Com- number from one to several hundred. pare morphology. piliferous layer The region of the root phytoalexin A chemical produced by a epidermis that gives rise to the root hairs. It is located just behind the zone of elonga- plant that inhibits the growth of a patho- tion and is the main absorptive area of the genic fungus. For example, when attacked root. by the fungus Ceratostomella, the sweet potato (Ipomoea batatus) concentrates the Pinaceae A family of conifers that bear terpenoid ipomeamarone in its tissues, and separate male and female cones, and in this concentration has been shown to be re- which the seeds are borne between the cone lated to the degree to which it resists the at- scales and do not have arils. Most species tack. are evergreen trees or shrubs, but the larches or tamaracks (Larix spp.) are de- phytochrome A proteinaceous pigment ciduous. One of the oldest living trees is a found in low concentrations in most plant bristlecone pine (Pinus aristata), thought organs, particularly meristems and dark- to be about 4900 years old. The family in- grown seedlings. It exists in two intercon- cludes cedars (Cedrus), firs (Abies), hem- vertible forms. PR (or P660 ) has an locks (Tsuga), and larches and pines absorption peak at 660 nm (red light) and (Pinus). PFR (or P730) at 730 nm (far-red light). Nat- ural white light favors formation of PFR, pin mold See Mucorales. the physiologically active form, while far- red light encourages reversion to PR if given pinna (pl. pinnae) See pinnate. soon after the red light. Light intensities re- quired for conversion are very low and it pinnate Describing a compound leaf occurs within seconds. In some species, PFR with leaflets (pinnae) arranged in two may revert to PR in the dark, a process in- rows, one on each side of the midrib (flow- hibited by low temperatures. See circadian ering plants, e.g. Vicia) or of a central stalk rhythm; photoperiodism. (ferns, e.g. Dryopteris). The pinnae may themselves be divided into leaflets, which phytohormone See hormone. are called pinnules. phytoplankton See plankton. pinnule See pinnate.

180 plant pinocytosis See endocytosis. placenta (pl. placentae) 1. The region of tissue occurring on the inner surface of the pioneer species A species that colonizes ovary wall of the carpels of flowering a new physical environment, for example plants where the ovules develop. The land exposed by retreating glaciers, felling arrangement of ovules within the ovary of forest, or bare mud on coasts or estuar- (see placentation) depends on whether ies. Pioneer species are the first stage in there are one or many carpels and whether plant succession. the carpels are free or fused. 2. A central swelling on the lower surface pistil In angiosperms, the seed-contain- of fern pinnules linking clusters of sporan- ing structure. In an apocarpous gynoecium gia with the main frond. See also sorus. it corresponds to the carpel, while in a syn- carpous gynoecium it is made up of two or placentation The position of the ovule- more fused carpels. bearing placentae in angiosperm seeds. Pla- centation varies according to whether there pit A gap in the secondary cell wall that are one or many carpels. If there is only one enables communication between thickened ovule in the ovary, it may be attached at cells, e.g. tracheids. According to whether the base of the ovary wall (basal placenta- or not the secondary wall forms a lip over tion), or at the apex (apical placentation). the pit, pits are described as bordered or If there is more than one ovule, and there is simple, respectively. If a pit occurs singly it only one carpel in the ovary, as in pea is termed a blind pit. Usually pits occur in (Pisum sativum), placentation is along the ventral suture and is termed marginal pla- pairs so that the only barrier separating ad- centation. Marginal placentation also oc- jacent cells is the middle lamella and the re- curs in apocarpous polycarpellary ovaries spective primary cell walls. The primary of LEGUMES but various types of placenta- cell wall adjacent to the cavity is called the tion are seen in syncarpous ovaries. Where pit membrane. the carpels are fused to give a unilocular ovary, the ovules may be found along the pitcher plant Any carnivorous plant two placentae at each line of fusion giving with pitcher-shaped leaves. Insects and parietal placentation, as in violet (Viola other small animals fall into the pitchers, spp.) and passionflower (Passiflora). If which are usually filled with rainwater or instead the ovules are borne on a central plant digestive enzymes, and drown; the column, placentation is termed central pla- plant obtains nitrates and perhaps other centation; a modification of this is free- nutrients from their decomposing bodies. central placentation, in which the column does not extend to the top of the ovary, pith (medulla) The central region of the as in primrose (Primula vulgaris). Where stem and, in some herbaceous plants, root other carpels to give a multilocular ovary, that is normally composed of parenchyma- the marginal placentae of the carpels may tous tissue. It occurs to the inside of the fuse in the center of the ovary, giving axile stele. placentation, as in tulip (Tulipa spp.). pitted thickening The most extensive plagiotropism See tropism. form of thickening found in XYLEM vessels and tracheids, where the secondary cell plankton A varied collection of aquatic wall is continuous apart from small areas organisms that drift freely, not being at- called PITS. The pits may be arranged in an tached to any substrate and not possessing alternate or opposite pattern. If they are any organs for locomotion. The most im- elongated horizontally and parallel to each portant components of the plant plankton other, this is described as scalariform (phytoplankton) are the diatoms upon thickening. which the planktonic animals (zooplank-

181 plant hormone ton) feed. Other members of the phyto- plasma membrane (cell membrane; plas- plankton include other microscopic algae malemma) The membrane that sur- and cyanobacteria.The plankton form the rounds all living cells. It ranges from basis of the food chain in the sea. Compare 7.5–10 nm in thickness and consists of benthic. phosphoglyceride molecules (about 40%) and proteins (about 60%). The membrane plant An organism that can (usually) backbone consists of phosphoglyceride make its own food by taking in simple in- molecules arranged in two rows with their organic substances and building these into hydrophilic polar heads facing outwards complex molecules by a process termed and the hydrophobic hydrocarbon tails in photosynthesis. This process uses light en- the center. This lipid layer is penetrated by ergy, absorbed by a green pigment called globular proteins, which in some places ex- chlorophyll, which is found in all plants tend from one side of the membrane to the but no animals. There are a few exceptions, other. The whole structure has fluidlike in the form of certain parasitic plants. properties, as the molecules are able to Most plants have cellulose cell walls, and move laterally – the so-called fluid-mosaic have starch as a storage polysaccharide, model. The membrane is probably being whereas animals have no cell walls, store constantly renewed. The plasma mem- glycogen, and do not have plastids or brane is selectively permeable, and controls chlorophyll; and fungi usually lack cellu- which substances can enter and leave the lose and also do not form plastids. Plants cell. lack motility, with the exception of the ga- metes of many species. One major charac- plasmasol (endoplasm) The sol-like teristic that distinguishes plants from other form of cytoplasm, located inside the PLAS- plantlike organisms, such as algae or fungi, MAGEL. It is free-flowing and contains the is the possession of an embryo that is re- cell organelles. tained and nourished by maternal tissue. Fungi and algae lack embryos and develop plasmid An extrachromosomal genetic from spores. Plants also differ from fungi element found within bacterial cells that in having a regular alternation of diploid replicates independently of the chromoso- and haploid generations. Plants also differ mal DNA. Plasmids typically consist of cir- from animals by generally responding to cular double-stranded DNA molecules of stimuli very slowly, the response often tak- molecular weight 106–108. They carry a ing a matter of days and occurring only if variety of genes, including those for antibi- the stimulus is prolonged. Compare algae; otic resistance (R factors), toxin produc- Fungi; Protoctista. tion, and enzyme formation, and may be advantageous to the cell. Some plasmids, plant hormone See hormone. called episomes, can reversibly insert them- selves into the bacterial DNA, replicating plasmagel (ectoplasm) The gel-like re- with it. gion of cytoplasm located in a thin layer just beneath the plasma membrane, partic- (pl. plasmodesmata)A ularly of unicells that move in an ameboid fine strand of cytoplasm that serves to con- fashion, and cells that exhibit cyclosis. nect the protoplasm of adjacent plant cells, allowing material to pass from one cell to plasmagene A gene contained in a its neighbor. structure outside the nucleus, e.g. the genes in mitochondria and chloroplasts. See cy- plasmodial slime molds See Myxomy- toplasmic inheritance. cota. plasmalemma (pl. plasmalemmae) See Plasmodiophora A phylum of protoc- plasma membrane. tists that are obligate endoparasites of

182 pneumatophore plants, fungi, and algae. Most plasmodio- tance. See cytoplasmic inheritance; plasma- phorans cause little damage to the host gene. plant, but a few cause serious diseases, e.g. Plasmodiophora brassicae causes clubroot pleiomorphism The occurrence of dif- of cabbage and related brassicas. ferent morphological stages during the life of an organism. Examples are the different plasmodium (pl. plasmodia) See Myx- spore forms of the rust fungi. Compare omycota. polymorphism. plasmogamy Fusion of protoplasm, Pleistocene The first epoch of the Qua- usually referring to the fusion of cytoplasm ternary period, which started with a glacia- but not nuclei. It usually occurs between tion about two million years ago and ended gametes or protoplasts. Plasmogamy in the with the last glaciation about 10 000 years absence of karyogamy (fusion of nuclei) ago. Several ICE AGES drove many organ- occurs between fungal mycelia of different isms toward the equator while others be- strains of some ascomycetes and basid- came extinct. For example, tundra covered iomycetes to form a heterokaryon. temperate parts of the USA and Central Europe where today deciduous forests are plasmolysis Loss of water from a the natural vegetation. The boreal forests walled cell (e.g. of a plant or bacterium) extended south almost to northern due to osmosis to the point at which the Louisana and the Mediterranean. Fossil protoplast shrinks away from the cell wall. pollen deposits provide evidence for the past distribution of plants. Some migrating The point at which this is about to happen plants found the tops of mountains further is called incipient plasmolysis. Here the cell south resembled the conditions they had wall is not being stretched; i.e. the cell has left, and survived in isolated areas (refu- lost its turgidity or become flaccid (wall gia). When the climate warmed again, pressure is zero). Wilting of herbaceous these became centers of diversity. The cycle plants occurs at this point. of southward then northward migration led to the mixing of plants from both plastid An organelle enclosed by two cooler and warmer regions, leading to new membranes (the envelope) that is found in patterns of competition and reoorganiza- plants and certain protoctists (e.g. algae), tion of plant communities. Modern man and develops from a proplastid. Various (Homo sapiens) evolved during this period, types exist, but all contain DNA and ribo- and probably caused further disruption of somes. Plastids range from 10 m m to less plant communuties by his use of fire and than 1.0 m m in diameter. They include the start of forest clearance. See also geo- chloroplasts, chromoplasts, amyloplasts, logical time scale; pollen analysis. etc., and are often interconvertible. plerome See histogen theory. plastid inheritance See plastogene. pleuropneumonia-like organisms See plastocyanin An electron carrier in mycoplasmas. PHOTOSYNTHESIS, involved in the transfer of electrons from photosystem II to photosys- Pliocene The epoch of the Tertiary pe- tem I. riod, about 7 to 2 million years ago, which followed the Miocene. In the Pliocene the plastogene A gene present in a self- hominids, such as Australopithecus and replicating plastid. Inheritance studies have Homo, became clearly distinguishable shown that plastogenes control leaf color from the apes. Grasses became more abun- in some plants, e.g. yellow-leaved Primula dant, perhaps because the climate was sinensis, is an example of plastid inheri- becoming drier. Modern conifers were

183 Poaceae spreading, and leptosporangiate ferns un- podsol (podzol) The type of SOIL found derwent adaptive radiation. See also geo- under heathland and coniferous forests in logical time scale. temperate and boreal climates where there is heavy rainfall and long cool winters, plumule The shoot apex and first rudi- such as the coniferous forests of northern mentary leaves in the mature embryo and North America and Eurasia. It is strongly the seedling. In seedlings showing epigeal acid and often deficient in nutrients, espe- germination, the plumule is taken above cially iron compounds and lime, as a result ground between the cotyledons. When ger- of leaching by the heavy rain or snow-melt mination is hypogeal, only the plumule in spring. Beneath the humus layer lies a emerges from the soil; it has a hooked tip leached horizon (A horizon) composed to protect it as it pushes up through the mainly of quartz sand. In the clay-rich B soil. Compare radicle. horizon below the leached layer iron com- pounds accumulate, staining the layer pneumatophore (aerophore; breathing brown and forming an impermeable hard root)A specialized negatively geotropic pan that prevents drainage. Where this root produced by certain aquatic vascular layer is sufficiently impermeable to main- plants (especially trees such as mangroves, tain the A horizon in a waterlogged condi- e.g. Sonneratia) that protrudes well above tion, the soil is termed a gley podsol. soil level. The aerial part is covered with point mutation See gene mutation. LENTICELS through which gases can diffuse to and from the highly developed system A lack of symmetry: a definite of intercellular airspaces. Pneumatophores polarity orientation that arises during the differen- are an adaptation to waterlogged, poorly tiation of a cell, tissue, or organ. For ex- aerated soils, allowing roots to exchange ample, in the development of the plant gases. embryo from the zygote, one end becomes the radicle, the other gives rise to cotyle- Poaceae (Gramineae, grasses) The dons. grasses, a family of monocotyledons with 9500 species. Most are annual or perennial polarizing microscope A light MICRO- herbs, although some are woody (but do SCOPE that uses polarized light to illuminate not have secondary thickening), e.g. bam- the specimen. The orientation of molecules boos. Grasslands are the dominant vegeta- in certain crystalline substances affects the tion on the prairies, steppes and savannas, transmission of polarized light. and in times past supported vast herds of grazing mammals. Today these areas are polar nuclei The, usually two, haploid also important are for growing cereal nuclei found midway along the EMBRYO grasses, such as barley (Hordeum vulgare), SAC, forming a binucleate central cell. They maize (Zea mays), millet (Panicum mili- may fuse to form the diploid definitive nu- aceum), oats (Avena sativa), rye (Secale cleus. The endosperm is formed from the cereale), sorghum (Sorghum), and wheat fusion of one or both polar nuclei (occa- (Triticum). Rice () and sugar sionally several) with one of the male ga- cane (Saccharum officinarum) are also metes from the pollen tube. grasses. pollen The MICROSPORES of seed plants, pod See legume. produced in large numbers in the pollen sacs, or stamens. They are formed by meio- Podocarpaceae A family of evergreen sis of somatic pollen mother cells. Pollen conifers. It includes one of the oldest living grains are rich in mitochondria, Golgi ap- organisms, a huon pine, Dacrydium paratus, and endoplasmic reticulum. In franklinii, from Tasmania, possibly 10 000 many gymnosperms the nuclei divide years old. within the pollen grain to produce a small

184 pollution

Pollen: section of a pollen grain showing the exine number of cells (2–40 depending on the information on the composition and extent group) representing the male prothallus. In of past floras and environments by exam- angiosperms and some conifers (e.g. Cu- ining fossil pollen grains, spores, and simi- pressus) the pollen grains contain just two lar structures in peat and sedimentary nuclei: a GENERATIVE NUCLEUS and a VEGE- deposits. The outer wall (exine) of the TATIVE NUCLEUS (tube nucleus).When a pollen grain is often diagnostic (see pollen) pollen grain germinates, a pollen tube and very resistant to decay. See also paly- pushes its way out through an aperture in nology. the pollen grain wall, and the nuclei mi- grate into the tube. The generative nucleus pollen culture See anther culture. divides to produce two sperm cells consist- ing of only nucleus and cytoplasm, one of pollen mother cell (PMC) In an- which will fertilize the ovum, while the giosperms and gymnosperms, a somatic other will fuse with the POLAR NUCLEI to cell that gives rise to four haploid pollen form the endosperm. The vegetative nu- grains by meiosis. cleus degenerates after fertilization. Pollen grains are highly variable in size and shape pollen sac A chamber in which the ranging from 5m to over 200m in diameter pollen is formed in the angiosperms and and from circular to tetrahedral or dumb- conifers. bell shaped. The wall of the pollen grain consists of two layers, a highly resistant pollen tube A filamentous outgrowth of outer exine, which may be intricately the pollen grain that in most seed plants sculpted and is punctured by apertures, grows out through a pore in the exine and and an inner intine, which sometimes pro- transports the male gametes to the ovule. trudes through these apertures. The size Germination of the pollen grain to give the and shape of the pollen grains and the pollen tube usually takes place only when number and orientation of the apertures the pollen is compatible with the stigma tis- and sculpturing is characteristic of particu- sue. The pollen tube may grow down the lar families, genera, or sometimes species. stylar canal, or it may secrete enzymes that Pollen grains are adapted according to the digest the tissues of the style, creating a method of pollination, those carried by in- path for the tube. It usually enters the ovule sects often being sticky or barbed, while through the micropyle, but in some species, wind-pollinated plants generally produce such as beech (Fagus) it may enter through smooth light pollen. See also pollen analy- the chalaza (chalazogamy). sis. pollination The transfer of pollen from pollen analysis A means of obtaining the anther to the stigma in angiosperms,

185 polyembryony and from the sporangiophores to the mi- biodegradable pollutants, such as certain cropyle in gymnosperms. If the pollen is chlorinated hydrocarbon pesticides (e.g. compatible (i.e. of the right type) then the DDT) and heavy metals, such as lead, cop- pollen grains germinate, producing a per, and zinc in some industrial effluents, pollen tube that grows down the style accumulate in the environment. Heavy carrying the pollen nuclei to the ovule. metals, even when present at low concen- Plants may be self-pollinating, e.g. barley trations, are toxic to plants, inhibiting (Hordeum vulgare), thus ensuring that water or nutrient uptake, damaging cell seed will be set, even in the absence of membranes, and inhibiting enzymes. other members of the same species. How- ever, self-pollination also leads to homozy- polyembryony The occurrence of many gosity and, potentially, less adaptable embryos in one ovule. See apomixis; plants in the long term; thus, in many plant parthenogenesis. species mechanisms exist to prevent it and promote CROSS-POLLINATION. In some spe- polymer A molecule made up of a series cies of angiosperms stamens and stigmas of similar units joined together, e.g. starch, mature at the same time and pollen may which is made up of glucose units. fall directly onto the stigma of the same flower, or the stamens and stigmas may ac- polymerase An enzyme that regulates tually touch. See also cleistogamy; di- the synthesis of a polymer. Examples in- chogamy; homogamy; incompatibility; clude RNA polymerases and DNA poly- protandry; protogyny. merases. pollution Any damaging or unpleasant polymerase chain reaction (PCR)A change in the environment that results technique for amplifying small samples of from the physical, chemical, or biological DNA rapidly and conveniently. It is used side-effects of human activities. Pollution widely in research and forensic science, e.g. can affect the atmosphere, rivers, seas, and to clone genes or to produce a suitable the soil. quantity of DNA for genetic fingerprinting Air pollution is caused by the domestic from the minute amounts present in traces and industrial burning of carbonaceous of blood or other tissue. See also genetic fuels, by industrial processes, and by vehi- fingerprinting. cle exhausts. Industrial emissions of sulfur dioxide cause acid rain, and the release into polymorphism A distinct form of vari- the atmosphere of chlorofluorocarbons, ation in which significant proportions of used in refrigeration, aerosols, etc., leads to different types of individuals exist within a the depletion of ozone in the stratosphere population of a species at the same time (see acid rain; ozone layer). Carbon diox- and in the same place, such that the fre- ide, produced by burning fuel and by quency of the rarest form cannot be ex- motor vehicle exhausts, is slowly building plained on the basis of recurring mutation. up in the atmosphere, and is one of the fac- The term is generally used where the fre- tors leading to an overall increase in the quency of the least common morph ex- temperature of the atmosphere (see green- ceeds 1 in 20, less frequent occurrences house effect). Vehicle exhausts also contain being usually due to rare mutation or re- carbon monoxide, nitrogen oxides, and combination events. If the differences per- other hazardous substances, such as fine sist over many generations then there is a particulate dusts. Water pollutants include balanced polymorphism. Compare pleio- those that are biodegradable, such as morphism. sewage effluent and nitrates leached from agricultural land, which if allowed to enter polypeptide A compound that contains water courses can lead to eutrophication many amino acids linked together by pep- (see eutrophic) and algal blooms. Non- tide bonds. The three-dimensional shape

186 potometer

(conformation) of the poplypeptide chain two or more different monosaccharides, is determined by its amino acid sequence, e.g. hemicelluloses. See also carbohydrates; but can be disrupted by extremes of pH or sugar. heat. Proteins are made up of one or more polypeptides. See peptide; denaturation; polysome See ribosome. conjugated protein. polysomy See aneuploidy. polyploid The condition in which a cell or organism contains three or more times polystely Having many steles, as in the the haploid number of chromosomes. It oc- stems of some Selaginella species. See also curs when chromosomes fail to separate distely. during anaphase I of meiosis, giving rise to gametes that are diploid instead of haploid. pome A fleshy pseudocarp fruit, charac- Fertilization results in triploid or tetraploid teristic of the family Rosaceae, e.g. apple individuals. Polyploidy is far more com- (Malus). The fleshy part of the fruit is de- mon in plants than in animals and very rived from the greatly enlarged urn-shaped high chromosome numbers may be found; receptacle, which encloses the entire fruit, for example in octaploids and decaploids and which is fused to the tough carpel (containing eight and ten times the haploid walls. Thus the apple core is the true fruit. chromosome number). Some 60% of monocotyledons and 40% of dicotyledons population dynamics The study of the are polyploid, as are 90% of ferns, many factors influencing the fluctuations in num- bryophytes, and some algae. See allopoly- bers in a population or of its gene pool: the ploidy; autopolyploidy; heterosis. factors affecting birth and mortality rates, immigration and emigration, and repro- Polypodiaceae A family of terrestrial or ductive potential (for example, seed size, epiphytic leptosporangiate ferns. dispersal, and dormancy). polyribosome See ribosome. population A group of interbreeding organisms of the same species (or other polysaccharide A polymer of 10 or groups within which individuals may ex- more monosaccharides joined by glyco- change genetic information) occupying a sidic links (see glycoside). They contain particular space. A population is continu- many repeated units in their molecular ally modified by increases (birth and immi- structures and are of high molecular gration) and losses (death and emigration), weight. They can be broken down to and is limited by the effects of environmen- smaller polysaccharides (oligosaccharides ), tal factors such as disease. Many plants DISACCHARIDES, and MONOSACCHARIDES by cannot be divided strictly into populations hydrolysis or by the appropriate enzyme. as they may reproduce vegetatively, with Important polysaccharides are inulin (hy- the result that one individual can occupy a drolyzed to fructose), starch (hydrolyzed to large area and parts can detach. glucose), and cellulose (hydrolyzed to glu- cose). Polysaccharides may form branching porogamy The usual method of fertil- chains, or they may be unbranched. They ization in angiosperms in which the pollen have many roles in plants, as storage mol- tube enters the ovule by the micropyle. ecules (e.g. starch), structural elements in Compare chalazogamy. cell walls (cellulose), and be components of gums and mucilages (glucuronic acid). A porphyrins Cyclic organic structures polysaccharide containing only one type of containing tetrapyrrole rings (four joined sugar residue is called a homopolysaccha- rings, each consisting of four CH units and ride, e.g. starch, which contains only glu- one NH unit) that have the important char- cose units. Heteropolysaccharides contain acteristic property of forming complexes

187 PPLO with metal ions. Examples of such metallo- start of the Cambrian, around 570 Ma porphyrins are the iron porphyrins (e.g. (million years ago). The term ‘Precam- heme in hemoglobin) and the magnesium brian’ is now used mainly descriptively, porphyrin, chlorophyll, the photosynthetic and has been largely discarded as a geolog- pigment in plants. In nature, the majority ical term in the light of greater knowledge of metalloporphyrins are conjugated to of the early evolution of life. Precambrian proteins to form a number of very impor- time is now divided into three eons: tant molecules, such as the cytochromes. Hadean, from the earth’s origin to about 3900 Ma; Archaean, 3900–2390 Ma; and potassium One of the essential elements Proterozoic, 2390–570 Ma (the Cambrian in plants. It is absorbed by plant roots as marks the start of the Phanerozoic eon, the potassium ion, K+, and in plants is the which extends to the present day). most abundant cation in the cell sap. Potas- The oldest fossils discovered so far are sium ions are required in high concentra- remains of bacterialike organisms, dating tions in the cell for efficient protein from about 3500 Ma. There is abundant synthesis, and for glycolysis in which they evidence of flourishing colonies of cyano- are an essential cofactor for the enzyme bacteria and other bacteria throughout the pyruvate kinase. Potassium ions balance Archaean and Proterozoic eons. This takes anions and organic acids, and are thus im- the form of STROMATOLITES, rock structures portant in osmoregulation, the control of representing the remains of sediment stomatal opening, and leaf movements trapped or precipitated by bacterial com- (through their effects on the turgor of pul- munities. Stromatolites are still found in a vini). A deficiency of potassium leads to a few special locations today. The ancient characteristic red or purple coloration of rocks also contained spherical carbona- the leaves, poor root growth, and flower ceous aggregates up to 20 mm diameter, and fruit formation. that resemble algae or the cysts of flagel- lates, and delicate carbonaceous filaments potometer An apparatus for measuring up to 150 m m long. The first appearance the rate of water uptake by a cut shoot or of multicellular animals is in the so-called whole plant. This is normally closely re- Ediacara fauna, in rocks dated to the last lated to the rate of water loss by transpira- 100 million years of Precambrian time. tion, and the potometer can be used to Around this time the stromatolites began compare transpiration rates under differ- to decline, perhaps as they were being ent conditions. The rate of uptake is mea- eaten by newly evolved multicellular ani- sured by the progress of an air bubble in a mals. capillary tube along a scale. More mean- ingful comparisons can be made by com- pressure potential See turgor pressure; paring water loss from the potometer with water potential. that from an atmometer (an apparatus used to measure the rate of evaporation prickle A protective multicellular out- from a porous pot or other nonliving wet growth from the surface of a plant. It is a surface), which allows uncontrolled evapo- modified trichome and thus completely ration. epidermal in origin. Compare spine, thorn.

PPLO See mycoplasmas. primary endosperm nucleus See en- dosperm. PR (P660) See phytochrome. primary growth Growth derived solely Precambrian The time in the earth’s ge- from meristems present in the embryo, i.e. ological history that precedes the Cam- apical meristems. Such growth generally brian period, i.e. from the origin of the increases the length of plant organs. All the earth, nearly 5 billion years ago, to the tissues of the young plant are the result of

188 promoter primary growth. Compare secondary procambium (procambial strand) The growth. layer of cells that gives rise to the vascular tissue. Part of the apical meristem, it is dis- primary phloem PHLOEM derived from cernable just below the apex as a strand of the procambium in the young plant. It con- flattened cells which, if traced back along sists of the PROTOPHLOEM (formed first) the shoot or root, may be seen to give rise and METAPHLOEM. In nonwoody plants it is to the primary vascular tissues. It is contin- the only food-conducting tissue, but in ma- uous with the intrafascicular cambium. In ture plants with secondary growth this roots it may also be called plerome. See his- function is taken over by the secondary togen theory; primary growth. phloem. See secondary growth. procaryote See prokaryote. primary structure The sequence of amino acids in a polypeptide chain. See producer The first TROPHIC LEVEL in a also conformation; secondary structure, food chain. Producers are those organisms tertiary structure; quaternary structure. that can build up foods from inorganic ma- terials, i.e. green plants, algae, and photo- primary tissue Plant tissue that is de- synthetic and chemosynthetic bacteria. rived solely from meristems present in the Producers are eaten by herbivores (primary embryo and their derivatives. See primary CONSUMERS). growth. proembryo A young plant after fertil- primary xylem Xylem derived from the ization but before differentiation into em- procambium in the young plant. It consists bryo and suspensor tissue. of the protoxylem (formed first) and metaxylem. In nonwoody plants the pri- profundal The deepwater zone of a lake mary xylem is the only water-conducting beyond a depth of 10 meters. Little light tissue, but in mature plants with secondary penetrates this zone and thus the inhabi- growth this function is mostly taken over tants are all heterotrophic, depending on by the secondary xylem. See secondary the littoral and sublittoral organisms for growth. basic food materials. Commonly found in- habitants include bacteria, fungi, mollusks, primordium (pl. primordia) A collec- and insect larvae. Species found in the pro- tion of cells that differentiates into a spe- fundal zone are adapted to withstand low cialized cell, tissue or organ, e.g. the apical oxygen concentration, low temperatures, shoot and apical root primordia of the em- and low pH. Compare littoral; sublittoral; bryo. The term is used particularly in rela- photic zone. tion to a part of the apical meristem that later differentiates further. prokaryote (procaryote) An organism whose genetic material (DNA) is not en- probability The chance that a given closed by membranes to form a nucleus but event will occur, or that over a series of ob- lies free in the cytoplasm. Organisms can servations a particular kind of observation be divided into prokaryotes and EUKARY- will occur regularly as a given proportion OTES, the latter having a true nucleus. In of the total number of observations. Statis- prokaryotes the nuclear material is simply tical probability is usually based on an a circular strand of DNA, not complexed infinite number of observations. For exam- with histone proteins, and cell division ple, in genetics, if a heterozygous plant is is amitotic. This is a fundamental division selfed, the probability of finding the double because it is associated with other major recessive is 1 in 4, or 25%. The greater the differences. Prokaryotes constitute the number of offspring the better the chance kingdom Bacteria (formerly the Prokary- that this actual percentage will be achieved. otae). Recent molecular evidence suggests

189 prophage the prokaryotes actually comprise two dis- Prophase may be divided into successive tinct kingdoms, or even domains, the Ar- stages termed leptotene, zygotene, pachy- chaea and the Eubacteria or Bacteria. tene, diplotene, and diakinesis. In leptotene Eukaryotes comprise all other organisms. the chromosomes, already replicated, start Prokaryote cells evolved first and gave rise to condense and appear as fine threads, al- to eukaryote cells. They are normally much though sister chromatids are not yet dis- smaller (about 1m m in diameter) than eu- tinct. The spindle starts to form around the karyote cells (about 20m m). Prokaryotes intact nucleus. Zygotene is characterized lack organelles such as nucleoli, mitochon- by the active and specific pairing (synapsis) dria, plastids, Golgi apparatus, endoplas- of homologous chromosomes leading to mic reticulum, and Golgi apparatus, and the formation of a haploid number of biva- do not exhibit cyclosis. Their ribosomes lents. In pachytene the paired homologous are smaller (70S) than those of eukaryotes chromosomes contract. At this point each (80S) – similar in size to the ribosomes of chromosome consists of a pair of chro- chloroplasts and mitochondria. This has matids and the two associated chromo- led to the theory (endosymbiont theory) somes are termed a tetrad. In diplotene the that eukaryotes arose as a result of symbi- pairs of chromatids begin to separate from otic associations between prokaryotes. See the tetrad formed by the association of ho- Bacteria; cell; endosymbiont theory. mologous chromosomes. Chiasmata can often be seen at this stage. At diakinesis the proline A nonpolar molecule synthe- sister chromatids of homologous pairs of sized from glutamic acid and broken down chromosomes complete their separation, to glutamic acid by the same pathway. The and the chromosomes coil tightly, shorten- derivative 4-hydroxyproline is an impor- ing and thickening. In practice, there is no tant component of cell walls. Proline has clear demarcation between these stages: been found in every protein studied to date. the process is continuous. The events oc- See amino acids. curring during these stages differ in meiosis and mitosis, notably in that bivalents (pairs promoter A specific DNA sequence of homologous chromosomes) are formed within an OPERON that initiates TRANSCRIP- in meiosis, whereas homologous chromo- TION by binding RNA polymerase. The somes remain separate in mitosis. See also promoter is situated between the operator meiosis; mitosis. and the structural genes or genes. It is the starting point for transcription of the struc- proplastid A self-duplicating undiffer- tural genes in the operon, but is not itself entiated plastid, about 0.5–1 m m in diam- transcribed. In Escherichia coli the RNA eter and found in the meristematic regions polymerase has a protein ‘sigma factor’ of plants. They grow and develop into plas- that recognizes the promoter; in the ab- tids of different types. sence of this factor the enzyme binds to, and begins transcription at, random points prop root An adventitious root, arising on the DNA strand. In eukaryotic cells, from one of the lower nodes of the stem binding of RNA polymerase to the pro- close to the soil surface, found at the junc- moter involves proteins called transcrip- tion of stem and soil that grows down to tion factors. the ground and gives additional support to the stem, as seen in maize (Zea mays). prophage See lysogeny. Some palms, mangroves and other tropical trees form woody prop roots, often called prophase The first stage of cell division stilt roots. See also buttress root. in meiosis and mitosis. During prophase the chromosomes become coiled, shorten- prosthetic group The nonprotein com- ing and thickening and becoming visible, ponent of a conjugated PROTEIN. Thus the and the nuclear membrane dissolves. heme group in hemoglobin is an example

190 Protista of a prosthetic group, as are the coenzyme complex tertiary structure, and are mostly components of a wide range of enzymes. enzymes. Fibrous proteins usually have a e.g. the heme group is an iron–porphyrin structural or contractile role. Proteins are complex that acts as a prosthetic group in vital compounds found in all living organ- the cytochromes, important components of isms. Their functions range from enzymes electron-transport chains in both plants and hormones to electron-transfer mol- and animals. See enzyme. ecules, contractile elements, and important genetic elements such as the histones that protandry The maturation of the an- coat DNA in eukaryotic chromosomes. By thers before the stigma, a very common coding for structural proteins and for the condition seen in many specialized insect- enzymes that catalyze the synthesis of pollinated flowers, especially those polli- other proteins and biological molecules, nated by bees. It helps to promote genes control the functioning and develop- cross-pollination and is often combined ment of all living organisms. with self-incompatibility. Compare pro- togyny. See also dichogamy. proteinase See protease. protease (proteinase) An enzyme that protein sequencing The determination catalyzes the hydrolysis of peptide bonds in of the primary structure of proteins, i.e. the proteins to produce peptide chains and type, number, and sequence of amino acids amino acids. Individual proteases are in the polypeptide chain. The most widely highly specific in the type of peptide bond used procedure is the Edman degradation. they hydrolyze. This uses a series of three chemical reac- tions, each taking place at a specific pH, to protein A large, complex molecule remove one amino acid at a time from the made up of one or more polypeptide amino terminus of a protein. The amino chains, i.e. it consists of amino acid mol- acids are then identified by ion-exchange ecules joined together by peptide links. The chromatography. The process is auto- molecular weight of proteins may vary mated in an amino acid analyzer. The from a few thousand to several million. About 20 amino acids are present in pro- Edman degradation can sequence peptides teins. Simple proteins contain only amino with only 5–10 picomoles of protein pre- acids. In conjugated proteins, the amino sent. acids are joined to other groups. The primary structure of a protein is the protein synthesis The process whereby particular sequence of amino acids present; proteins are synthesized on the ribosomes this determines the three-dimensional of cells. The sequence of bases in MESSEN- shape of the molecule. The secondary GER RNA (mRNA), transcribed from DNA, structure is the way in which this chain is determines the sequence of amino acids in arranged; for example, coiled in an alpha the polypeptide chain: each CODON in the helix or held in beta-pleated sheets. The mRNA specifies a particular amino acid. secondary structure is held by hydrogen As the ribosomes move along the mRNA in bonds. The tertiary structure of the protein the process of translation, each codon is is the way in which the protein chain is ‘read’, and amino acids bound to different folded. This may be held by cystine bonds transfer RNA molecules are brought to (disulfide bridges) and by weak polar and their correct positions along the mRNA hydrophobic attractive forces between molecule. These amino acids are polymer- atoms. The quaternary structure is the ized to form the growing polypeptide combination of more than one polypeptide chain. See also transfer RNA; translation. chain. There are two main groups of pro- teins: globular proteins and fibrous pro- proteolysis The hydrolysis of proteins teins. Globular proteins have the more into their amino acids. Enzymes that cat-

191 protocooperation alyze this are proteases or proteolytic en- kingdoms. Most are aerobic, some are ca- zymes. pable of photosynthesis, and most possess undulipodia (flagella or cilia) at some stage proteomics The compilation and char- of their life cycle. Protoctists are typically acterisation of all the proteins in an organ- microscopic single-celled organisms, such ism – its proteome. as the amebas, but the group also has large multicellular members, for example the proteoplast A colorless plastid (leuko- seaweeds and other large algae, some of plast) that stores protein. which are sometimes classified as plants. Protoctists show a wide range of nutri- Proterozoic See Precambrian. tional habits, including photoautotrophs, , phagotrophs and mixo- prothallus (pl. prothalli) A flattened trophs. disk of cells that forms the free-living hap- loid gametophyte generation of certain protoderm The outermost layer of the ferns, e.g. Dryopteris. In homosporous apical meristem: the tissue that develops plants, there is only one type of prothallus from the tunica initials of the apical meris- with both male and female sex organs. In tem and gives rise to the epidermis and, in heterosporous plants, the microspores give some plants, the root cap. In the root it rise to small male prothalli bearing male may also be called the dermatogen. See his- sex organs (antheridia), and larger female togen theory. prothalli bearing female sex organs (archegonia). The prothallus is greatly re- protogyny The maturation of the duced in . See alternation stigma before the anthers. It is a mecha- of generations. nism that helps prevent self-pollination and is most common among wind-polli- Protista In some classifications, a king- nated and unspecialized insect-pollinated dom of simple eukaryotic organisms in- flowers. as in figwort. It is less common cluding the algae and protozoans. It was than protandry. Compare protandry. See introduced to overcome the difficulties of also dichogamy. assigning such organisms that may show both animal and plantlike characteristics protonema (pl. protonemata) The young to the kingdoms Animalia or Plantae. gametophyte that develops fol- Today the grouping is considered to be ar- lowing spore germination. The mature ga- tificial and many taxonomists support the metophyte plants develop from buds that FIVE KINGDOMS CLASSIFICATION whereby the form at several points along the pro- bacteria and fungi are both assigned to sep- tonema. arate kingdoms, while algae and proto- zoans constitute various phyla of the proton pump An energy-requiring meta- kingdom PROTOCTISTA. bolic process that causes protons (hydro- gen ions) to move across a membrane protocooperation See mutualism. against their concentration gradient. This creates a potential difference across the Protoctista A kingdom of simple eu- membrane, a source of potential energy karyotic organisms that includes the algae, that can be used to drive other metabolic slime molds, funguslike oomycetes, and the processes. For example, the inner mem- organisms traditionally classified as proto- brane of the mitochondrion is imperme- zoa, such as flagellates, ciliates, and sporo- able to the movement of protons. During zoans. The members of this kingdom do respiratory electron transport, protons are not share clear phylogenetic links, but are drawn from the internal matrix into the grouped together simply because they do space outside the membrane, creating a po- not not belong in any of the other four tential difference across the membrane. In

192 proximal respiration and photosynthesis, the move- thesis, using the enzyme ATPase. The en- ment of protons back across a membrane ergy thus trapped can be transported to down a diffusion gradient takes place at other parts of the cell. See chemiosmotic sites where it can be coupled to ATP syn- theory; electron-transport chain.

Pseudocarp: various types of pseudocarp

193 pseudoallele protophloem The first-formed PRIMARY protoxylem The PRIMARY XYLEM el- PHLOEM, differentiated from the procam- ements that are formed from the procam- bium in the region just behind the meri- bium first. In seed plants the protoxylem stem. occurs at the innermost edge of the vascu- lar bundles in stems, but external to the protoplasm The living contents of a later-formed xylem in roots. cell, comprising the cytoplasm plus nucleo- plasm. The term usually excludes the large protozoa An old name for animal-like vacuoles. See cytoplasm; nucleus. PROTOCTISTA that included some photosyn- thetic organisms (e.g. Euglena, Volvox). protoplast The protoplasm and plasma membrane of a cell after removal of the cell proximal Denoting the part of an organ wall, where present. This can be achieved that is nearest the origin or point of attach- by physical means or by enzymic digestion. ment. Compare distal. protostele A simple form of stele, unin- pseudoallele A mutation in a gene that terrupted by leaf gaps, consisting of xylem produces an effect similar to another muta- in the center completely surrounded by tion at a different site in the same gene phloem. Most roots have protosteles. The locus. They are closely linked, so recombi- stems of some lower plants (e.g. Ly- nations between them are rare. See cis– copodium) also have protosteles. trans effect.

Ptyxis: various types of leaf ptysis

194 purine pseudocarp (false fruit)A fruit that in- 2. In slime molds, a projection of the body cludes parts of the flower, e.g. the bracts, used for locomotion. inflorescence, or receptacle, other than 3. A temporary fingerlike projection or those derived from the ovary. See also hip; lobe on the body of an ameboid cell. It is fruit; pome. See illustration on page 193. formed by a flowing action of the cyto- plasm and functions in locomotion and pseudogene A DNA sequence that can- feeding. not be transcribed. Although they have no immediate function, pseudogenes have Psilophyta (whisk ferns) In the Five high potential to form new genes by further Kingdoms classification scheme, a phylum mutation as they already have useful se- of vascular seedless plants, the whisk quences, such as those signaling transcrip- ferns. In most modern classifications they tion. Pseudogenes are genes that have are placed in the class Psilopsida of the become ‘switched off’ during the course of Filicinophyta. There are only two living evolution. subtropical genera (Psilotum and Tmesip- teris). pseudoparenchyma A fungal or algal tissue resembling parenchyma but made up psychrophilic Describing microorgan- of interwoven hyphae (fungi) or filaments isms that can live at temperatures below (algae). The of the mushroom and the 20°C. Compare mesophilic; thermophilic. thallus of red algae (e.g. Porphyra) are pseudoparenchymatous tissues. (Pteridophyta) A general term, now largely obsolete, to include any pseudoplasmodium See Acrasiomy- vascular nonseed- bearing plant. Pteri- cota. dophytes include the club mosses (phy- lum Lycophyta), horsetails (Sphenophyta), pseudopodium (pl. pseudopodia) 1. A ferns (Filicinophyta), and whisk ferns leafless stalk that bears the capsule in (Psilophyta). Sphagnum and other mosses that lack a seta. ptyxis The way in which young leaves

Punnet squares

195 pyramid of biomass

purine A simple nitrogenous organic molecule with a double-ring structure – a 6-membered pyrimidine ring fused to a 5- membered imidazole ring. Members of the purine group include adenine and guanine, which are constituents of the nucleic acids, Purine and certain plant alkaloids, e.g. and theobromine. are folded or rolled in the bud. See also ver- nation. pyramid of biomass A type of ecologi- cal pyramid based on the total amount of pulvinus (pl. pulvini) 1. A specialized living material at each trophic level in the group of cells with large intercellular community, which is normally measured spaces that are located at the bases of by total dry weight or calorific value per leaves or leaflets in certain plants. They are involved in nongrowth nastic movements, bringing these about by rapid changes in turgor through loss of water to the inter- cellular spaces. See nyctinasty. 2. A thickened region at the node of a grass stem, which often contains an INTERCALARY MERISTEM.

Pyrimidine punctuated equilibrium A theory of evolution proposing that there have been long periods of geological time, lasting for unit area or volume, and shown diagram- several million years, when there is little matically. The biomass depends on the evolutionary change, punctuated by short amount of carbon fixed by green plants periods of rapid speciation of less than and other producers. The pyramid of bio- 100 000 years. This is in contrast to the mass usually has a more gentle slope than traditional theory (see neo-Darwinism) in the PYRAMID OF NUMBERS because organ- which it is postulated that species have isms at successively higher levels in the evolved gradually throughout geological pyramid tend to be larger than those time. below. A chequerboard dia- pyramid of numbers A type of ecolog- gram used in genetics to illustrate the fre- ical pyramid in which the number of indi- quencies of various genotypes in the vidual organisms at each stage in the food offspring of a particular cross. chain of the ecosystem is depicted dia- grammatically. The producer level forms pure line The succession of descendants the base, and successive levels the tiers. The of an individual that are identical to each shape of the pyramid of numbers depends other and continue to breed true, i.e. they upon the community considered; generally, produce genetically identical offspring. the organism forming the base of a food They are assumed to be homozygous. chain is numerically very abundant, and

196 Q

quadrat A square area of variable size Quaternary (Neogene) The most recent depending on the study (standard sizes are period of the Cenozoic era from about two 0.5 or one meter square to 10 m square or million years ago to the present day and more) taken at random, within which the composed of the Pleistocene and Holocene composition of organisms is noted. The epochs. It was a period of cyclical climate quadrat sampling technique is mostly used change, including several ice ages in high in to study the distribution and mid-latitudes, cycles of varying aridity and abundance of species in plant commu- and humidity in low latitudes, and signifi- nities. cant fluctuations in sea level. This had im- portant implications for plant speciation, qualitative inheritance See discontinu- as species ranges expanded and contracted, ous variation. new species radiated out as climate amelio- rated, and retreating ice afforded opportu- qualitative variation See discontinu- nities for colonization of newly exposed ous variation. land. The Quaternary is also characterized quantitative inheritance See continu- by the emergence of humans and the start ous variation. of human modification of the environ- ment. Increased rates of evolution and ex- quantitative variation See continuous tinction of mammals (due to both climate variation. change and human activity) also affected plant distribution. See also Ice Age. quaternary structure The pattern in which two or more polypeptide chains are quiescent center A group of cells in the linked together to form a protein molecule. center of the apical meristem in which mi- This involves hydrogen bonds, ionic totic divisions are rare or absent. The cells bonds, disulfide bridges and hydrophobic may begin dividing if another part of the interactions. See conformation; protein. meristem is damaged. Compare primary structure; secondary structure; tertiary structure. quillworts See Isoetales.

197 R

race In classification, a taxon below the radioactive dating See radiometric dat- rank of SPECIES, sometimes being placed be- ing. tween subspecies and variety. The term is sometimes used instead of ECOTYPE, for radiometric dating (isotopic dating; ra- groups of individuals that have uniform dioactive dating) Any method of dating ecological preferences or physiological re- that uses the decay rates of naturally oc- quirements. curring radioactive isotopes to assess the age of a specimen or material. Organic raceme (racemose inflorescence; indefinite matter less than 7000 years old can be inflorescence) See inflorescence. dated using radiocarbon dating. This uses the fact that the isotope carbon-14 is found rachis 1. The main stalk on which the in the atmosphere and taken in by plants leaflets are borne in a compound leaf, or on when they photosynthesize, and subse- which the pinnae are attached in the fern quently assimilated by the animals that leaf. feed on them. When plants and animals 2. The central axis of certain inflores- die, no more carbon is taken in and the ex- 14 cences, e.g. wheat (Triticum), that bears isting C decays to the nonradioactive iso- tope carbon-12. If the proportion of 14C to the spikelets. 12C in the atmosphere and the decay rate of 14C to 12C are both known, as they are, radial 1. Aligned along the radius of a then the sample may be dated by finding circle or cross-section. the present proportion of 14C to 12C. Spec- 2. See anticlinal. imens of rocks over 7000 years old can be dated by other radioisotope methods, radial symmetry The arrangement of e.g. potassium–argon dating and rubid- parts in an organism in such a way that ium– strontium dating. cutting in any plane across the diameter splits the structure into similar halves (mir- rainforest See forest. ror images). Many stems and roots show radial symmetry. The term actinomorphy random amplified polymorphic DNA is generally used to describe radial symme- (RAPD) The generation of copies of seg- try in flowers. See also bilateral symmetry. ments of DNA by means of the polymerase chain reaction, whereby the segments mul- radicle The embryonic root. It is the first tiplied are those that lie between two iden- organ to emerge from the seed on germina- tical binding sites for a particular primer, tion. The radicle is joined to the hypocotyl usually about 10 base pairs long. Several and both tissues are derived from the four different segments of DNA may fall into octants nearest the suspensor. The root cap this category, and the amplified DNA can over the tip of the radicle is derived from be sorted using ELECTROPHORESIS. Different the closest cell of the suspensor. The radi- individuals may differ in the number and cle may persist to form a taproot, or it may location of such sites, giving rise to a kind be replaced by adventitious roots. of genetic fingerprint. See genetic finger-

198 receptacle printing; molecular systematics; poly- friends at Cambridge, especially Francis merase chain reaction. Willoughby, helped to finance him in his further studies. His life’s work was an at- Ranunculaceae A highly variable fam- tempt to describe all the known flora and ily of about 2450 species of dicotyledonous fauna. With Willoughby, he traveled plants that includes the buttercups, monks- throughout Europe to study a wide range hoods, anemones, clematis, delphiniums, of plants and animals. In 1667 he pub- and hellebores. lished a catalog of British plants and in the same year was elected to the Royal Society. raphe 1. In some seeds, a ridge formed On Willoughby’s death in 1672 he in- by the portion of the funicle that is fused herited enough money to continue their with the integument. endeavor. His three-volume Historia plant- 2. A slit in the valve of motile diatoms, arum was published between 1686 and thought to be involved in movement by al- 1704. He is best known for fixing the lowing contact between cytoplasm and species as the basic unit in taxonomy and substrate. also for realizing the importance of distin- guishing between monocotyledons and di- raphides Bunches of needlelike crystals cotyledons. of calcium oxalate found in certain plant cells. ray floret See inflorescence.

Raunkiaer’s plant classification A ray fungi See Actinobacteria. classification of growth forms based on the persistence of the shoots and the position ray initial See initial. of the resting buds. The system simplifies assessment of the percentages of different reaction center In PHOTOSYNTHESIS, a plant forms in any given type of vegetation, hypothetical site at which absorbed light specially in temperate communities. See energy is used to transport electrons for also chamaephyte; cryptophyte; helophyte; photophosphorylation. It contains a mol- hemicryptophyte; phanerophyte; thero- ecule of the primary pigment (in plants this phyte. is chlorophyll a, which transfers the elec- trons. Plants have two reaction centers in Ray, John (1627–1705) English natu- the thylakoids of the chloroplast: Photo- ralist and taxonomist. Ray, from a humble systems I and II. background, was appointed a Cambridge fellow in 1648 but lost his fellowship in Recent See Holocene. 1662, having refused to take the oath re- quired by the Act of Uniformity. His receptacle 1. The tip of the angiosperm

Raunkiaer’s plant classification

199 recessive flower stalk upon which the floral organs eign gene is inserted into a VECTOR, e.g. a are inserted. If the ovary is on top of the re- bacterial plasmid or virus particle, which ceptacle with the other organs it is said to then infects the host cell. Inside the host be superior but if it is sunk in the center, cell the recombinant vector replicates and e.g. in the daffodil (Narcissus), it is said to the foreign gene is expressed. One of the be inferior. See also epigyny; hypogyny; most common vectors used with plants perigyny. is the Ti (tumor-inducing) plasmid of the 2. The swollen portion of the lamina bacterium Agrobacterium tumefaciens. (blade) bearing the conceptacles in certain This causes crown gall tumors in plants, algae (e.g. Fucus). and its plasmid has been used on a range of crop plants. recessive An allele that is only expressed in the phenotype when it is in the homozy- recombination The regrouping of genes gous condition. Compare dominant. See that regularly occurs during meiosis as a re- also double recessive. sult of the independent assortment of chro- mosomes into new sets, and the exchange reciprocal cross A cross that tests of pieces of chromosomes (crossing over). whether the inheritance of a particular Recombination results in offspring that character is affected by the sex of the par- differ both phenotypically and genotypi- ent. The cross is thus made both ways, i.e. cally from both parents (recombinants) the character under consideration is car- and is the most important means of pro- ried by the female in one cross and by the ducing variation in sexually reproducing male in the second cross. The procedure organisms. can demonstrate which characters are con- trolled by sex-linked genes. red algae See Rhodophyta. recombinant DNA 1. DNA produced Red Data Book An updatable set of in- by recombinant DNA technology. formation on rare, threatened, and endan- 2. DNA formed naturally by recombina- gered species worldwide, compiled by the tion, e.g. by crossing over in meiosis or by World Conservation Monitoring Center, conjugation in bacteria. See recombina- and published at regular intervals by the tion. Species Survival Commission of the World Conservation Union (WCU). recombinant DNA technology The technique by which foreign DNA, whether red desert soil A type of coarse SOIL rich from another organism or genetically engi- in salts and lime but poor in humus, neered, is inserted into another DNA frag- formed in hot deserts. ment or molecule. The product – recombinant DNA – is fundamental to red light Electromagnetic radiation of many aspects of GENETIC ENGINEERING, par- wavelength approximately 630 nm. It is ticularly the introduction of foreign genes the most effective wavelength for initiating to cells or organisms. There are now many many light-dependent reactions in plants, techniques for creating recombinant DNA, such as the germination of light-sensitive depending on the nature of the host cell or seeds. See photoperiodism; phytochrome. organism receiving the foreign DNA. Par- ticular genes or DNA sequences are cut red tropical soil A type of SOIL formed from the parent molecule using specific under rainforests and in savannas in areas type II RESTRICTION ENDONUCLEASES, or are of alternating wet and dry seasons. It is assembled using a messenger RNA tem- heavily leached, but nutrients released by plate and the enzyme REVERSE TRANSCRIP- decomposition help to maintain the min- TASE. In GENE CLONING using cultures of eral balance. The red color is due to oxides bacterial or eukaryote tissue cells, the for- of iron, magnesium, and aluminum, which

200 replication in some such soils form a hard crust (LAT- rendzina A type of brown earth soil typ- ERITE) at the soil surface. ical of humid to semi-arid grasslands over calcareous rocks. The rendzina is rich in reducing sugar A sugar that can reduce lime and has an upper humus-rich horizon. certain inorganic ions in solution, such as the cupric ions in Benedict’s and Fehling’s repetitive DNA DNA that consists of solutions. Examples are glucose, lactose, multiple repeats of the same nucleotide se- and meltose. quences (repeated sequences). In some plants, such as lilies, repeats may constitute reduction division The first division of over 50% of the genome, leading to unex- meiosis, including prophase, metaphase I, pectedly large genomes. The repeats may and anaphase I. It results in a haploid num- occur together, or they may be widely scat- ber of chromosomes gathering at each end tered. Unlike prokaryotic cells, eukaryotic of the nuclear spindle. See meiosis. cells contain appreciable amounts of repet- itive DNA. For example, satellite DNA reflexed Bent backwards. (tandem DNA) is made up of up to one million adjacent repeats of the same short regeneration The regrowth by an or- sequence, usually not more than 10 base ganism of an organ or tissue that has been pairs long. It occurs in the region of cen- lost by injury or other means. It applies to tromeres and telomeres, and is described as natural regrowth from living roots or cop- highly repetitive DNA. This DNA often piced stems as well as artificial regrowth in has a nucleotide composition significantly different from the rest of the cell’s DNA, the laboratory. Many plant propagation giving it a different density, so it shows up techniques make use of the ability of cut- as a separate band (the ‘satellite’) when tings of shoots, roots, or leaves to regener- centrifuged. Microsatellite DNA, with ate into complete plants. short sequences repeated only 10 to 100 times, is used in DNA fingerprinting. Some regulator gene A gene whose product repetitive DNA is accounted for by multi- can promote or prevent the TRANSCRIPTION ple copies of particular genes, e.g. genes en- of structural genes, which may or may not coding histones and ribosomal RNA. See be adjacent to other regulator genes, and also junk DNA; transposon. may even be on another chromosome. The regulator may code for a REPRESSOR protein replica In a general sense an exact copy. that switches off an operator gene (in bac- In biology specifically used for a thin de- teria). In eukaryotes it may bind to proteins tailed copy of a biological specimen, ob- in a transcription complex to inhibit its ac- tained by spraying the surface with a layer tion, or it may bind to a TRANSCRIPTION fac- of plastic and carbon. Replicas are used in tor. See also operon; promoter. electron-microscope work. relative humidity The ratio of the replication The mechanism by which amount of water vapor present in a body of exact copies of the genetic material are air at a given temperature compared to the formed. Replicas of DNA are made when maximum quantity of water vapor that the double helix unwinds as a result of he- body can contain at the same temperature, licase enzyme action and the separated i.e. when saturated at that temperature. strands serve as templates along which Relative humidity affects the diffusion gra- complementary nucleotides are assembled dient for TRANSPIRATION and evaporation, through the action of the enzyme DNA which proceed more slowly with increase polymerase. The result is two new mol- in relative humidity. ecules of DNA each containing one strand of the original molecule, and the process release factor See translation. is termed semiconservative replication.

201 repressor

Replication of DNA occurs in interphase; group to ADP to form the short-term en- by the end of interphase the replicated ergy carrier ATP, which can then be used DNA and associated histones are in the to power energy-requiring processes within form of chromosomes each consisting of the cell. ATP is not transported between two chromatids joined at the centromere. cells, but is made in the cell where it is re- In RNA viruses an RNA polymerase is in- quired. volved in the replication of the viral RNA. The chemical reactions of respiration normally require oxygen from the environ- repressor A protein molecule that pre- ment (AEROBIC RESPIRATION). Some organ- vents protein synthesis by binding to the isms are able to respire, at least for a short operator sequence of the gene and prevent- period, without the use of oxygen (ANAER- ing transcription. The molecule is pro- OBIC RESPIRATION), although this process duced by a regulatory gene and may act produces far less energy than aerobic res- either on its own or in conjunction with a piration. A few bacteria can survive in- second protein called a corepressor. In definitely in anaerobic conditions. The some cases another molecule, an inducer, complex reactions of cell respiration fall may bind to the repressor, weakening its into two stages, GLYCOLYSIS and the KREBS bonds with the operator and derepressing CYCLE. The first stage, glycolysis, occurs the gene, allowing transcription to pro- within the cytoplasm, but the Krebs cycle ceed. Such a molecule is termed an effector. enzymes are localized within the mitochon- See operon. dria of eukaryotes; cells with high rates of respiration have many mitochondria. Gly- reproduction See asexual reproduction; colysis results in partial oxidation of the sexual reproduction. respiratory substrate to the 3-carbon com- pound pyruvate. It does not require oxy- resin One of a group of substances oc- gen, so can occur in anaerobic conditions, curring in many trees and shrubs, including when the pyruvate is converted to ethanol conifers and certain families of flowering in plants and most bacteria, or to lactic plants. They are usually secreted by special acid in certain bacteria. The yield of gly- cells into long resin ducts or canals. Some- colysis is 2 molecules of ATP plus 2 mol- times they are produced in response to in- ecules of the reduced coenzyme NADH2 jury or infection. They sometimes form a for each molecule of glucose respired. In sticky covering to buds (e.g. horse chest- the presence of oxygen, the pyruvate is fur- nut), reducing transpiration and giving ther oxidized in the Krebs cycle. In the protection. Krebs cycle, which requires free oxygen, pyruvate is converted into the 2-carbon resolving power (resolution) The abil- acetyl group, which becomes attached to a ity of an optical system to form separate coenzyme forming acetyl coenzyme A. This images of closely spaced objects. It depends then enters a cyclic series of reactions dur- upon the perfection of the lens, and is theo- ing which carbon dioxide is evolved and retically limited by the wavelength of light, hydrogen atoms are transferred to the being greater with decreasing wavelength. coenzymes NAD and FAD. The energy re- See microscope. leased by the Krebs cycle is transferred via the reduced coenzymes NADH2 and respiration The oxidation of organic FADH2 to an ELECTRON-TRANSPORT CHAIN molecules to provide energy in plants and embedded in the inner mitochondrial animals. Autotrophic plants respire mol- membrane. According to the chemiosmotic ecules that they have synthesized by photo- theory, electrons flow through the various synthesis. Respiration occurs in all cells, components of the chain, while a gradient whether or not they are capable of photo- of hydrogen ions is maintained by an en- synthesis. The energy from respiration is ergy-requiring (active) proton pump that used to attach a high-energy phosphate drives protons into the intermembrane

202 Rf value space. This gradient drives the formation members of a population in the sites at of ATP. Overall, 38 molecules of ATP are which restriction enzymes cleave the DNA, generated for each molecule of glucose ox- and hence in the size of the resulting DNA idized during aerobic respiration, com- fragments. It results from differences be- pared with only 2 molecules of ATP during tween individuals in nucleotide sequences anaerobic respiration. At the end of the at the cleavage sites (restriction sites). The electron-transport chain the electrons react presence or absence of particular restric- with protons and oxygen to give water. In tion sites can be ascertained using DNA aerobic respiration, therefore, pyruvate is probes in the technique called SOUTHERN completely oxidized to carbon dioxide and BLOTTING. Restriction sites vary enor- water. mously, and this variation is exploited in analyzing and comparing the genomes of respiratory chain See electron-trans- different individuals, e.g. to establish how port chain. closely related they may be. Restriction sites are also invaluable as genetic markers respiratory quotient (RQ) The ratio of in chromosome mapping, and can be used the volume of carbon dioxide evolved dur- to track particular genes. RFLPs are used in ing RESPIRATION to the volume of oxygen GENETIC FINGERPRINTING. See chromosome absorbed: map. RQ = volume CO2/volume O2 The complete breakdown of sugars by aer- restriction map A map of a segment of obic respiration gives an RQ of 1. This DNA showing the cleavage sites of restric- rises if anaerobic respiration is involved. tion endonucleases and their physical dis- The respiration of proteins or fats pro- tance apart, usually measured in base duces less carbon dioxide, so the RQ falls pairs. It can be used to reveal variations in below one. restriction sites between individuals of the same species or between different species response A change in an organism or in (see restriction fragment length polymor- part of an organism that is produced as a phism). This variation serves as a key to the reaction to a stimulus. organism’s genes, since the restriction sites can be used as markers to identify closely restriction endonuclease (restriction en- linked genetic loci and allow investigation zyme)A type of enzyme, found mainly in of deletions, insertions, or other mutations. bacteria, that can cleave and fragment RFLPs are an essential tool in chromosome DNA internally (see endonuclease). Their mapping. function is to protect the cell against for- eign DNA, such as the DNA of an invading reticulate thickening The type of virus. Some restriction endonucleases secondary-wall formation in which an ir- cleave DNA at random, but a particular regularly branching mass of lignified de- group of enzymes, known as class II posits covers the inner wall of the cell. It is restriction endonucleases, cleave DNA at usually found in metaxylem tracheids and specific sites. The discovery of these en- vessels. Compare annular thickening; zymes formed the basis for the develop- scalariform thickening. ment of genetic engineering, since they enable the isolation of particular gene se- reverse transcriptase An enzyme that quences and the DNA fragments are repli- catalyzes the synthesis of DNA from RNA cated by means of base pairing and DNA (i.e. the reverse of transcription, in which ligases. See also recombinant DNA tech- mRNA is synthesized from a DNA tem- nology. plate). The enzyme occurs in certain RNA viruses and enables the viral RNA to be restriction fragment length polymor- ‘transcribed’ into DNA, which is then inte- phism (RFLP) Variation among the grated into the host DNA and replicates

203 Rhizobium with it. It is also used in genetic engineering and in the photooxidation of endogenous to make complementary DNA (cDNA) auxins. It is one of the water-soluble from an RNA template. B-group of VITAMINS. It is found in cereal grains, peas, and beans. Riboflavin is a RFLP See restriction fragment length constituent of several enzyme systems polymorphism. (flavoproteins), acting as a coenzyme for hydrogen transfer in the reactions cat- Rf value In chromatography, a ratio alyzed by these enzymes. that expresses the degree to which a solute moves in a solvent. It is calculated by di- ribonuclease (RNase) Any nuclease en- viding the distance moved by the spot of zyme that cleaves the phosphodiester solute on the chromatogram column by the bonds between adjacent nucleotides in distance traveled by the solvent front. The RNA. Exoribonucleases cleave nucleotides Rf value is constant for a particular mol- from one or both ends of the RNA mol- ecule and serves to identify it. ecule, while endoribonucleases cleave bonds within the molecule. RNase is Rhizobium A spherical or rod-shaped thought to be involved in the incompatibil- nitrogen-fixing bacterium that can live ei- ity system that prevents pollen of certain ther freely in the soil or symbiotically in the genetic composition from germinating on root nodules of leguminous plants and a the style of a flower. few other species, such as alder (Alnus). See also nitrogen cycle; nitrogen fixation. ribonucleic acid See RNA. A unicellular rootlike structure ribose A monosaccharide, C5H10O5; a found in certain algae and in the gameto- component of RNA. phyte generation of the bryophytes and some ferns. It serves to anchor the plant ribosomal RNA (rRNA) See ribosome. and absorb water and nutrients. ribosome A small organelle that is the rhizome A stem that grows horizontally site of protein synthesis in the cell. Ribo- below ground. Rhizomes may be fleshy, somes are found in large numbers in all e.g. Iris, or wiry, e.g. couch grass (Elytrigia cells, free in the cytoplasm or bound to EN- repens) and may serve as an organ of DOPLASMIC RETICULUM. In most species they perennation or vegetative propagation, or are composed of roughly equal amounts of occasionally both. Compare corm; tuber; protein and ribosomal RNA (rRNA). The stolon. See also rootstock. ribosome consists of two unequally sized rounded subunits arranged on top of each Rhodophyta (red algae) A phylum of other like a cottage loaf. Eukaryotic cells protoctists comprising aquatic, mainly ma- have larger ribosomes than prokaryotic rine, algae characterized by their red color. cells but the ribosomes in mitochondria The color results from photosynthetic pig- and chloroplasts are about the same size as ments, phycoerythrin and phycocyanin prokaryotic ribosomes. (see phycobilins). Most species are marine, The ribosome comprises different types a few freshwater or terrestrial; their distri- of rRNA and associated proteins. The bution ranges from the shore to quite deep rRNA interacts directly with the mRNA water. during initiation of translation, and there are specific sites on the rRNA that interact rhytidome See bark. with anticodons on the transfer RNAs (tRNA). The proteins have various roles, riboflavin (vitamin B2)A flavin pigment including keeping the mRNA in a single- that is thought to be involved in the recep- stranded state so that the bases are exposed tion of the light stimulus in phototropism, for TRANSLATION. During translation the

204 root ribosome moves along the messenger RNA similar in composition to a single strand of (mRNA), enabling the peptide linkage of DNA except that the sugar ribose replaces amino acids delivered to the site by trans- deoxyribose and the pyrimidine base uracil fer RNA molecules according to the code replaces thymine. RNA is synthesized in in mRNA. Several ribosomes may be ac- the nucleus, using DNA as a template, and tively engaged in protein synthesis along exists in three main forms (messenger the same mRNA molecule, forming a RNA; transfer RNA; ribosomal RNA). In polyribosome, or polysome. certain viruses RNA is the genetic material. ribozyme Any RNA molecule that acts RNA polymerase See polymerase. as an enzyme. RNase (ribonuclease) An enzyme that ribulose A 5-carbon sugar that is phos- catalyzes the hydrolysis of the sugar– phorylated to form RIBULOSE BISPHOSPHATE. phosphate bonds of RNA. There are sev- eral types, each having a specific action. ribulose bisphosphate (RUBP) A 5- For example ribonuclease T1 degrades carbon compound that accepts carbon RNA to mono- and oligonucleotides termi- dioxide during PHOTOSYNTHESIS. Each mol- nating in a 3¢-guanine nucleotide, while ecule is then converted into two molecules those produced by ribonuclease T2 termi- of 3-carbon phosphoroglyceric acid. The nate in a 3¢-adenine nucleotide. ribulose bisphosphate is regenerated in the Calvin cycle when the carbon dioxide is RNA splicing In eukaryote cells, the converted into carbohydrate. process whereby noncoding segments (IN- TRONS) of the primary RNA transcript are ribulose bisphosphate carboxylase removed and the remaining coding seg- (RuBP carboxylase; rubisco) An enzyme ments (EXONS) joined together to form the that catalyzes the carboxylation of ribulose functional mRNA molecule. It is catalyzed bisphosphate to two molecules of glycerate by a complex of proteins and RNA mol- 3-phosphate, the carbon dioxide-fixing ecules (a spliceosome), or sometimes by first step in the Calvin cycle of PHOTOSYN- mRNA itself (see ribozyme). See transcrip- THESIS. It may be as much as 15% of the tion. total protein in the chloroplast. Rubisco can also act as an oxygenase, breaking root The organ that anchors a plant to down ribulose bisphosphate in the pres- the ground and that is responsible for the ence of oxygen to glycerate 3-phosphate uptake of water and mineral nutrients and phosphoglycolic aid. This reaction from the soil. Roots develop from the radi- forms the basis of PHOTORESPIRATION . The cle of the embryo and, according to the relative concentrations of oxygen and car- nature of branching from the seedling root, bon dioxide in the surrounding air deter- a fibrous or tap root system develops. mine the balance of photosynthesis and Roots differ from shoots in lacking chloro- photorespiration and thus the efficiency of phyll and not producing buds or leaves. carbon fixation. See also C3 plant; C4 Roots also differ from shoots in the plant. arrangement of xylem and phloem, having a solid central strand of vascular tissue ring-porous Describing wood in which rather than a hollow cylinder of conduct- the largest vessels are in the early wood, ing tissue. giving distinct growth rings, as seen in elm. The growing point of the root is pro- Compare diffuse-porous. See also annual tected by a root cap to withstand the abra- ring. sion that occurs as the root grows through the soil. The direction of root growth is RNA (ribonucleic acid)A nucleic acid controlled by both gravity and water sup- comprising a single polynucleotide chain ply. The main absorptive region of the root

205 root hair is just beyond the zone of elongation be- derground stem or rhizome. It is found in hind the root tip, where the ROOT HAIRS are many angiosperms, e.g. rhubarb (Rheum formed. Further back along the root, lat- spp) and strawberry (Fragaria ananassa), eral roots are formed, which develop from and in certain ferns, e.g. Osmunda. It can within the vascular tissue and grow out reproduce vegetatively. through the cortex (endogenous root for- mation). All roots whose derivation can Rosaceae A large family of dicotyle- eventually be traced back to the radicle are donous trees, shrubs, and herbs with alter- called primary roots. Roots that arise in nately arranged (opposite in Rhodotypos), any other way are termed adventitious simple, compound, or dissected leaves with roots, for example those that develop from stipules, and actinomorphic flowers with the stem in bulbs and corms. In many parts in fives and stamens in one to several plants, particularly biennials, the root may whorls of five or more. The ovary is infe- become swollen with carbohydrates and rior or partly inferior or the carpels lie free act as an underground food store during in a hollow receptacle. The fruit may be a the winter. Many other root modifications head of follicles or achenes, as in avens are seen (see buttress root; contractile root; (Geum), achenes on a fleshy receptacle, as prop root). The roots of many plant species in strawberry (Fragaria); a drupe, as in exist in association with fungi as myco- cherries, plums, and peaches (Prunus); a rrhizae or, as in the Fabaceae, with nitro- head of drupelets, e.g. blackberry and rasp- gen-fixing bacteria to form root nodules berry (Rubus); or a pome, as in apple (see nitrogen fixation). (Malus). There are about 2825 species. Many species and cultivated hybrids, espe- root hair An outgrowth, or trichome, cially roses and cherry trees, are grown from the piliferous layer of the root. Root as ornamentals for their showy flowers. hairs are projections of single epidermal Cotoneaster and Spiraea are also popular cells that make contact with the soil for up- garden shrubs. Other edible fruits include take of water and solutes, increasing the loquats (Eriobotrya japonica), pears surface area of the root for absorption. (Pyrus communis), quinces (Cydonia ob- They may also have an anchoring role, longa), and (Fragaria). helping to retain contact between the root tip and the soil under dry conditions when rosette plant Any plants whose leaves the soil tends to shrink away from the radiate outward from a short stem at soil roots. level, e.g. daisy (Bellis perennis). This growth form helps plants to withstand root cap See calyptra. trampling or grazing and in exposed habi- tats avoids exposure to strong winds. root nodule See nitrogen fixation. royal ferns See Osmundales. root pressure The pressure that may build up in a plant root system due to the Rubiaceae A large family of dicotyle- osmotic potential of the root cells, which is donous trees, shrubs, and herbs, including thought to help force water upward in the some lianas, epiphytes, and ant plants (e.g. xylem vessels. It is demonstrated by the Myrmecodia). They have decussately oozing of water from the surface of a cut arranged simple leaves often bearing elab- stem. Root pressure tends to build up at orate stipules, and flower parts fused, in night, when the rate of transpiration is low, fours or fives (sometimes including one and is a cause of GUTTATION in some extra-large, brightly colored sepal), and species, especially grasses. See osmosis; 4–5 stamens that alternate with the corolla water potential. lobes. The ovary is usually inferior, with two fused carpels. The fruit is a capsule, rootstock A vertical, usually short, un- berry, or schizocarp. There are about

206 Rutaceae

10 200 species, mostly tropical, but with runner A branch, formed from an axil- some in temperate and arctic regions. lary bud, that grows horizontally along the Many species have long corolla tubes, an ground. Runners often have greatly elon- adaptation to pollination by butterflies, gated inernodes. The axillary buds of the moths, or birds. Most temperate species runner may develop into daughter plants are herbs, e.g. bedstraws (Galium spp.), with adventitious roots growing from the typically with square stems and whorls of node, as seen in the creeping buttercup. leaves. The family includes a few light tim- Conversely, only the terminal bud may ber trees, coffee (Coffea arabica), and Cin- form a new plant, as in the strawberry. A chona, whose bark is a source of quinine. new runner then develops from a branch of There are some cultivated ornamentals, this daughter plant. If the intervening run- such as Gardenia jasminoides and Ixora. ner breaks or rots away, the daughter plants become independent, so this is also rubisco See ribulose bisphosphate car- a means of . See boxylase. also offset; stolon.

RuBP carboxylase See ribulose bispho- rusts Parasitic basidiomycete fungi of sphate carboxylase. the order Uredinales. The name derives

207 S

saccharide See sugar. S alleles See multiple allelism.

Saccharomycetales An order of the As- salt gland See hydathode. comycota, containing the yeasts, in which the mycelium is poorly developed or ab- saltmarsh Vegetation growing in saline sent, and the vegetative stage comprises or brackish marshy places, such as river es- mainly single cells that reproduce asexually tuaries and sheltered, muddy coasts, and by budding or fission. The commercially consisting mainly of HALOPHYTES. Salt important yeasts of the genus Saccha- marshes can be productive grazing land, romyces are used in brewing (S. cerevisiae), and serve as important barriers against in- wine-making (S. ellipsoideus), and bread- undation of the hinterland by the sea. making (s.cervisiae): they feed on sugars, and in anaerobic conditions produce samara A nut or achene whose pericarp ethanol (alcohol) and carbon dioxide. is extended to form a wing for wind dis- persal, e.g. ash (Fraxinus excelsior). Sachs, Julius von (1832–97) German botanist. Having obtained his PhD in sand Mineral particles consisting mainly botany at the University of Prague in 1856, of quartz, felspar, and mica, and measur- Sachs occupied a number of posts, culmi- ing between 2 and 0.05 mm in diameter. nating in his appointment as professor of Sandy soils contain at least 85% sand and botany at Würzburg University. He estab- not more than 10% clay. They are light lished that chorophyll is not univerally dis- and drain well. However, retention of nu- tributed in plants, but is confined inside trients and water is poor. Compare silt. definite structures that he named ‘chloro- plasts’, and showed that starch is the first sap 1. The solution of mineral salts and product of photosynthesis. He also discov- sugars that is found in xylem and phloem ered that plants respire in the same way as vessels and oozes out of cut stems. animals, taking up oxygen and giving out 2. The liquid contents of a plant cell vac- carbon dioxide. His work on plant growth uole. movements led him to invent the KLINOS- TAT. saponins Any of a class of bitter-tasting GLYCOSIDES that form colloidal solutions in safranin In microscopy, a permanent water and foam when shaken. Saponins red stain used to stain nuclei in plant cells. occur in many plants, such as soapwort It also stains lignin and cutin red and (Saponaria offinicalis). chloroplasts pink. It is often used with a green counterstain such a fast green or a saprobe (saprophyte) An organism that blue counterstain, e.g. hematoxylin. See derives its nourishment by absorbing the staining. products or remains of other organisms. Saprobes usually secrete enzymes on their Salicaceae A family of dicotyledonous food and absorb the soluble breakdown trees, shrubs, and creeping forms including products. Many fungi and bacteria are the willows (Salix) and poplars (Populus). saprobes, and are important decomposers

208 Schultze’s solution in food chains, returning nutrients to the scanning electron microscope See mi- soil by putrefaction and decay. croscope.

Saprolegniales (water molds) An order Schiff’s reagent A reagent used to de- of the OOMYCOTA containing aquatic or tect the presence of aldehyde and ketone soil-dwelling funguslike microorganisms, groups in certain compounds. It is pro- e.g. the water mold, Saprolegnia, which duced by the reduction of fuchsin by sul- may be saprobes or parasites. Some form furous acid, and is oxidized by aldehydes mycelia; others are unicellular. and ketones, restoring the magenta color of the fuchsin. saprophyte See saprobe. schizocarp A dry fruit, formed from two or more carpels, that divides at matu- saprotrophic (saprophytic) The mode rity into one-seeded achenelike segments of nutrition of a SAPROBE. See decomposer. termed mericarps. Such fruits are seen in hollyhock (Alcea). sapwood (alburnum) The outer living xylem cells in a tree trunk, consisting of Schleiden, Matthias Jakob (1804–81) secondary xylem elements, parenchyma, German botanist. Having abandoned a ca- and medullary rays, that are actively in- reer in law, Schleiden eventually became volved in water transport and food stor- professor of botany at the University of age. Compare heartwood. Jena in 1839. After years studying the mi- croscropic structure of plants, he proposed satellite DNA See repetitive DNA. his cell theory Contributions to Phytogen- esis (1838), in which he stated that the var- savanna Tropical GRASSLAND with scat- ious plant structures are all composed of tered trees and/or tall bushes, found in all individual cells. He also recognized the sig- parts of the tropics where there is a pro- nificance of the nucleus. Mistakenly, he nounced dry season. Some savannas have believed that new cells formed by being many grazing animals that affect the veg- budded off the nuclei. His cell theory was etation structure; these and other savannas also adopted and expanded by SCHWANN, have periodic or irregular fires in the dry who applied it to animals also. season. See climax. Schultze, Max Johann Sigismund scab Any plant disease that produces (1825–74) German zoologist. During his dry, raised, scablike lesions due to the for- time as professor of anatomy at Bonn Uni- mation of cork layers. The disease is usu- versity, Schultze studied the cells of many ally caused by a bacterium, e.g. common different species of animals. This led to the publication of the famous paper (1861) in scab of potatoes (Streptomyces scabies), or which he concluded that protoplasm is the a fungus, e.g. apple scab (Venturia inae- physical basis of life. He described cells as qualis). being composed of ‘nucleated protoplasm’ and maintained that this constituent of scalariform thickening A type of sec- cells was more important than cell walls (of ondary wall formation consisting of inter- plants) or membranes. He supported his laced helical bands of thickening giving a conclusion by pointing out that certain em- ladderlike formation. It allows for very lit- bryonic cells do not possess membranes. tle further extension, and is found in tissues such as metaxylem tracheids and vessels, Schultze’s solution A solution of zinc which do not elongate after maturation. chloride, potassium iodide, and iodine See xylem. Compare pitted thickening; spi- used mainly for testing for cellulose and ral thickening. hemicellulose. Both materials stain a blue

209 Schwann, Theodor color with the reagent, that of hemicellu- purpurea) and root-rot (Sclerotinia spp), lose being weaker. See also staining. formed from a mass of hyphae. Sclerotia are often rounded or club-shaped. They do Schwann, Theodor (1810–82) Ger- not contain spores, and eventually produce man physiologist. Schwann was educated either a mycelium or fruiting bodies. See at a number of German universities before also stroma. eventually moving to Belgium. He is best known for his work on different animal tis- Scrophulariaceae A family of dicotyle- sues, which he suggested were composed of donous plants, mainly herbs, but including individual cells, rather than forming di- some trees, shrubs, climbers, and aquatics rectly from molecules. He thus confirmed with a wide range of leaf form and flower the cell theory of Schleiden and extended it structure. A few are semiparasitic on the to animals. His earlier work in Germany, roots of angiosperms, especially grasses, attempting to disprove the theory of spon- such as eyebright (Euphrasia) and louse- taneous generation, led him to discover wort (Pedicularis). There are about about that yeast is involved in fermentation. He 5100 species. Leaves of the foxglove (Dig- was so discredited for this work that he left italis purpurea) are the source of drugs Germany. When Pasteur’s work on fer- such as digitalin. mentation was published in the 1850s, his good name was restored. scutellum The part of the embryo of Poaceae (grasses) that lies next to the scion A shoot or bud from one plant endosperm. It is thought to be the modified that is joined to another plants with roots cotyledon. (the STOCK) by grafting or budding. The scion supplies only aerial parts to the secondary cell wall See apposition. GRAFT. secondary growth Plant growth de- sclereid Any SCLERENCHYMA cell except rived from secondary or lateral meristems, the long fibers. The cell wall has undergone i.e. the vascular and cork cambia. It is usu- secondary thickening, and is often ligni- ally absent in monocotyledons, but occurs fied. The various forms of sclereid include in most dicotyledons and conifers, and in a the star-shaped astrosclereid, the rod- few lower plants. In dicotyledons, the re- shaped macrosclereid, and the isodiametric sult of secondary growth is termed sec- stone cell. ondary thickening since there is usually an increase in width rather than length. The sclerenchyma The main supporting tis- activity of the vascular CAMBIUM gives rise sue in plants, made up of cells with heavily to the secondary xylem and secondary thickened, often lignified, walls and empty phloem. Wood is mostly secondary xylem. lumina. The cells usually have simple un- The cork cambium (PHELLOGEN) gives rise bordered pits. Unlike collenchyma, it is not to the PERIDERM, a protective layer of tissue very extensible and is thus not formed in on the outside of the stem or root, which quantity until after the young tissues have consists of the cork (PHELLEM), the phel- fully differentiated. Sclerenchyma is often logen, and the PHELLODERM (secondary found associated with vascular tissue and cortex). The parts of the plant formed by exists as two distinct types of cell: the long secondary growth are called the secondary FIBER and the shorter SCLEREID. It may de- plant body. Compare primary growth. See velop by thickening of the secondary walls annual ring. of parenchyma cells, or it may arise directly from meristematic cells. secondary phloem See secondary growth. sclerotium (pl. sclerotia) The resting secondary plant body See secondary body of certain fungi, e.g. ergot (Claviceps growth.

210 self-compatibility secondary structure In a PROTEIN, the tive dormant state until conditions are fa- basic shape of the polypeptide, i.e. the way vorable for germination. In annual plants, it folds. There are two main forms: the seeds provide the only mechanism for sur- a-helix and the b-pleated sheet. The viving the cold or dry seasons. Seeds may a-helix is an extended spiral with one turn be formed asexually in certain plants by for every 3.6 amino acids; the turns are APOMIXIS, e.g. in dandelion. held in position by HYDROGEN BONDS be- The development of the seed , tween adjacent C=O and NH groups. Such which makes water unnecessary for fertil- a secondary structure allows for flexibility, ization, is one of the most significant ad- the extent of which is determined by the vances in . It has enabled amount of further linking of the structure gymnosperms and angiosperms to colonize by DISULFIDE BRIDGES. In the b-pleated sheet dry terrestrial habitats where lower plants the polypeptides are more extended, and are unable to establish themselves. adjacent chains of amino acids are ar- ranged antiparallel to each other, linked by seed ferns Seed-bearing gymnosperms hydrogen bonds between the C=O and NH of the extinct order Pteridospermales groups of adjacent chains. b-pleated sheets (Cycadofilicales), represented only by fos- have high tensile strength, but are inflexi- sil forms that flourished in the Carbonifer- ble. Many proteins contain both a-helices ous but became extinct in the Cretaceous. and b-pleated sheets in different parts of The plant body resembled a fern and did the molecule. See conformation; primary not produce flowers, the seeds developing structure; quaternary structure; tertiary from megasporangia borne on the margins structure. of the fronds. Their stems showed some secondary thickening. secondary thickening See secondary growth. seed plants See . secondary xylem See secondary growth. segregation The separation of the two alleles of a gene into different gametes, secretion The process of discharging brought about by the separation of ho- materials from cells. The term is usually mologous chromosomes at anaphase 1 of applied to materials that have been synthe- meiosis. sized by the cell. seismonasty (seismonastic movements) seed The structure that develops from A NASTIC MOVEMENT in response to shock. the OVULE following fertilization in an- An example is the collapse of leaves of the giosperms or gymnosperms. In flowering sensitive plant (Mimosa pudica) when plants one or more seeds are contained touched or shaken. within a fruit developed from the ovary wall. The individual seeds are composed of Selaginella See Selaginellales; Lyco- an embryo and, in those seeds in which phyta. food is not stored in the embryo cotyle- dons, a nutritive endosperm tissue. This Selaginellales An order of the Lyco- difference enables seeds to be classified as phyta (clubmosses) containing the single nonendospermic or endospermic. The living genus Selaginella, sometimes called whole is surrounded by a testa developed spike-mosses, with about 700 species, from the integuments of the ovule. In gym- found worldwide, but especially in the nosperms the seeds do not develop within tropics. They range from erect, sometimes a fruit but are shed ‘naked’ from the plant. tufted, forms to prostrate creeping plants Following dispersal from the parent plant, and climbers. seeds may germinate immediately to form a seedling or may remain in a relatively inac- self-compatibility The capacity to self-

211 self-incompatibility fertilize. Many flowers are capable of self- ganism between maturity and natural fertilization if pollination fails, and a few death. It is usually characterized by a re- species do it regularly. duction in capacity for self-maintenance and repair of cells, and hence deteriora- self-incompatibility (self-sterility) The tion. There is often accumulation of waste condition in whereby male gametes cannot metabolic products, a decrease in dry fertilize female gametes from the same in- weight (as substances are withdrawn from dividual and give rise to a viable embryo. the affected part), and a rise in the rate of The pollen tube may be blocked by callose respiration (see climacteric). as it tries to grow down the style, be pre- vented from penetrating the stigma sur- sensitive plant A plant (Mimosa pu- face, abort on reaching the ovule or the dica) of the family Fabaceae (pea family) pollen grain may fail to germinate. See in- whose leaves are divided into many small compatibility; heterostyly. leaflets or pinnae, which progressively col- lapse and fold when the leaf is touched. selfish DNA DNA that can move This is the result of turgor changes in cells around within the genome of an organism in swellings (see pulvinus) in the petiole at or insert copies of itself at various sites the bases of the leaflets. See also nastic without serving any apparent useful func- movements. tion. The prime examples of selfish DNA are the mobile genetic elements called sepal One of the structures situated im- transposons. Some also regard mediately below the petals of a flower. INTRONS as selfish DNA. Its name derives Their collective name is the calyx. They are from the hypothesis that selection acts often green and hairy and enclose and pro- within the genome, favoring anything that tect the flower bud. Sometimes they are results in increased replication of DNA. brightly colored and attract insects for pol- The apparently surplus DNA does not ap- lination. Sepals contain several vascular pear to confer any advantage on the organ- bundles, and are thought to be derived ism, so the DNA is considered to be acting from leaves. ‘selfishly’. Compare junk DNA; repetitive DNA. See transposon. septum (pl. septa) A wall, partition, or membrane separating two cavities. For ex- self-pollination See pollination. ample, the capsule of a poppy (Papaver) is divided by septa, as are the individual com- self-sterility See self-incompatibility. partments of the mycelia of many fungi. Seliwanoff’s test A standard test for the sere Any plant community in a succes- presence of fructose or other ketose sugars sion in which each community itself effects in solution. A few drops of Seliwanoff’s changes in the habitat that determine the reagent, resorcinol in 50% hydrochloric nature of the following stage. The succes- acid, are heated with the test solution. A sive stages are known as seral stages. Seres red color or red precipitate indicates fruc- result eventually in a . tose. Stages in a secondary succession, which ap- pear when the biotic components of a pri- semiconservative replication See repli- mary sere are destroyed, e.g. by fire, are cation. called subseres; seres in microhabitats are called microseres. The initial (pioneer) semipermeable membrane See osmo- community in a succession is termed a pris- sis. ere. See also climax; halosere; hydrosere; succession. senescence The phase of the aging process of an organism or part of an or- serine An AMINO ACID synthesized from

212 siblings glycerate 3-phosphate. Serine is also a haploid gametes fuse, the diploid number product of photorespiration and other of chromosomes is restored. In this way metabolic reactions. It is a component of sexual reproduction permits genetic re- several phosphoglycerides. It is broken combination, which results in greater vari- down by removal of the amino group to ety in offspring and so provides a form pyruvic acid. mechanism for evolution by natural selec- tion. In organisms showing ALTERNATION sessile Unstalked, for example an acorn, OF GENERATIONS, the gametes may be pro- leaf, flower, or other organ that is attached duced by mitosis in the haploid generation, to the main body of the plant. or by meiosis in the diploid generation and give rise directly to gametes or gamete- seta (pl. setae) The part of the SPOROGO- producing cells. NIUM that forms the stalk between the foot and the capsule in bryophytes, e.g. Funaria shade plant A plant that can tolerate and Pellia. It may contain vascular tissue, and thrive in low light intensity. Some enabling the sporophyte capsule to draw shade plants are sensitive to very bright nutrients from the parent gametophyte. light and cannot live in open habitats.

Sewall Wright effect See genetic drift. shadowing A method of preparation of material for electron microscopy enabling sex chromosomes Chromosomes that surface features to be studied. It can be carry sex-determining genes. They are rare used for small entire structures, subcellular in the plant kingdom, and many plants are organelles, or even large molecules (e.g. hermaphrodite. Sex chromosomes occur in DNA). The specimen is supported on a some dioecious plants, e.g. Silene. In other plastic or carbon film on a small grid and dioecious plants a single pair of alleles de- sprayed with vaporized metal atoms from termines sex. one side while under vacuum. The coated specimen appears blacker (more electron- sexual reproduction The formation of opaque) where metal accumulates, and the new individuals by fusion of two nuclei or lengths and shapes of ‘shadows’ cast (re- sex cells (GAMETES) to form a diploid gions behind the objects not coated with ZYGOTE. In unicellular organisms whole in- metal) give structural information. It is dividuals may unite but in most multicellu- often used in association with freeze frac- lar organisms only the gametes combine. In turing. See freeze fracturing; microscope. organisms showing sexuality, the gametes are of two types: male and female. They shoot The aerial photosynthetic portion may be produced in special organs (e.g. of a plant that generally consists of a stem carpel and anther in angiosperms, archego- upon which leaves, buds, and flowers are nia and antheridia in lower plants and borne. some algae). The gametes may be derived from the same parent (AUTOGAMY) or from short-day plant (SDP) A plant that ap- two different parents (ALLOGAMY). Individ- pears to require short days (days with less uals producing both male and female ga- than a critical length of daylight) in order metes are termed hermaphrodite if in the to flower. In fact, the plants require a min- same flower or MONOECIOUS if in separate imum period of darkness (night length). flowers; those in which male and female Short-day plants are typically found in gametes are borne on different individuals temperate latitudes. Examples include are termed DIOECIOUS. spring-flowering plants such as strawberry Generally meiosis occurs before gamete (Fragaria) and autumn-flowering plants formation, resulting in the gametes being such as Chrysanthemum. See critical day haploid (having half the normal number of length; photoperiodism. Compare long- chromosomes). At fertilization, when the day plant; day-neutral plant.

213 sieve cell siblings (sibs) Two or more offspring walls of many higher plants, especially from the same cross. The term sibs is usu- grasses, where it can be a useful taxonomic ally used in a narrower sense to mean feature. plants derived by selfing or by crossing be- tween genetically similar parents. The term silicula A capsular fruit that is formed sibling species is sometimes used to de- from a bicarpellary ovary. It is flattened, scribe species that are extremely similar be- short, and broad and is divided into two or cause of recent common ancestry. Such four loculi (valves) by a false septum, species may be almost impossible to distin- which bears the seeds. Siliculas are typical guish in the field. of the Brassicaceae, e.g. honesty (Lunaria annua). Compare siliqua. sieve cell See sieve element. siliqua () A fruit of some Brassi- sieve element A vascular cell in the caceae, similar to the SILICULA but longer PHLOEM, whose function is to transport and thinner, for example the wallflower sugars and other nutrients from the site of fruit (Erysimum cheiri). production to the site of utilization or stor- age. The term includes both sieve cells and silt Mineral particles between 0.05 and sieve tubes. Sieve cells are long, narrow 0.002 mm in diameter. A silt soil is one cells with specialized sieve areas in the pri- with more than 80% silt and less than 12% mary wall, perforated by several pores, de- clay, and has a smooth, soapy texture. See rived from pits. The pores are lined with soil. Compare sand. callose, and allow cytoplasmic strands to pass from one sieve cell to the next. They Silurian The period, some 440–405 mil- are found in gymnosperms and lower vas- lion years ago, between the Ordovician cular plants. The sieve tubes of an- and the Devonian periods of the Paleozoic. giosperms are shorter and wider, and have It was a relatively warm period in the highly specialized sieve areas called sieve Earth’s history, during which the first land plates, in some species (e.g. Nicotiana) plants appeared. It is characterized by sim- with several sieve areas, usually on the end ple plants such as liverworts; fossils of walls of the elements. They are usually plants similar to vascular plants, e.g. joined end to end, allowing continuous Cooksonia, are known from the later part passage of materials up and down the of the period, some of which had simple plants. In contrast to xylem components, strands of tracheids. Cooksonia had a slen- sieve tube elements have nonlignified cell der dichotomously branching stem with no walls, and a living enucleate (without a nu- leaves, terminal sporangia and spores im- cleus) protoplast. They are usually associ- pregnated with cutin. It resembles the ated with specialized parenchyma cells psilophyte Rhynia from the Rhynie chert called COMPANION CELLS, whose proto- of the Devonian period. See also geological plasts are connected with the protoplasm time scale. of the sieve tube by plasmodesmata. simple pit See pit. sieve plate See sieve element. siphonostele A stele with a central core sieve tube See sieve element. of pith internal to the xylem. The phloem may lie external to the xylem (ectophloic), silicon A MICRONUTRIENT found in many or both internal and external to the xylem animals and plants, although not essential (amphiphloic). See stele. for growth in most plants. It is found in large quantities in the cell walls of certain SI units (Système International d’Unités) algae (e.g. desmids, diatoms) and horse- The internationally adopted system of tails, and in smaller amounts in the cell units used for scientific purposes. It has

214 soil structure seven base units (the meter, kilogram, sec- changes from red to orange and yellow ond, kelvin, ampere, mole, and candela) with a slight increase in acidity, and thus and two supplementary units (the radian indicates change in pH. and steradian). Derived units are formed by multiplication and/or division of base soil The accumulation of mineral parti- units; a number have special names. Stan- cles and organic matter that forms a super- dard prefixes are used for multiples and ficial layer over large parts of the Earth’s submultiples of SI units. See Appendix. surface. It provides support and nutrients for plants and is inhabited by numerous sliding growth A pattern of plant and various microorganisms and animals. growth seen, for example, in many epider- A section down through the soil is termed mal cells where, in order to accommodate a soil profile and this can characteristically growth by adjoining cells, expanding cell be divided into three main layers or hori- walls of adjacent cells slide over each other. zons. Horizon A, the topsoil, is darker than Thus growth is achieved without disrup- the lower layers due to the accumulation of tion of neighboring cells, although it does organic matter as humus. It is the most fer- result in breakage of plasmodesmata. tile layer and contains most of the soil pop- Compare symplastic growth. ulation and a high proportion of plant roots. Horizon B, the subsoil, contains ma- slime bacteria See myxobacteria. terials washed down from above and may be mottled with various colors depending slime molds See Acrasiomycota; Myx- on the iron compounds present. Horizon C omycota. is relatively unweathered parent material from which the mineral components of the smuts Plant diseases caused by basid- above layers are derived. The depth and iomycete fungi of the order Ustilaginales. content of the horizons are used to classify Many are important parasites of cereals soils into various types, e.g. podsols and and form a mass of sooty black spores in brown earths. The texture, structure, and place of the grain. In other smuts the spores porosity of soil depends largely on the sizes form an uncovered mass of black powder. of the mineral particles it contains and on Examples of smuts are Tilletia and Usti- the amount of organic material present. lago. Soils also vary depending on environmen- tal conditions, notably rainfall. See soil snake’s tongue ferns See Ophioglos- structure. sales. soil profile See soil. snurp See spliceosome. soil structure The arrangement of the society A minor plant community within mineral particles in the soil – whether they a larger community, characterized by a are free or bound into aggregates by de- specific dominant species, for example, a composing organic matter. These units Trillium-dominated society in a commu- may be further grouped into crumbs, nity such as an oak woodland. blocks, plates, and so on. The structure af- fects fertility, drainage, aeration, and ease sodium An element found in all terres- of cultivation. A blocky soil will tend to trial plants, although it is believed not to be dry out and suffer from leaching of miner- essential in most, with the exception of als from the upper horizons. The soil struc- some salt-tolerant C4 plants. ture is affected by soil texture – the sizes of the different mineral particles, which also sodium bicarbonate indicator A mix- influences drainage and aeration, and the ture of the dyes cresol red and thymol blue retention of water by capillarity. The size in a sodium bicarbonate solution, which of the particles depends in part on the par-

215 soil texture ent rock and how it weathers. The main The sorus is covered by a flap of tissue, the size classes of mineral particles are gravel indusium. and stones (greater than 2 mm in diame- 2. The reproductive area of certain algal ter), sand (0.05 to 2 mm in diameter), silt thalli, e.g. Laminaria. (0.002 to 0.05 mm in diameter), and clay 3. The spore-bearing body of a rust fungus. (smaller than 0.002 mm in diameter). Clay soils, which have small particles, tend to be Southern blotting A technique for wet and sticky, with poor drainage and transferring DNA fragments from an elec- aeration. However, they tend to be electro- trophoretic gel to a filter or statically charged and attract humus, form- nylon membrane, where they can be fixed ing a clay–humus complex that can attract in position and probed using DNA probes. and retain minerals. The humus sticks the Named after its inventor, E. M. Southern clay particles together, forming larger ag- (1938– ), it is widely used in genetic gregates and improving drainage and aera- analysis. The DNA is first digested with re- tion. Clay soils can be further improved by striction enzymes and the resulting mixture adding lime, which promotes clumping of of fragments separated according to size particles. Sandy soils have large particles, by electrophoresis on an agarose gel. The and therefore larger spaces between them double-stranded DNA is then denatured to single-stranded DNA using sodium hy- for drainage and aeration. They may need droxide, and a nitrocellulose filter pressed addition of humus or mulches to improve against the gel. This transfers, or blots, the water and mineral retention and prevent single-stranded DNA fragments onto the leaching. nitrocellulose, where they are permanently bound by heating. The DNA probe can soil texture See soil structure. then be applied to locate the specific DNA fragment of interest, while preserving the Solanaceae A family of dicotyledonous electrophoretic separation pattern. See herbs, shrubs, lianas, and small trees. They DNA probe. Compare Western blotting. include commercial food plants such as aubergine (Solanum melongena), potato SP (suction pressure) See osmosis. (Solanum tuberosum), sweet peppers (Cap- sicum), tomato (Lycopersicon esculen- spadix (pl. spadices) See inflorescence. tum), and Nicotiana (the source of tobacco). spathe See inflorescence. solenostele See stele. speciation The formation of one or more new SPECIES from an existing species. solute potential See water potential. Speciation occurs when a population sepa- rates into isolated subpopulations that de- somatic Describing the cells of an or- velop distinctive characteristics as a result ganism other than germ cells. Somatic cells of NATURAL SELECTION or random GENETIC divide by mitosis producing daughter cells DRIFT, and cannot then reproduce with the identical to the parent cell. rest of the population, even if there are no geographical or other physical reasons to soredium (pl. soredia) See lichens. prevent them from doing so. Speciation oc- curs when barriers to interbreeding arise, sorus (pl. sori) 1. A reproductive struc- such as geographical changes (e.g. in river ture found in ferns comprising a collection courses), habitats becoming uninhabitable of sporangia borne on a cushion of tissue (e.g. following urbanization), changes in termed the placenta. The placenta develops timing of flowering, or when populations over a vein ending on the underside of a at the margins of the main population foliage leaf or on a specialized fertile leaf. experience different selection pressures

216 spiral thickening as they colonize different environments. spermatophyte (seed plant) Any seed- Another cause of rapid speciation in plants bearing plant. In many older classifications is polyploidy. See also adaptive radiation. they constituted the division (phylum) Spermatophyta, subdivided into the classes species A taxonomic unit comprising Angiospermae and Gymnospermae. Com- one or more populations, all the members pare tracheophyte. of which are able to breed amongst them- selves and produce fertile offspring. They spermatozoid See antherozoid. are normally isolated reproductively from all other organisms, i.e. they cannot breed S phase See cell cycle. with any other organisms. Two or more re- lated species unable to breed because of Sphenophyta A phylum of vascular geographical separation are called allo- nonseed-bearing plants that contains one patric species. Related species that are not living order, the Equisetales, comprising geographically isolated, and which could one genus, Equisetum (horsetails or scour- interbreed but in practice do not because of ing rushes), and three extinct orders, the differences in behavior, breeding season, Calamitales, , and Pseudo- etc., are called sympatric species. Some borniales. Sphenophytes were particularly species can interbreed with other related abundant in the Carboniferous period species, but the hybrids are often wholly or when the genus Calamites formed a large partially sterile. For some species there proportion of the forest vegetation, some may be an almost complete continuum be- treelike species reaching heights of 30 m. tween closely related species, as in certain spherosome A small spherical organelle orchids. This may also happen in disturbed of plant cells, about 0.5–1.0 m m in diame- areas or in regions where the range of two ter, bounded by a single membrane and or more species or even genera overlap, storing lipid. producing hybrid swarms. Where species form apomictic clones that are very diffi- spike See inflorescence. cult to distinguish from each other, they aggregate species are termed , e.g. bramble spindle The spindle-shaped structure (Rubus fruticosus agg.). In general, species formed in the cytoplasm during MITOSIS are regarded as distinct if they remain re- and MEIOSIS that is responsible for moving productively isolated for most of their geo- the chromatids and chromosomes to oppo- graphical range and have recognizably site poles of the cell. The spindle consists of distinct morphological characteristics. a longitudinally orientated system of pro- Within a species, there may be subgroups tein MICROTUBULES whose synthesis starts with distinct morphological or (especially late in interphase. A special region of the in microorganisms) physiological charac- centromeres of each pair of sister chro- teristics; these groups are termed SUBSPECIES matids, the kinetochore, becomes attached or RACES. Groups of similar species are to one or a bundle of spindle microtubules. classified together in genera. During anaphase, the kinetochore itself acts as the motor, disassembling the at- spectrophotometer An instrument for tached microtubules and hauling the chro- measuring the amount of light of different matid toward the spindle pole. Later in wavelengths absorbed by a substance. It anaphase, the unattached interpolar micro- gives information about the identity or tubules actively slide past each other, elon- concentration of the specimen and can be gating the entire spindle. used to plot absorption spectra. It can also be used to investigate how electromagnetic spine A modified leaf reduced to a radiation interacts with matter, e.g. the ab- sharply pointed structure. In some xero- sorption of light in photosynthesis. phytes, such as cacti, most of the leaves are

217 spirillum modified into spines, greatly reducing the ing process and the assembly of the spliceo- leaf area for transpiration; the photosyn- somes. See transcription. thetic function of the leaves is taken over by the green stems. The dense mass of spongy mesophyll See mesophyll. whitish spines also helps to reflect incom- ing radiation, further reducing transpira- The erroneous tion. belief that modern living organisms can be formed directly and spontaneously from spiral thickening (helical thickening) inorganic material, given the right condi- The type of secondary wall formation in tions (). This belief was dis- which a spiral band of lignified deposits is proved by Redi and Pasteur in the 17th and formed on the inner wall of the cell. It is 19th centuries. found in protoxylem and first metaxylem tracheids and vessels. Like annular thick- sporangiophore A structure bearing ening, it allows for continued elongation of sporangia (see sporangium). In fungi it the xylem. See annular thickening; xylem. may be a simple hypha or erect branch of a Compare reticulate thickening; scalariform hyphae; in lower vascular plants it may be thickening. a leaf (SPOROPHYLL) or a STROBILUS. spirillum A helically shaped bacterium. sporangium (pl. sporangia) A repro- ductive body in which asexual spores are spirochetes Long spirally twisted bacte- formed. It may be unicellular, as in simple ria surrounded by a flexible wall. An axial algae and fungi, or multicellular, as in plants. The spores are liberated by rupture filament, with a similar structure to a bac- of the sporangium wall. terial flagellum, is spirally wound about the protoplast inside the cell wall. They spore A unicellular or multicellular swim actively by flexing the cell. They are plant reproductive body. Generally the found in mud and water and can withstand term is applied to reproductive units pro- low oxygen concentrations. Many spiro- duced asexually, such as the spores of chetes are pathogens causing yaws, bryophytes and ferns. A prefix is often syphilis, and relapsing fever. added, providing information as to the na- ture of the spore, for example coni- Spirogyra A genus of filamentous green diospores arise on a conidium, megaspores algae found in freshwater and having a and microspores are the larger and smaller characteristic spiral chloroplast. Repro- spores produced by heterosporous plants, duction is by conjugation: two filaments etc. In plants that undergo ALTERNATION OF become aligned and pairs of cells, one from GENERATIONS, spores are produced by each filament, become joined by a conjuga- meiosis by the sporophyte generation, so tion tube. The contents of one cell pass are usually haploid; they germinate to pro- through the tube and fuse with the contents duce the haploid gametophyte generation. of the second cell to form a zygote. See Gamophyta. spore mother cell A cell that gives rise to four haploid spores by meiosis. In het- spliceosome A unit in the nucleus that erosporous species, a distinction is usually removes the noncoding RNA (INTRONS) made between megaspore mother cells and from the initial RNA transcript and splices microspore mother cells. Of the four prod- the RNA molecule back together again. It ucts of a megaspore mother cell, three usu- consists of several small ribonucleopro- ally abort. teins called SNURPS, comprising RNA com- bined with proteins. The RNA molecules sporocarp A hard multicellular spore- act as RIBOZYMES, catalyzing both the splic- containing structure found in water ferns

218 starch

(e.g. Marsilea and Pilularia). The sori be- are stained by acidic or basic dyes respec- come enclosed by the growing together of tively. Basic stains are suitable for nuclei, fertile fronds, and the spores are not liber- staining DNA. Stains for light microscopy ated until the sporocarp decays and rup- are colored dyes; those for electron mi- tures. croscopy contain heavy metals, e.g. os- mium tetroxide. sporogonium The sporophyte genera- For immediate observation, temporary tion in mosses and liverworts. It develops stains may be used. These stains may fade from the zygote and comprises the foot, in time, e.g. ruthenium red. Or the stain seta, and capsule. The sporogonium is par- may damage the specimen if left, for exam- asitic on the gametophyte generation. ple SCHULTZE’S SOLUTION, which dissolves cell walls and causes swelling. Permanent sporophore The aerial spore-producing stains are used for specimens that are to be body of certain fungi, e.g. the mushroom of kept. They do not fade or damage speci- Agaricus. mens. Examples include fast green and the counterstain safranin. See also negative sporophyll The sporangium-bearing staining. structure of vascular plants. In some ferns sporangia develop on the normal foliage stamen The male reproductive organ in leaves but in higher plants, the sporophylls flowering plants consisting of a fine stalk, are highly modified leaves. They may be the filament, bearing the pollen producing grouped together in a strobilus or, in the anther. It is equivalent to the microsporo- angiosperms, located in the flowers. See phyll present in the gymnosperms and het- megasporophyll; microsporophyll. erosporous members of the Filicinophyta and Lycophyta. The collective term for the sporophyte The diploid generation in stamens is the androecium. See also micro- the life cycle of a plant, which gives rise sporophyll. asexually to haploid spores. It arises by fu- sion of haploid gametes produced by the A sterile stamen. It may be haploid generation (GAMETOPHYTE). In vas- rudimentary, consisting of only the fila- cular plants, the sporophyte is the domi- ment, as in figwort (Scrophularia), or it nant generation, while in bryophytes it is may form a conspicuous part of the flower, parasitic on the gametophyte. See alterna- as in Iris. tion of generations. standard deviation In statistics, a mea- staining A procedure that is designed to sure of the dispersion of a frequency distri- heighten contrast between different struc- bution: it is the average magnitude of tures. Normally biological material is lack- deviations from the center of a normal ing in contrast, protoplasm being curve, calculated by squaring all the devia- transparent, and therefore staining is es- tions, calculating their mean, then finding sential for an understanding of structure at the square root of the mean. This gives a the microscopic level. VITAL STAINS are used value s, which is the point of maximum to stain and examine living material. Most slope either side of the center of the curve. stains require dead or nonliving material. It is more suitable for use than the mean Staining is done after fixation and either deviation, because it avoids problems of during or after dehydration. Double stain- positive and negatives values on either side ing involves the use of two stains; the sec- of the center. ond is called the counterstain. Acidic stains have a colored anion, basic stains have a standing crop The total weight of all colored cation. Some stains are neutral. the living organisms present in an ecosys- Materials can be described as acidophilic tem at a given moment, usually expressed or basophilic depending on whether they as dry weight per unit area.

219 starch sheath starch A polysaccharide that occurs ex- starch–statolith hypothesis A hypoth- clusively in plants. It is an early end prod- esis concerning the mechanism of gravity uct of photosynthesis, and is stored perception in plants. See gravitropism. temporarily in the chloroplasts. In the dark this starch is broken down by enzymes to statocyte A gravity-sensitive plant cell sucrose and transported to other parts of containing STATOLITHS. See gravitropism. the plant. Starch is a storage reservoir for statolith One of a number of large the plant: it is broken down by enzymes to starch grains found in the statocytes, plant simple sugars and then metabolized to sup- cells that are thought to be gravity sensi- ply energy needs. It is present in plant cells tive. They move through the cytoplasm to as granules dispersed in the cytoplasm. The the lowermost cell surface. See gravitro- shape of these granules is used in taxon- pism. omy. In most plants it is found mainly in the root cortex, and in storage organs such stearic acid (octadecanoic acid) A satu- as swollen taproots, corms, etc. Starch is rated carboxylic acid, which is widely dis- extracted commercially from maize, tributed in nature as the glyceride ester. It wheat, barley, rice, potatoes, sorghum, is present in most fats and oils of plant and cassava, and other crops. It is also the main animal origin, particularly the so-called storage material in many seeds. Starch is a hard fats, i.e. those of higher melting point. polymer of glucose. It is not a single mol- Stearic acid is formed from palmitic acid. It is an intermediate in the synthesis of oleic, ecule, but a mixture of amylose (straight linoleic, and linolenic acids. chains of glucose subunits) and amy- lopectin (branching chains of subunits). stele The vascular tissue and associated parenchyma tissue and (if present) the sur- starch sheath The innermost layer of rounding pericycle and endodermis of a cells of the cortex replacing the endodermis stem or root. The arrangement of tissues in in some stems, especially young herba- the stele changes from stem to root. In di- ceous dicotyledonous stems. Its cells con- cotyledons, for example, the stem has a tain prominent starch grains. See also ring of discrete vascular bundles, which al- endodermis. lows it to resist bending, while the root has

Stele: types of stele

220 stoma a central core of vascular tissue, adapting it and lower rainfall in the east. The species to resist pulling stresses. Stelar arrange- composition varies with the climate. Large ments vary considerably in plants from the areas of the steppe are under cultivation for simple protostele, in which leaf gaps are grain production. absent, to the complex dictyostele, in which there are many closely spaced LEAF sterigma (pl. sterigmata) A fingerlike GAPS. Intermediate between these is the projection upon which spores are formed solenostele in which the leaf gaps are more in most basidiomycete fungi. Usually four widely spaced vertically so that only one is sterigmata, each bearing one basidiospore, seen per cross section of the stem. See poly- are borne on each basidium. The cultivated stely. mushroom is an exception, with only two sterigmata on each basidium. stem A longitudinal axis upon which are borne the leaves, buds, and reproductive steroid Any member of a group of organs of the plant. The stem is generally compounds having a complex basic ring aerial and erect but various modifications structure. Many plant steroids are alcohols are found, for example underground stems (sterols); some are alkaloid-like com- like RHIZOMES, BULBS, and CORMS, and hor- pounds. See also sterol. izontal structures, such as RUNNERS. The stem serves to conduct water and food ma- sterol A steroid with long aliphatic side terials up and down the plant and, particu- chains (8–10 carbons) and at least one hy- larly in young plants and some succulents, droxyl group. They are lipid-soluble and it may serve as a photosynthetic organ. The often occur in membranes (e.g. cholesterol stem is generally cylindrical and consists of and ergosterol). regularly arranged conducting (vascular), strengthening, and packing cells, the whole stigma 1. The receptive tip of the carpel being surrounded by a protective epider- of plants. It may be elevated on a stalklike mis. protrusion called a style. 2. See eyespot. steppe Grassland dominated by drought- stilt root An enlarged form of PROP ROOT, seen in some mangroves and a few palms and other trees, that helps support plants in unstable soils.

stimulus A change in the external or in- ternal environment of an organism that elicits a response in the organism. The stimulus does not provide the energy for the response.

stipe 1. In the more highly differentiated Steroid: skeleton of the molecular structure Phaeophyta (brown algae), such as wracks and kelps, the stalk between the holdfast resistant species of perennial grasses, found and the blade. in regions of LOESS soil and extreme tem- 2. The stalk of a mushroom or toadstool, perature range, often on CHERNOZEM soils, which bears the cap or pileus. in a zone from Hungary eastwards through the Ukraine and southern Russia to Cen- A modified leaf found as an out- tral Asia and China. There are pronounced growth from the petiole or leaf base. seasons, with hot summers and cold win- ters, with greater temperature extremes stock A plant onto which SCIONS (shoots

221 stomatal apparatus or buds) of another plant are grafted. The strain Any group of similar or identical stock provides the underground parts of individuals, such as a CLONE, mating strain, the plant, and affects its size, vigor, and the physiological RACE, or PURE LINE. timing of flowering and fruiting. See graft. stratification 1. A seed treatment that stolon An initially erect branch that, due enables seeds that require vernalization to to its great length, eventually bends over germinate the following spring: the seeds toward the ground. Where a node touches are placed between layers of moist sand or the soil adventitious roots develop and the peat and exposed to low temperatures, axillary bud at that node grows out to form usually by leaving them outside through a new plant. See also layering; offset; run- the winter. ner. 2. The existence of layers of water of dif- ferent density in a lake or other body of stoma (pl. stomata) One of a large num- water. The differences in density may be ber of pores in the epidermis of plants due to temperature or to salinity. In many through which gaseous exchange occurs. lakes in summer that is a warm low-density Each stoma is surrounded by two crescent- layer (EPILIMNION) lying above a colder shaped GUARD CELLS, attached to each denser layer (the HYPOLIMNION), the zone other at their ends, which regulate the of rapid temperature change between the opening and closing of the pore by changes two layers being called the thermocline. in their turgidity. The arrangement of cel- lulose microfibrils in the guard-cell walls strobilus (pl. strobili) A group of sporo- causes the guard cell to increase in length phylls (spore-bearing leaves) arranged rather than width as it takes in water. This around a central axis, found in gym- extra length is accommodated by the cell nosperms, Lycophyta (clubmosses), and bulging away from the aperture, so widen- Sphenophyta (horsetails). In the club- ing the pore. Adjacent to the guard cells mosses and horsetails only one type of stro- there are usually modified epidermal cells bilus is formed, while in the gymnosperms called subsidiary or accessory cells. The female megastrobili and male microstrobili stoma and guard cells are collectively both develop. The strobili of gymnosperms called the stomatal apparatus. In most and cycads are also termed cones. plants stomata are located mainly in the lower epidermis of the leaf, where evapo- stroma (pl. stromata) 1. The colorless ration rates are lower. In many xerophytes ground matter between the grana lamellae and in some conifers of cooler climates, in a CHLOROPLAST. It is the site of the dark where soil water may be difficult to obtain (light-independent) reactions of photosyn- in winter, the guard cells – and hence the thesis. stoma – are sunk in pits in the epidermis, 2. A solid mass of fungal hyphae, some- often protected by hairs from passing air times including host tissue, in which fruit- currents. TRANSPIRATION rates are thus ing bodies may be produced. An example is greatly reduced, while allowing gaseous the compact black fruiting body of the exchange for photosynthesis. In vertical ergot fungus (Claviceps purpurea). leaves, such as those of grasses, they may be equally distributed on both surfaces. stromatolite A layered cushionlike mass of carbonate-rich rock formed by the stomatal apparatus See stoma. actions of CYANOBACTERIA. Communities of these organisms secrete calcium carbon- stomium (pl. stomia) The site at which a ate, often building up concentric rings of sporangium or pollen sac ruptures to re- deposits forming cushions up to a meter in lease the spores or pollen. diameter. Modern stromatolite-building communities are confined to salt flats or stone cell (brachysclereid) See sclereid. shallow salty lagoons where invertebrate

222 Sudan stains predators cannot survive, but the fossil number of specialized epidermal cells of a record demonstrates a much more wide- plant that are found adjacent to the guard spread distribution in the Precambrian cells, and may help in opening and closing period. Some stromatolites date back the stomata. The number, arrangement nearly 4000 million years, making them and character of subsidiary cells is often of the oldest known fossils. value in taxonomy. style The stalklike portion of a carpel, subspecies The taxonomic group below joining the ovary and the stigma. The style the SPECIES level. Crosses can generally be may be elongated in plants relying on made between subspecies of a given species wind, insect, or animal pollination so that but this may be prevented in the wild by the stigma has a greater chance of coming various isolating mechanisms, e.g. geo- into contact with the pollinating agent. graphical isolation or different flowering times. Subspecies usually differ from each subclimax See climax. other in morphology, and occupy distinct suberin A fatty acid polyester produced habitats or regional locations. in the walls of the endodermis and in bark, where it renders the tissue impermeable to substrate 1. The molecule or molecules water and resistant to decay. The Caspar- upon which an enzyme acts. ian strip in roots and some stems contains 2. The nonliving material upon which an suberin, lignin, or similar substances. By organism lives or grows. preventing the passage of water through the cell walls of the endodermis, the Cas- succession A progressive series of parian strip forces it to pass through the cy- changes in vegetation and animal life of an toplasm, thus allowing selective uptake or area over time from initial colonization to exclusion of dissolved substances. See the final stage, or CLIMAX. The climax is a cutin. dynamic equilibrium because, although the succession can progress no further under sublittoral 1. (neritic) The marine zone the environmental factors present at the extending from low tide to a depth of time, the populations present change, e.g. about 200 m, usually to the edge of the trees die, creating gaps for other species to continental shelf. Light penetrates to the colonize. In addition, the climate is seldom seabed, and the water is well oxygenated. completely stable – at best it is cyclical, Large algae (e.g. kelps) are found in shal- with variations from year to year. See sere. lower waters while certain Rhodophyta (red algae) may be found in deeper water. succinic acid (butanedioic acid) A di- Compare littoral; benthic. carboxylic acid formed by fermentation of 2. The zone in a lake or pond between the sugars. It is an intermediate in the KREBS LITTORAL and PROFUNDAL zones, extending from the edge of the area occupied by CYCLE, and is formed from succinyl CoA in rooted plants to a depth of about six to ten a reaction linked to the formation of GTP. meters, where the water temperature de- clines. Its depth is limited by the compen- succulent A fleshy plant. Succulents, sation level – the depth at which the rate of such as cacti, store water in large paren- photosynthesis is equalled by the rate of chyma cells in swollen stems and leaves. respiration, and below which plants can- not live (but some phytoplankton can). sucker An adventitious underground shoot that at some stage emerges above the subsere See sere. soil surface and gives rise to a new plant, which initially is nourished by the parent subsidiary cell (accessory cell) One of a plant until it becomes established.

223 sugar sucrase An enzyme that breaks down trisaccharides, etc. See also fructose; glu- sucrose into glucose and fructose. cose; polysaccharide; sucrose. sucrose A SUGAR that occurs in many sugar acid An acid formed from a plants. Sucrose is a disaccharide formed monosaccharide by oxidation. Oxidation from a glucose unit and a fructose unit. of the aldehyde group (CHO) of the aldose Sucrose is the main transport sugar in monosaccharides to a carboxyl group higher plants. It is formed in the chloro- (COOH) gives an aldonic acid; oxidation plasts from fructose 6-phosphate and glu- of the primary alcohol group (CH2OH) to cose, and transported in the phloem to the COOH yields uronic acid; oxidation of rest of the plant. It is hydrolyzed to a mix- both the primary alcohol and carboxyl ture of fructose and glucose by the enzyme groups gives an aldaric acid. The uronic invertase. Since this mixture has a different acids are biologically important, being optical rotation (levorotatory) from the components of many polysaccharides, for original sucrose, the mixture is called in- example glucuronic acid (from glucose) is a vert sugar. Invert sugar is found in many major component of gums and cell walls, fruits. while galacturonic acid (from galactose) makes up pectin. Ascorbic acid or vitamin suction pressure (SP) See osmosis. C is an important aldonic acid found uni- versally in plant tissues, particularly in cit- Sudan stains Any of various stains rus fruits. Another aldonic acid, gluconic based on aniline dyes, and used to color acid, is an intermediate in the pentose fats and waxes, and hence cutinized (e.g. phosphate pathway. cuticle) and suberized (e.g. Casparian strip) sugar alcohol (alditol) An alcohol de- tissues. See staining. rived from a monosaccharide by reduction of its carbonyl group (CO) so that each sugar (saccharide) One of a class of car- carbon atom of the sugar has an alcohol bohydrates that are soluble in water. Sugar group (OH). For example, glucose yields molecules consist of linked carbon atoms sorbitol, common in fruits, and mannose with –OH groups attached, and either an yields MANNITOL. GLYCEROL is important in aldehyde or ketone group. The simplest lipid metabolism, and INOSITOL is an inter- monosaccharides sugars are the , such as mediate in the synthesis of cell wall poly- glucose and fructose, which cannot be hy- saccharides. drolyzed to sugars with fewer carbon atoms. They can exist in a chain form or in sugar phosphate A phosphate deriva- a ring formed by reaction of the ketone or tive of a MONOSACCHARIDE. aldehyde group with an –OH group on one of the carbons at the other end of the chain. sulfur An essential element in living It is possible to have a six-membered (pyra- tissues, being contained in the amino nose) ring or a five-membered (furanose) acids cysteine and methionine and hence in ring. Monosaccharides are classified ac- nearly all proteins. Sulfur atoms are also cording to the number of carbon atoms: a found bound with iron in ferredoxin, pentose has five carbon atoms and a hex- one of the components of the electron- ose six. Monosaccharides with aldehyde transport chain in photosynthesis. It is groups are aldoses; those with ketone a component of coenzyme A. Plants take groups are ketoses. Thus, an aldohexose is up sulfur from the soil as the sulfate ion 2– a hexose that has an aldehyde group; a SO4 . The sulfides released by decay of or- ketopentose is a pentose with a ketone ganic matter are oxidized to sulfur by SUL- group, etc. FUR BACTERIA of the genera Chromatium Two or more monosaccharide units can and Chlorobium, and further oxidized to be linked in disaccharides (e.g. sucrose), sulfates by bacteria of the genus Thiobacil-

224 syncarpy lus. There is thus a cycling of sulfur in na- ates from the proembryo by mitosis and ture. pushes the embryo into the nutritive en- dosperm after fertilization. In certain Lyco- sulfur bacteria Filamentous autotrophic phyta (e.g. Selaginella), a few ferns (e.g. chemosynthetic bacteria that derive energy Botrychium), and in gymnosperms it by oxidizing sulfides to elemental sulfur pushes the embryo into the female gameto- and build up carbohydrates from carbon phyte tissue. It also serves as a passage for dioxide. They use sulfides instead of water nutrients to pass from the parent tissue to as a source of electrons in photosynthesis, the embryo. releasing sulfur instead of oxygen. An ex- ample is Beggiatoa. They are found mainly swamp An area of vegetation domi- in sulfur-rich muds and springs, including nated by trees that develops on ground that hydrothermal vents. There are two main is normally waterlogged or covered by groups: the (Chloro- water all year round, such as the margin of bia) and the purple sulfur bacteria (certain a lake, a river floodplain, or an area of members of the Proteobacteria). A few Ar- water-retentive clay in an arid region. chaea, e.g. Sulfolobus, can oxidize elemen- Swamp vegetation represents the early tal sulfur. As well as sulfides, some bacteria stages of a HYDROSERE or HALOSERE. The oxidize thiosulfates, polythionates, and presence of trees distinguishes it from a sulfites. Sulfur bacteria play an important marsh. Coastal and estuarine swamps in role in the cycling of sulfur in the eco- warm climates are dominated by MAN- system. GROVES. The swamp vegetation slows the flow of water, and dead plant matter builds supergene A collection of closely linked up. genes that determine a particular trait or series of inter-related traits and behave as a syconium (syconus) A composite inflo- single unit because crossing over between rescence or fruit in which the small indi- them is very rare. An example is the S/s vidual flowers are borne on the inside of a gene, which determines pin (s) and thrum hollow receptacle, as in figs (Ficus). These (S) style length in certain species of Prim- develop into achenes (the ‘pips’ of the ula. fruit). See pseudocarp. superior In plant science, the term is symbiosis (pl. symbioses) Any close as- used with reference to the position of the sociation between two or more different ovary in relation to the other parts of the organisms, as seen in parasitism, mutual- flower. When the ovary is superior, the ism, and commensalism. Often one or both petals, sepals, and stamens are inserted at organisms is dependent on the other. The the base of the ovary where it joins the term is usually used more narrowly to flowerstalk, as in buttercup (Ranunculus) mean MUTUALISM, but many mutualistic as- flowers. In a floral formula a superior sociations may have once been parasitic or ovary is denoted by a line below the carpel may become so at some stage in their life number. Compare inferior. See also hypo- cycle, e.g. plants and their mycorrhizal gyny; perigyny. fungi. suspension culture A method of grow- sympatric species See species. ing free-living single cells or small clumps of cells in a liquid medium; the liquid symplast The living system of intercon- medium is agitated to keep the cells in sus- nected protoplasts extending through a pension. Microorganisms or cells of plant plant body. Cytoplasmic connections be- callus tissue may be grown in this way. tween cells are made possible by the plas- modesmata. The symplast pathway is an suspensor A temporary stalklike row of important transport route through the cells, found in angiosperms, that differenti- plant. Compare apoplast.

225 synchronous culture symplastic growth A form of plant synapsis (pairing) The association of growth in which neighboring cell walls homologous chromosomes during the stay in contact and grow at the same rate. prophase stage of meiosis that leads to the production of a haploid number of biva- sympodial Describing the system of lents. branching in plants in which the terminal bud of the main stem axis stops growing syncarpous See carpel. and growth is taken over by lateral buds. syncarpy The condition in which an These in turn lose their dominance and lat- ovary is made up of fused carpels, as in the eral buds take over their role. Compare primrose (Primula vulgaris). Compare monopodial. apocarpy. symport A process involving a mem- synchronous culture A culture of cells brane protein that transports two kinds of in which all the individuals are at approxi- substance (substrates) across a membrane, mately the same point in the cell cycle. one of which is usually an ion. Compare Cells can be synchronized by a variety of antiport; proton pump. means, e.g. temperature, shock, or drugs.

226 T

tactic movement See taxis. taxis (pl. taxes)(tactic movement) Move- ment of an entire cell or organism (i.e. taiga See forest. locomotion) in response to an external stimulus, in which the direction of move- tandem DNA See repetitive DNA. ment is dictated by the direction of the stimulus. Movement toward the stimulus is tangential Orientated at a tangent to a positive taxis and away from the stimulus circle, the circle being, for example, the is negative taxis. It is achieved by proto- surface of a . plasmic streaming, extrusion of cell sub- stances, or by locomotory appendages, tannin One of a mixed group of sub- such as flagella and undulipodia. See aero- stances that, as defined by industry, precip- taxis; chemotaxis; phototaxis. Compare itate the gelatin of animal hides to form nastic movements; tropism. leather. Tannins are also used in dyeing and ink manufacture. They are polymers taxol A diterpene originally isolated derived either from carbohydrates and from the bark of the Pacific yew (Taxus phenolic acids by condensation reactions, brevifolia), that has proved an effective or from flavonoids. Many plants accumu- treatment for solid tumors, especially late tannins, particularly in leaves, fruits, breast cancer. It blocks tumor cells by sta- seed coats, bark, and heartwood. Their as- bilizing and polymerizing microtubules. tringent taste may deter animals from eat- ing the plant and they may discourage taxon (pl. taxa) A group of any rank in infection. Species that produce large taxonomy. Ranunculaceae (a family) and amounts of tannin include tea (Camellia Triticum (a genus) are examples. sinensis) and the bark of oak (Quercus). taxonomy The area of systematics that tapetum (pl. tapeta)A food-rich layer covers the principles and procedures of surrounding the spore mother cells in the classification, specifically the classification anthers of vascular plants. These cells usu- of variation in living organisms. See classi- ally disintegrate, liberating food substances fication; systematics. that are subsequently absorbed by the spore mother cells and the developing TCA cycle See Krebs cycle. spores. telophase The final stage in mitosis and taproot A persistent primary root that meiosis, in which the separated chromatids grows vertically downward. Swollen tap- (mitosis) or homologous chromosomes of roots used for food storage are common in bivalents (meiosis) collect at the poles of many biennial plants, such as carrot (Dau- the spindle and the nuclei of the daughter cus carota). cells are formed. During this stage in mito- sis and the second division of meiosis the Taxaceae A family of evergreen - chromosomes uncoil and disperse, the nu- ous trees and shrubs, the yews, with clear spindle degenerates, nucleoli reap- needlelike or linear leaves. pear, and a new nuclear membrane forms.

227 temperate phage

The cytoplasm may also divide during this by the rapid evolution and expansion of phase. angiosperms and the emergence of mam- mals. The ferns underwent adaptive radia- temperate phage A DNA bacterio- tion, and the angiosperms underwent an phage that becomes integrated into the even greater explosion of adaptive radia- bacterial DNA and multiplies with it, tion, in which flowers became more spe- rather than replicating independently and cialized. This went hand in hand with the causing lysis of the bacterium. A bacterium evolution of specialist insect pollinators, thus infected is said to be lysogenic. See especially bees, butterflies, and moths. The lysogeny. Compare virulent phage. climate was warm and wet at the start of the period, but became colder and drier temporary stain See staining. later, and this was accompanied by the ex- pansion in range of grasses. Many modern A slender structure found in genera of conifers arose in the Tertiary, but plants, used for twining and support. It other gymnosperms declined. See also geo- may be a modified terminal bud, as in logical time scale. the grapevine (Vitis); a modified lateral branch, as in the passion flower (Passi- tertiary structure The mode of folding flora); a modified leaf, as in Clematis mon- of a polypeptide (a-helix or b-pleated tana; a modified leaflet, as seen in many of sheet) in a PROTEIN. The shape may be sta- the pea family (Fabaceae); or a modified in- bilized by HYDROGEN BONDS, disulfide florescence, as in Virginia creeper (Par- bridges, and hydrophobic interactions, thenocissus). Tendrils may be branched, or whereby hydrophobic parts of the unbranched, and may have terminal adhe- polypeptide are protected from the sur- sive disks, as in the Virginia creeper. rounding aqueous solution by hydrophilic parts. Such folding determines the shape of terminal bud See bud. the active site of enzymes. See conforma- tion; primary structure; quaternary struc- terminalization The movement of chi- ture; secondary structure. asmata to the end of the bivalent arms, a process that may occur during late testa The hard dry protective covering of prophase I of meiosis. The chiasmata can a seed, formed from the integuments of the slip off the ends of the bivalents, and thus ovule. After fertilization the layers of the chiasma frequency may be reduced by ter- integuments fuse and become thickened minalization. and pigmented. terpene One of a complex group of test cross See backcross. lipids based on the hydrocarbon skeleton C5H8 (isoprene). Monoterpenes are built tetrad 1. A group of four cells formed as from two C5 residues (C10H16), diterpenes a result of meiosis in a spore mother cell. from four, etc. The C10 to C20 terpenes are The four cells may develop into spores, as present in essential oils, giving the charac- in the formation of pollen grains. teristic scent of some plants (e.g. mint, 2. In meiosis, the association of four ho- lemon). Some terpenoid substances are mologous chromatids seen during the physiologically active, e.g. vitamins A, E, pachytene stage of prophase. and K. tetraploid A cell or organism containing Tertiary The larger and older period of four times the haploid number of chromo- the Cenozoic, being composed of the Pale- somes. Tetraploid organisms may arise by ocene, Eocene, Oligocene, Miocene, and the fusion of two diploid gametes that have Pliocene epochs (65–2 million years ago). resulted from the nondisjunction of chro- Literally the ‘third age’, it is characterized mosomes at meiosis, or by nondisjunction

228 thorn of the chromatids during the mitotic divi- day temperatures. See also photoperi- sion of a zygote. Many stable tetraploids odism; vernalization. arise initially from hybridization. See also polyploid; allotetraploid; autotetraploid. thermophilic Describing microorganisms that require high temperatures (around thallus (pl. thalli) A simple plant body 60°C) for growth. It is exhibited by certain showing no differentiation into root, leaf, bacteria that grow in hot springs or com- and stem and lacking a true vascular sys- post and manure. Compare mesophilic; tem. It may be uni- or multicellular, and is psychrophilic. found in the algae, lichens, bryophytes, and the gametophyte generation of the Fil- therophyte A plant that survives part of icinophyta, Lycophyta, and Sphenophyta. the year as a seed and completes its life cycle during the remainder of the year. See of Eresus (372–287 BC) also Raunkiaer’s plant classification. botanist and philosopher. Theophrastus was a pupil of Plato and as- thiamine (vitamin B1) One of the water- sisted at the in Athens soluble B-group of VITAMINS. It is a pyrim- after Plato’s death. He became head of the idine compound that is synthesized by school when Aristotle died. He is often plants but not by some microorganisms considered to be the father of scientific and most vertebrates. It is therefore re- botany because of his work on classifying quired in the diet of mammals. Good and naming plants, which he recorded in sources of thiamine include unrefined ce- his prolific writings. His pupils were en- real grains. couraged to observe plants growing near their homes, which were often far away thigmotropism (haptotropism) A tro- from Athens, and possibly helped him to pism in which the stimulus is touch. The make the connection between plants and tendrils of climbing plants are thig- the environment, especially their adapta- motropic. The part touching the substrate tions to soil and climate. His Enquiry into bends toward the stimulated side, so bring- Plants (nine volumes) is important even ing more of the tendril into contact with today. It is evident from his writings that the substrate, until it encircles it. See tro- he had distinguished between monocotyle- pism. dons and dicotyledons and had discovered that seeds and fruits were derived from thin-layer chromatography A chro- flowers. matographic method in which a glass plate is covered with a thin layer of inert ab- thermocline See stratification. sorbent material (e.g. cellulose or silica gel) and the materials to be analyzed are spot- thermonasty (thermonastic movements) ted near the lower edge of the plate. The A NASTIC MOVEMENT in response to change base of the plate is then placed in a solvent, in temperature. For example, Crocus flow- which rises up the plate by capillary action, ers open rapidly if the temperature in- separating the constituents of the mixtures. creases by 5 to 10°C, as the inner side of The principles involved are similar to those the petals grows faster than the outer side. of paper chromatography and, like paper chromatography, two-dimensional meth- thermoperiodism The phenomenon ods can also be employed. See paper chro- shown by certain plants, e.g. Chrysanthe- matography. mum and tomato (Lycopersicon esculen- tum) in which there is a response to daily thorn A stiff sharply pointed woody alternations of low and high temperature. process that may be found on the stems of Such plants flower earlier and more pro- vascular plants. It is a modified branch and fusely if subjected to low night and high is supplied with vascular tissue, e.g.

229 thorn forest hawthorn (Crataegus monogyna). Com- rounds the large central vacuole of plant pare prickle; spine. cells. thorn forest See forest. torus (pl. tori) A disk-shaped structure formed from lignified primary cell-wall threonine An AMINO ACID derived from material on the middle lamella of a bor- aspartic acid. It is broken down to form dered pit. The structure is found mainly in glycine and acetyl CoA. Isoleucine can be the conifers. It is thought to act like a valve, synthesized from threonine. sealing the pit when the pressure on the two sides is unequal, e.g. when an adjacent thylakoid An elongated flattened fluid- tracheid is damaged. filled sac that forms the basic unit of the photosynthetic membrane system in chloro- totipotency The ability shown by many plasts and photosynthetic bacteria. See living cells to form all the types of tissues chloroplast. that constitute the mature organism. This may be achieved, even if the cells have thymidine The nucleoside formed when completely differentiated, provided that thymine is linked to D-ribose by a b-glyco- the appropriate balance of nutrients and sidic bond. See nucleoside. hormones is given. The best example of this phenomenon is the formation of ad- thymine A nitrogenous base found in ventitious embryos in carrot tissue cul- DNA, but not in RNA. It has a PYRIMIDINE tures. ring structure. It base pairs with adenine in the complementary strand of DNA. See toxin A chemical produced by a base pairing; thymidine. pathogen (e.g. bacteria, fungi) that causes damage to a host cell in very low concen- tissue In a multicellular organism, a trations. The toxin may be formed within group of cells that is specialized for a par- the pathogen and released when it dies (en- ticular function, e.g. palisade mesophyll. dotoxin), or secreted through its cell wall Several different tissues may be incorpo- rated into an organ, e.g. a leaf. Compare (exotoxin). Toxins are often similar to the organ. enzymes of the host and interfere with the appropriate enzyme systems. tissue culture The growth of cells, tis- sues, or organs in suitable media in vitro. trabecula (pl. trabeculae) An elongated Such media must normally be sterile, cor- cell wall, cell, or line of cells across a cav- rectly pH balanced, and contain all the nec- ity. In plants, trabeculae may be found in essary micro and macronutrients and Selaginella stems, where they suspend the hormones for growth. Studies of such cul- steles in large air spaces. tures have shed light on physiological processes that would be difficult to follow trace element See micronutrient. in the living organism. For example, the cy- tokinins were discovered through work on tracheid An elongated xylem conduct- tobacco pith tissue culture. ing element with oblique end walls. Tra- cheids have heavily lignified walls and the tocopherol (vitamin E) A terpenoid-like only connection between adjacent tra- substance found especially in certain seeds, cheids is through paired pits, which are such as cereals, where it probably prevents mainly concentrated in the end walls. oxidation of lipids, thus prolonging seed Many tracheids lack protoplasm at matu- viability. rity and have lignified secondary cell walls. Tracheids form the only xylem conducting tonoplast The membrane that sur- tissue of vascular plants other than an-

230 transfer RNA giosperms, some Gnetales, and a few ferns plementary RNA molecule. The poly- such as bracken. Compare vessel. merase enzyme proceeds until reaching a stop signal, when formation of the tracheophyte (Tracheophyta) Any plant RNA strand is terminated. Behind the en- with a differentiated vascular system; i.e. zyme, the DNA double helix re-forms, all plants except the liverworts, mosses, stripping off the newly synthesized RNA and hornworts. strand. In eukaryotes, transcription is initi- ated and regulated by a host of proteins trama The inner tissue of the gills in ba- called transcription factors; in prokaryotes sidiomycete fungi that is made up of an accessory sigma factor is essential for loosely packed hyphae. transcription. See also translation. transamination The transfer of an transduction The transfer of part of the amino group from an AMINO ACID to an a- DNA of one bacterium to another by a keto acid, producing a new a-keto acid and temperate bacteriophage. The process does a new amino acid. This is catalyzed by a occur naturally but is mainly known as a transaminase enzyme in conjunction with technique in RECOMBINANT DNA TECHNOL- the coenzyme pyridoxal phosphate. The OGY, and has been used in mapping the amino group becomes attached to the bacterial chromosome. coenzyme to form pyridoxamine phos- phate, and is then transferred to the a-keto transect A line or belt designed to study acid, which is usually pyruvic acid, ox- changes in species composition across a aloacetic acid, or a-ketoglutaric acid. particular area. The transect sampling Transamination is a key step in the biosyn- technique is most often used in plant ecol- thesis and breakdown of most amino acids. ogy to study changes in the composition of vegetation. See also quadrat. transcription The process in living cells whereby RNA is synthesized according to transferase An enzyme that catalyzes the template embodied in the base se- reactions in which entire groups or radicals quence of DNA, thereby converting the are transferred from one molecule to an- cell’s genetic information into a coded mes- other. For example, hexokinase catalyzes sage (messenger RNA, mRNA) for the as- the transfer of a high-energy terminal sembly of proteins, or into the RNA phosphate group from ATP to glucose to components required for protein synthesis give glucose 6-phosphate and ADP. (ribosomal RNA and transfer RNA). The term is also applied to the formation of sin- transfer cell A specialized type of plant gle-stranded DNA from an RNA template, cell in which the cell wall forms protuber- as performed by the enzyme reverse tran- ances into the cell, thus increasing the sur- scriptase, for example in retrovirus infec- face area of the wall and plasma tions. Details of DNA transcription differ membrane. Transfer cells are active cells between prokaryote and eukaryote cells, containing many mitochondria, and are but essentially it involves the following concerned with short-distance transport of steps. Firstly, with the aid of a helicase en- solutes. They are common in many situa- zyme, the double helix of the DNA mol- tions, for example as gland cells and epi- ecule is unwound in the region of the site dermal cells, and in xylem and phloem marking the start of transcription for a par- parenchyma, where they are concerned ticular gene. The enzyme RNA polymerase with active loading and unloading of ves- moves along one of the DNA strands, the sels and sieve tubes. transcribed strand (or anticoding strand, since the code is carried by the comple- transfer RNA (tRNA)A type of RNA mentary base sequence of the RNA), and that participates in protein synthesis in liv- nucleotides are assembled to form a com- ing cells. It attaches to a particular amino

231 transformation acid and imports this to the site of polypep- cular tissue is intermediate between that of tide assembly at the ribosome when the ap- the root and shoot. See hypocotyl. propriate codon on the messenger RNA is reached. Each tRNA molecule consists of translation The process whereby the roughly 80 nucleotides; some regions of genetic code of messenger RNA (mRNA) is the molecule undergo base pairing and deciphered by the machinery of a cell to form a double helix, while in others the make proteins. Molecules of mRNA are in two strands separate to form loops. When effect coded messages based on informa- flattened out the tRNA molecule has a tion in the cell’s genes and created by the characteristic ‘cloverleaf’ shape with three process of TRANSCRIPTION. They relay this loops; the base of the ‘leaf’ carries the information to the sites of protein synthe- amino acid binding site, and the middle sis, the RIBOSOMES. In eukaryotes these are loop contains the ANTICODON, whose base located in the cytoplasm, so the mRNA triplet pairs with the complementary must migrate from the nucleus. The first codon in the mRNA molecule. Hence, be- stage in translation is initiation, in which cause there are 64 possible codons in the the two subunits of the ribosome assemble genetic code, of which about 60 or so code and attach to the mRNA molecule near the for amino acids, there may be up to 60 or initiation codon, which signals the begin- so different tRNAs in a cell, each with a ning of the message. This also involves var- different anticodon, although some of ious proteins called initiation factors, and them will bind the same amino acid. The the initiator TRANSFER RNA (tRNA), which correct amino acid is attached to a tRNA always carries the amino acid N-formyl molecule by an enzyme called an amino- methionine. The next stage is elongation, acyl-tRNA transferase. There are 20 of in which the peptide chain is built up from these, one for each type of amino acid. This its component amino acids. This involves attachment also involves the transfer of a the participation of several proteins called high-energy bond from ATP to the amino elongation factors, and tRNA molecules acid, which provides the energy for peptide that successively occupy two sites on the bond formation during TRANSLATION. larger ribosome subunit in a sequence de- termined by consecutive codons on the transformation A permanent genetic mRNA. As each pair of tRNAs occupies recombination in a bacterial cell, in which the ribosomal sites, their amino acids are a DNA fragment is incorporated into the joined together by a peptide bond. As the DNA of the cell. This may be demonstrated ribosome moves along the mRNA to the by growing bacteria in the presence of dead next codon, the next tRNA enters the first cells, culture filtrates, or extracts of related ribosomal site, and so on, leading to elon- strains. The bacteria acquire genetic char- gation of the peptide chain. Having deliv- acters of these strains. ered its amino acid, the depleted tRNA is released from the second ribosomal site, transgenic Describing organisms, espe- which is then occupied by the tRNA with cially eukaryotes, containing foreign ge- the growing chain. This process continues netic material. Genetic engineering has until the ribosome encounters a termina- created a wide range of transgenic animals, tion codon. A protein called a release fac- plants, and other organisms for both ex- tor binds directly to the stop codon, perimental and commercial purposes. Ex- causing the addition of a water molecule amples include -resistant crop instead of an amino acid to the polypeptide plants and plants that secrete pharmaceuti- chain. The polypeptide chain is then cals. See recombinant DNA. released and the ribosome complex dissoci- ates, marking the termination of transla- transition zone The zone in a vascular tion. Following its release, the polypeptide plant where the root and shoot structures may undergo various changes, such as the merge and where arrangement of the vas- removal or addition of chemical groups, or

232 trichome even cleavage into two parts. This post- moving from place to place, or by produc- translational modification produces the ing a copy that inserts elsewhere. The sim- fully functional protein. Folding of the pro- plest types are called insertion sequences; tein is assisted by a class of molecules these comprise about 700–1500 base pairs. called chaperones. More complex ones, called composite transposons, have a central portion, which translocation 1. The movement of min- may contain functional genes, flanked by eral nutrients, elaborated food materials, insertion sequences. Transposons can af- and hormones through the plant. In vascu- fect both the genotype and phenotype of lar plants, the xylem and phloem serve to the organism, e.g. by disrupting gene ex- translocate such substances. pression, or causing deletions or inver- 2. See chromosome mutation. sions. They are common in bacteria, where they usually contain only one or two genes. transpiration The loss of water vapor In eukaryotes, transposons account for from the surface of a plant. Most is lost much of the repetitive DNA in the genome. through stomata when they are open for Compare plasmid. See repetitive DNA. gaseous exchange. It has been shown that transpiration through the stomata tree ferns See Filicinophyta. amounts to 90% of the transpiration that occurs from a water surface of the same triacylglyceride (triglyceride) An ester area as a the leaf. Typically, about 5% is of glycerol in which all the –OH groups are lost directly from epidermal cells through esterified; the acyl groups may be the same the cuticle (cuticular transpiration) and a or different. Plant triacylglycerols are usu- minute proportion through lenticels. A ally unsaturated. They are synthesized continuous flow of water, the transpiration from glycerol phosphate and fatty acyl stream, is thus maintained through the CoAs, usually in the microsomes. Lipases plant from the soil via root hairs, root cor- catalyze the breakdown of triacylglycerols tex, xylem, and tissues such as leaf meso- to glycerol and free fatty acids. Triacyl- phyll served by xylem. Evaporation of glycerols are important food reserves, espe- water from the leaf surface sets up a water cially in seeds such as rape (Brassica potential gradient that extends to the napus), linseed (Linum usitatissimum), roots, helping to draw up water; water castor bean (Ricinus communis) and co- flow is enhanced by the narrow diameter of conut (Cocos nucifera). They are commer- the xylem vessels, which favor capillary ac- cial sources of fats (solid triacylglycerols) tion. ROOT PRESSURE may be a contributing and oils (liquid triacylglycerols). See also factor. Water evaporates from wet cell carboxylic acid; glyceride; lipid; micro- walls into intercellular spaces and diffuses some. out through stomata. Transpiration may be useful in maintaining a flow of solutes Triassic The oldest period of the Meso- through the plant and in helping to cool zoic era, 250–215 million years ago. Dur- leaves through evaporation, but is often ing the Triassic, the climate gradually detrimental under conditions of water changed from arid to more temperate. shortage, when wilting may occur. It is fa- Conditions for preservation in the drier vored by low humidity, high temperatures, period were poor, so there are few fossils. and moving air. A major control is the de- There was a diversification of the gym- gree of opening of the stomata. Compare nosperms, and fossil groups such as the guttation. See antitranspirant; cohesion Bennettitales and evolved dur- theory. ing this period. See also geological time scale. transposon (transposable genetic element; jumping gene)A segment of an A taxonomic rank consisting of a organism’s DNA that can insert at various group of closely related genera. A number sites in the genome, either by physically of tribes comprise a subfamily. The tribe is

233 triglyceride introduced only in classifications of very in response to an external stimulus. Tro- large families such as the grasses (Poaceae). pisms are named according to the stimulus. The organ is said to exhibit a positive or tricarboxylic acid cycle See Krebs negative tropic response, depending on cycle. whether it grows toward or away from the stimulus respectively, e.g. shoots are posi- trichocyst See Dinomastigota. tively phototropic but negatively gravit- ropic. Growth straight toward or away trichome See hair. from the stimulus (0°and 180°orientation respectively) is called orthotropism. Pri- triglyceride See triacylglyceride. mary roots and shoots are orthotropic to light and gravity. By contrast, growth at triplet A sequence of three nucleotide any other angle to the direction of the stim- bases on a DNA or messenger RNA mol- ulus as by branches or lateral roots is called ecule. See genetic code. plagiotropism. The mechanism involved in the latter is poorly understood. In diatro- triploid A cell, tissue or organism con- pism the plant part is orientated at right taining three times the haploid number of angles to the direction of the stimulus. For chromosomes. In flowering plants the en- example, rhizomes grow horizontally in re- dosperm tissue is usually triploid, resulting sponse to gravity (diagravitropism), and from the fusion of one of the pollen nuclei leaves often grow at right angles to the in- with the two polar nuclei. Triploids are cident light. Since the receptor for the stim- usually sterile. ulus is often separate from the region of growth, tropic movements are often medi- trisomy See aneuploidy. ated by hormones. See phototropism; grav- itropism. Compare nastic movements; trophic level In complex natural com- taxis. munities, organisms whose food is ob- tained from plants by the same number of tryptophan See amino acids. steps are said to belong to the same trophic or energy level. The first and lowest trophic tuber A swollen underground stem or level contains the producers, green plants root that contains stored food and acts as that convert solar energy to food by photo- an organ of perennation and vegetative synthesis. Herbivores occupy the second propagation. Stem tubers, e.g. potato trophic level and are primary consumers: (Solanum tuberosum) develop at the end of they eat the members of the first trophic underground stems by swelling of nodes level. At the third level carnivores eat the and internodes. There is an increase in pith herbivores (the secondary consumer level), tissue to form a round tuber that bears and at the fourth level secondary carni- buds in the axils of greatly reduced scale vores eat the primary carnivores (the ter- leaves. The stem connecting the tuber to tiary consumer level). These are general the parent plant then severs. Root tubers categories, as many organisms feed on sev- may develop in the same way from adven- eral trophic levels, for example omnivores titious roots, as in Dahlia.A root tuber can eat both plants and animals. Decomposers be distinguished for a stem tuber by the ab- or transformers occupy a separate trophic sence of buds or ‘eyes’. See perennating level, which consists of organisms such as organ. fungi and bacteria, which break down dead organic matter into nutrients usable tubulin See microtubule. by the producers. See food chain. tundra A region or type of vegetation at tropism (tropic movement) A direc- high latitudes and high altitude. Tundra is tional growth movement of part of a plant widespread in the Arctic north of the tree

234 tyrosine lines of North America and Eurasia, but as pressure potential – the hydrostatic pres- occurs in only scattered localities on sure to which water in a liquid phase is Antarctic islands, where it consists mainly subjected. of lichens and mosses. turion 1. A swollen detached winter bud tunica–corpus theory A theory of api- that contains stored food and is protected cal organization and development that dis- by an outer layer of leaf scales and mu- tinguishes two separate tissue zones, the cilage. It is an organ of perennation or veg- tunica and the corpus, in the apex of a etative propagation, and is characteristic of flowering plant. The tunica is made up of various water plants (e.g. Sagittaria). Turi- one or more peripheral layers in which cell ons are usually released from the plant and division is mostly anticlinal. The corpus is lie dormant on the bed of the pond or river the inner area of tissue in which cell until conditions become favorable for arrangement and division is irregular. The growth. epidermis originates in the tunica region 2. Any vegetative shoot or sucker. and the other stem tissues arise from either the tunica or the corpus, depending on the tylose A bladderlike ingrowth from a species. Compare histogen theory. parenchyma cell into an adjacent tracheid or vessel through a paired pit. Tyloses are turgor The state, in a plant or prokary- often found in injured tissue, older wood, ote cell, in which the protoplast is exerting and below an abscission layer, and can a pressure on the cell wall owing to the in- completely block the conducting vessel. take of water by osmosis. Being slightly They often become filled with tannins, elastic, the cell wall bulges, but is rigid resins or pigments, giving the heartwood enough to prevent water entering to the its dark color, and helping to preserve and point of bursting. The cell is then said to be strengthen it. Some of these pigments are turgid. Turgidity is the main means of sup- used commercially as dyes, e.g. hema- port of herbaceous plants and other unlig- toxylin. nified tissues. See osmosis; plasmolysis; turgor pressure. type 1. The material used to define a species. It is usually a dried specimen turgor pressure (pressure potential) The stored in a herbarium but may also be a hydrostatic pressure exerted by the con- drawing. Type specimens of microorgan- tents of a cell against the cell wall. It results isms are often living cultures. from the uptake of water into the cell by 2. The representative species of a genus, the osmosis. It may be alternatively expressed representative genus of a family, etc. For

235 U

ubiquinone See coenzyme Q. motion (e.g. motile gametes) or feeding (e.g. ciliate protoctists). Undulipodia in- ultracentrifuge A high-speed cen- clude all eukaryotic cilia and flagella, trifuge, operating at up to a million revolu- which share the same essential structure, tions per second that is used to sediment and differ markedly from bacterial flagella protein and nucleic acid molecules. Ultra- (see flagellum). They are complex struc- centrifuges operate under refrigeration in a tures, containing over 500 different pro- vacuum chamber and forces 50 million teins. The shaft comprises a cylindrical times that of gravity may be reached. The array of nine doublet microtubules sur- rate of sedimentation depends on the mol- rounding a central core of two single mi- ecular weight of the molecule and thus the crotubules. The outer wall of the shaft is an ultracentrifuge can be used to separate a extension of the cell membrane. Its struc- mixture of large molecules, such as large ture is similar to that of a centriole. Strictly, colloidal proteins, and estimate particle the term flagella should be used only in re- sizes. lation to bacteria. Long undulipodia (over 10 m m), formerly called flagella, occur ultramicrotome See microtome. singly or in pairs and produce successive waves of bending that are propagated to ultrastructure (fine structure) The de- the tip of the shaft, e.g. Chlamydomonas. tailed structure of biological material as it can be revealed, for example, by electron unicellular Describing organisms that microscopy, but not by light microscopy. exist as a single cell. Such a state is charac- teristic of most protoctists and bacteria umbel See inflorescence. and is also found in many fungi. Compare multicellular; acellular. undulipodium (pl. undulipodia)A whiplike organelle that protrudes from a unisexual Describing organisms that eukaryotic cell and is used chiefly for loco- have either male or female sex organs, but not both. Unisexual plants are said to be dioecious. Compare hermaphrodite; mo- noecious.

universal indicator See indicator.

uracil A nitrogenous base that is found in RNA but not in DNA. During TRANSCRIP- TION, the thymine of the DNA template is substituted by a uracil in the RNA copy. Uracil has a PYRIMIDINE ring structure, and is derived from sugars and amino acids.

uridine The NUCLEOSIDE formed when uracil is linked to D-ribose by a b-glyco- Undulipodium sidic bond.

236 V

vacuole A fluid-filled organelle of vari- gen, mineral deficiency, or genetic or phys- able size found in the cytoplasm and sepa- iological differences between patches of rated from it by a single membrane, the cells. It occurs naturally in some species, TONOPLAST. Many mature plant cells have such as the dumb canes (Dieffenbachia). a single large central vacuole that confines the cytoplasm to a thin peripheral layer. variety The taxonomic group below the Newly formed plant cells often have a subspecies level. The term is often loosely number of small vacuoles derived from the used to describe breeds of livestock or var- endoplasmic reticulum. As they fill with ious cultivated forms of agricultural and sap, they enlarge and coalesce to form a horticultural species. Varieties are mor- single vacuole. The vacuole is filled with phological variants, which may differ in cell sap, which contains substances in solu- color or growth habit. See also cultivar. tion, e.g. sugars, salts, and organic acids, often in high concentrations resulting in a vascular bundle A strand of conducting high osmotic pressure. Water therefore tissue found in vascular plants. In plants moves into the vacuole by osmosis making showing secondary thickening (e.g. many the cell turgid. Vacuoles may also contain dicotyledons) the vascular bundles join to- storage products such as starch grains, oil gether to form a ring and contain meris- droplets, crystals, and waste substances tematic cambium tissue between the xylem that are byproducts of the cell’s metabo- and phloem, but this is generally lacking in lism. plants without secondary thickening (e.g. most monocotyledons). See also vascular valine An AMINO ACID synthesized from plants. pyruvate. It is broken down to yield suc- cinyl CoA. vascular cambium The ring of CAM- BIUM formed by the activity of the inter- variation The extent to which the char- fascicular cambium and the intrafascicular acteristics of the individuals of a species cambium that gives rise to secondary can vary. Variation can be caused by envi- xylem and secondary phloem. It is a lateral ronmental and genetic factors. Environ- meristem and contains fusiform initials mental variation (phenotypic plasticity) giving rise to secondary xylem and phloem results in differences in the appearance of and ray initials giving rise to medullary individuals of a species because of differ- rays. See also secondary growth. ences in nutrition, disease, light intensity, etc. Genetic variation is caused by recom- vascular plants Plants containing dif- bination and occasionally by MUTATION. ferentiated cells forming conducting tissue These differences may be favored or dis- (vascular tissue, also known as fascicular criminated against by NATURAL SELECTION. tissue), which comprises the xylem and See discontinuous variation; continuous phloem. Vascular tissue transports water variation; recombination. and nutrients through the plant and also provides strength and support. It consists variegation The occurrence of patches principally of XYLEM and PHLOEM (water- of different colors on leaves or flowers. It and food-conducting tissues respectively), may be caused by infection with a patho- and also contains strengthening tissue

237 vascular system

(SCLERENCHYMA) and packing tissue mon vector used to transfer cloned DNA to (PARENCHYMA). The arrangement of vascu- plant cells is the bacterium Agrobacterium lar tissue (the vascular system) in the stem tumefasciens. The foreign DNA is spliced is very varied, giving a number of different into a plasmid, which is taken up by the types of STELE. Primary vascular tissue, bacterium. The bacterium then infects a which is found in all vascular plants, is plant cell, and the plasmid DNA may be- formed from the procambium. Secondary come integrated into the plant chromo- vascular tissue, found only in plants with some. secondary thickening, develops from the vascular cambium. The vascular cambium vegetative cell See vegetative nucleus. extends to form a complete ring of meri- stematic tissue around the stem, the sepa- vegetative growth (vegetative propaga- rate vascular bundles being linked by tion; vegetative reproduction) Growth of interfascicular cambium. Vascular plants any parts of the plant other than flowers. are able to achieve considerable vertical This involves both upwards growth and growth both upwards and into the soil. outwards. Sometimes outward growth in- They have thus been able to colonize the volves STOLONS or RHIZOMES or the bud- drier habitats that are inaccessible to the ding of BULBS or CORMS. These parts may nonvascular bryophytes. The vascular get detached from the parent and become plants may be grouped into a single divi- independent, although they are identical sion, the Tracheophyta. genetically. Some plants have specialized organs such as GEMMAE or BULBILS for such vascular system (vascular tissue) See spread. See also budding; cutting; graft; vascular plants. layering; perennating organ. vascular tissue See vascular plants. vegetative nucleus (tube nucleus; vegeta- tive cell) One of the two or three nuclei in vector 1. An agent that carries a disease- a young pollen grain that are formed after causing organism to a healthy plant or division of the haploid nucleus. After the animal, causing the latter to become in- pollen grain germinates on the surface of fected;. In its widest sense, the term in- the stigma, the vegetative nucleus migrates cludes agents such as wind, rain, etc., but it to the tip of the pollen tube. It is thought to is usually restricted to vectors that are ani- regulate the growth and development of mals or microorganisms. Insects such as the pollen tube. The tube nucleus disinte- aphids carry viruses from plant to plant as grates as the pollen tube penetrates the nu- they feed, and mites and nematodes can cellus. also carry disease. Humans, including gar- deners and scientists, may carry fungal and vegetative parts All parts of a plant ex- bacterial spores on their clothing or skin. cept for those involved in sexual reproduc- 2. (cloning vector) An agent used as a vehi- tion. cle for introducing foreign DNA, for ex- ample a new gene, into host cells. Several vein One of the vascular bundles in a types of vector are used in GENE CLONING, leaf. The pattern in which the veins are notably bacterial plasmids and bacterio- arranged in a leaf is called the VENATION. phages. The segment of DNA is first spliced into the DNA of the vector, such as velamen A layer surrounding the aerial a plasmid, then the vector is transferred to roots of epiphytic plants (e.g. orchids) the host cell (e.g. the bacterium, E. coli), which, due to the spongy nature of the where it replicates along with the host cell. cells, is able to soak up surface water. It is The result is a clone of cells, all of which made up of several layers of dead empty contain the foreign gene, which is ex- cells situated on the epidermis. The cells pressed by the host cell machinery. A com- are spirally thickened, and are translucent

238 vesicle to allow light through to the photosyn- thetic tissue beneath. velum See annulus. venation The distribution of veins (vas- cular strands) in a leaf. Dicotyledons usu- ally show a netlike arrangement whereas monocotyledons generally show a parallel distribution of veins. Venation is a charac- ter sometimes used in taxonomy. venter The swollen base of an archego- nium that contains the egg cell (oosphere). ventral 1. In thallose plants, such as some liverworts, describing the lower sur- face closest to the substrate. 2. Designating the upper or adaxial surface of the lateral organs of plants, such as leaves. The term is not in common use in this context. Compare dorsal. vernalization The cold treatment of ungerminated or partially germinated seeds. Certain plants will germinate and other flower only if exposed to low tem- peratures (1–2% C) at an early period of growth, i.e. they have a chilling require- ment. Thus winter varieties of cereals will only flower in summer if sown the previous autumn. Spring-sown winter varieties re- main vegetative throughout the season un- less they have been vernalized. vernation The arrangement of leaves in relation to each other in the bud, or the arrangement of perianth segments in a flower bud. See illustration overleaf. See also ptyxis. versatile Describing an anther that is at- tached to the tip of the filament about halfway along its length in such a way that it can turn freely in the wind, thus aiding pollen dispersal. Compare basifixed; dorsi- fixed. vesicle A small vacuole of variable ori- gin and shape. It may contain secretory products, e.g. a vesicle budded off from the Golgi apparatus, or particles, e.g. endocy- Venation

239 vessel

order Gnetales, and a few other plants such as bracken (Pteridium aquilinum).

vestigial organ An organ that is func- tionless and generally reduced in size but bears some resemblance to the correspond- ing fully functioning organs found in re- lated organisms. Examples include the scale leaves of parasitic flowering plants or the reduced stamens in females of dio- ecious species.

virion The extracellular inert phase of a virus. A virion consists of a protein coat surrounding one or more strands of DNA or RNA. Virions may be polyhedral or helical and vary greatly in size.

viroid A tiny infectious agent found in plants that is similar to a virus but lacks a capsid, consisting simply of a circle of RNA, 300–400 nucleotides long. Viroids replicate within and cause characteristic disease symptoms; examples are the potato spindle tuber viroid and the hop stunt viroid.

virulence The relative pathogenicity of an organism: its ability to cause disease. Virulence may vary between strains of the same organism.

virulent phage A bacteriophage that in- Vernation: types of vernation fects a bacterial cell and immediately repli- cates, causing lysis of the host cell. totic vesicle. The membrane envelope iso- Compare temperate phage. lates the contents from the surrounding cy- toplasm. virus An extremely small infectious agent that causes a variety of diseases in vessel A form of xylem conducting tis- plants, such as tobacco mosaic disease. sue composed of vertically arranged vessel Viruses can only reproduce in living tis- elements (the individual cells that make up sues; outside the living cell they exist as in- active particles consisting of a core of DNA a xylem vessel) joined end to end, generally or RNA surrounded by a protein coat (cap- broader than TRACHEIDS. The end walls of sid). Most plant viruses are single-stranded the vessel elements have broken down to RNA viruses. The inert extracellular form leave perforation plates. These are usually of the virus, termed a VIRION, penetrates horizontal rather than the slanting end the host membrane and liberates the viral walls of tracheids. Vessel elements have lig- nucleic acid into the cell. Usually, the nu- nified secondary cell walls and at maturity cleic acid is translated by the host cell ribo- lack a living protoplast. Vessels are found somes to produce enzymes necessary for in most angiosperms, the gymnosperm the reproduction of the virus and the for-

240 Volvocales mation of daughter virions. The virions are tabolism but are not synthesized by ani- released by lysis of the host cell. Other mals, which therefore need to obtain them viruses remain dormant in the host cell be- from plant or microorganisms. The vita- fore reproduction and lysis, their nucleic mins have no energy value; most of them acid becoming integrated with that of the seem to act as coenzymes for essential host. Viruses are transmitted by VECTORS chemical changes in the body, each one in- such as aphids and nematodes. Some pro- fluencing a number of vital processes. duce visible symptoms, such as leaf mo- Vitamins A, D, E, and K are fat-soluble vi- saics or deformed growth. Others have less tamins, while vitamins B and C are water- obvious effects, but can significantly re- soluble vitamins. duce crop yields. A virus that infects a bac- terium is termed a BACTERIOPHAGE (phage). vivipary The germination of seeds or spores that are still attached to the parent vital stains Nontoxic coloring materials plant. An example is mangroves, in which that can be used in dilute concentrations to the seeds germinate into good-sized stain living material without damaging it. seedlings before they are released from the See also staining. plant, enabling them to plunge far enough into the mud to establish themselves. vitamins Organic chemical compounds that are essential in small quantities for me- Volvocales An order of the Chloro-

241 W

Wallace, Alfred Russel (1823–1913) water molds See Saprolegniales. British naturalist. After a basic education and a short career as a surveyor, Wallace water pore See aquaporin. met the entomologist Henry Bates and to- gether they embarked on a scientific expe- water potential Symbol: y A measure dition as naturalists to the Amazon. of the energy available in an aqueous solu- Wallace described this expedition in A tion to cause water to move across a selec- Narrative of Travels on the Amazon and tively permeable membrane during the River Negro (1853). His second expe- osmosis. Water potential may be defined as dition, to , took the chemical potential of water in a biolog- eight years during which he collected over ical system compared to the chemical 125 000 specimens. He noticed the marked potential of pure water at the same tem- differences between the Australian and perature and pressure. It is measured in Asian flora and fauna and drew a line on kilopascals (kPa), and pure water has the the map separating the regions, still re- value 0 kPa; solutions with increasing con- ferred to as Wallace’s line. His observa- centrations of solute have more negative tions of the marsupials in Australia led him values of y, since the solute molecules in- to conclude that Australian animals were terfere with the water molecules. This ef- more primitive than Asian ones. He be- fect is termed the solute potential (or lieved that these animals had been cut off osmotic potential), denoted by ys; it is mea- from the Asian animals when the land mass sured in kPa and always has a negative of Australia had broken away from Asia. value, with increasing concentrations of This resulted in less competition for the solute having increasingly negative values Australian animals than those on the larger of ys. In turgid plant cells there is also a Eurasian land mass. These conclusions led pressure exerted by the walls of the cell; this is called the pressure potential; it is de- him to formulate a theory of evolution by natural selection, which he communicated fined by yp, and has a positive value (al- to DARWIN; as a result the two men pre- though in xylem cells it may be negative sented joint papers at the famous Linnaean due to water movement in the transpira- Society meeting of 1858. Unlike Darwin, tion stream). Water potential is also Wallace never believed that the theory of affected by capillary forces, especially in evolution could be applied to humans. the narrow tubes of the xylem, and in the Wallace continued to gather evidence for interstices of cell walls, and by imbibitional the theory and published his results in Ge- forces. These forces are particularly impor- ographical Distribution of Animals (1876) tant in cell walls and intercellular spaces, and Island Life (1880). increasing in wilting plants. wall pressure See turgor pressure. wax One of a group of water-insoluble substances with a very high molecular water culture See hydroponics. weight; they are esters of long-chain alco- hols with fatty acids. Waxes form protec- water ferns See Marsileaceae. tive coverings to leaves, stems, fruits, seeds,

242 WWF animal fur, and the cuticles of insects, serv- show secondary thickening, namely the ing principally as waterproofing. conifers, Gnetum and dicotyledons. Water and nutrients are only transported in the WCMC See World Conservation Moni- outermost youngest wood, termed the SAP- toring Centre. WOOD. The nonfunctional compacted wood of previous seasons’ growth is called WCU See World Conservation Union. the HEARTWOOD and it is this that is impor- tant commercially. Western blotting A technique analo- gous to SOUTHERN BLOTTING used to sepa- woody perennial See perennial. rate and identify proteins instead of nucleic acids. The protein mixture is separated by World Conservation Monitoring Cen- electrophoresis and blotted onto a nitrocel- tre (WCMC) An information service set lulose filter. Antibodies specific to the pro- up by the World Conservation Union, the tein of interest are applied and bind to their Worldwide Fund for Nature and the target proteins. A second antibody, specific United Nations Environment Program to to the first antibody, is then applied. This compile information on conservation and carries a radioactive label, so enabling it to the sustainable use of the world’s natural be located by autoradiography wherever it resources. The WCMC collects data on binds to the antibody–protein complex. threatened and endangered species and habitats, national parks and nature re- wild-type The most commonly found serves, international agreements, and con- form of a given gene in wild populations. servation and environment programs. See Wild-type alleles, often designated +, are Red Data Book. usually dominant and produce the ‘nor- mal’ phenotype. World Conservation Union (WCU) Formerly the International Union for the wilt A plant disease characterized by Conservation of Nature (IUCN), founded wilting, usually due to pathogenic fungi or in 1948, an organization that brings to- bacteria, or to parasites. It is often a root gether states, government agencies, and a disease that eventually prevents the plant range of nongovernmental organizations to taking up water. Other wilts occur when promote conservation and the sustainable the vascular tissues become blocked, e.g. use of natural resources. Over 10 000 ex- Dutch elm disease, caused by the fungus pert volunteers collect information on Ceratocystis ulmi, which causes the plant particular species and biodiversity con- to block off its own infected tissues. servation projects, providing a base of ex- pertise that enables the WCU to advise wilting The loss of turgor due to lack of countries on national conservation strate- water. The tissues become limp and leaves gies. and flowers droop. Wilting may be due to the rate of transpiration exceeding the rate Worldwide Fund (for Nature) (WWF) at which the roots can take up water in the The largest independent conservation or- soil during drought periods, or to infection ganization in the world, comprising a (see wilt). The permanent wilting point is global network of national organizations, reached when the plant has wilted to a associates, and program offices. It provides point where it will not recover, even if conservation services based on global pol- more water is added, because the cells have icy, fieldwork, and scientific information. plasmolyzed. See turgor. Its goal is to prevent genetic, species, and ecosystem diversity, to ensure the sustain- wood The hard fibrous structure found able use of resources, and to promote ac- in woody perennials such as trees and tions to reduce pollution. shrubs. It is normally formed from the sec- ondary xylem and thus found in plants that WWF See Worldwide Fund (for Nature).

243 XYZ

xanthophyll One of a class of yellow to tains additional supporting tissue in the orange pigments derived from carotene, form of fibers and sclereids and some living the commonest being lutein. In most parenchyma. The proto- and metaxylem, plants, xanthophylls function as accessory which together constitute the primary pigments in photosynthesis: they absorb xylem, are formed from the procambium light at wavelengths where chlorophyll has of the apical meristems, while the sec- low absorption and pass the absorbed en- ondary xylem is differentiated from the ergy to chlorophyll. In the Phaeophyta cambium or lateral meristem. See sec- (brown algae) the xanthophylls fucoxan- ondary growth. thin and peridinin act as the primary light- absorbing pigments. See carotenoids; yeasts See Saccharomycetales. photosynthetic pigments. zinc A micronutrient that is needed for xeromorphic Structurally adapted to the activity of the enzyme alcohol dehydro- withstand dry conditions. See xerophyte. genase, which catalyzes the conversion of acetaldehyde to ethanol in anaerobic respi- xerophyte Any plant adapted to grow- ration. See micronutrient. ing in dry conditions or in a physiologically dry habitat, such as an acid bog or a salt zoosporangium (pl. zoosporangia)A marsh, or an exposed, very windy situa- sporangium that produces ZOOSPORES. tion, by storing available water, reducing water loss, or possessing deep root sys- zoospore An asexual motile spore pro- tems. Succulents, such as cacti and agaves duced by a zoosporangium. It has one or (family Agavaceae), have thick fleshy stems more undulipodia. These motile spores or leaves that store water. Features associ- may encyst in adverse conditions or may be ated with reducing water loss include: the means by which a fungus or parasitic shedding or dieback of leaves, e.g. ocotillo protoctist penetrates a new host. (Fouquieria splendens); waxy leaf coatings coupled with closure or plugging of stom- zwitterion An ion with both a positive ata, e.g. Kalanchoe; sunken or protected and a negative charge. Amino acids can stomata e.g. marram grass (Ammophila form zwitterions: the amino group has the + arenaria); folding, rolling or repositioning form –NH3 and the acid group is ionized of leaves to reduce sunlight absorption, e.g. as –COO–. marram grass; a dense covering of white re- flective spines, e.g. cacti; and the develop- zygomorphy See bilateral symmetry. ment of a dense hairy leaf covering, e.g. Espeletia. Compare hydrophyte; meso- zygomycete Any fungus belonging to phyte. See also halophyte. the phylum Zygomycota. xylem The water-conducting tissue in Zygomycota A phylum of fungi whose vascular plants. It consists of dead hollow members have haploid, nonseptate hyphae cells (the TRACHEIDS and VESSELS), which (i.e. hyphae that lack cross walls). Their are the conducting elements. It also con- cell walls contain chitin or chitosan. They

244 zygotene do not produce motile spores at any stage (which include such fungi as Mucor and of the life cycle. Zygomycetes are mostly Rhizopus) and the GAMOPHYTA (conjugat- saprophytic, absorbing nutrients from de- ing green algae, e.g. Spirogyra and Zyg- caying vegetation and other organic mat- nema). ter; the bread molds Rhizopus and Mucor are examples. zygote The diploid cell resulting from the fusion of two haploid gametes. A zy- zygospore A resistant sexual spore gote usually undergoes mitosis immedi- formed when a zygote develops a thick ately. See sexual reproduction. wall. Zygospores are characteristic of the ZYGOMYCOTA, especially the Mucorales zygotene See prophase.

245

APPENDIXES

Appendix I

SI Units

BASE AND DIMENSIONLESS SI UNITS Physical quantity Name of SI unit Symbol for SI unit length meter m mass kilogram(me) kg time second s electric current ampere A thermodynamic temperature kelvin K luminous intensity candela cd amount of substance mole mol *plane angle radian rad *solid angle steradian sr *supplementary units

DERIVED SI UNITS WITH SPECIAL NAMES Physical quantity Name of SI unit Symbol for SI unit frequency hertz Hz energy joule J force newton N power watt W pressure pascal Pa electric charge coulomb C electric potential difference volt V electric resistance ohm W electric conductance siemens S electric capacitance farad F magnetic flux weber Wb inductance henry H magnetic flux density tesla T luminous flux lumen lm illuminance (illumination) lux lx absorbed dose gray Gy activity becquerel Bq dose equivalent sievert Sv

DECIMAL MULTIPLES AND SUBMULTIPLES USED WITH SI UNITS Submultiple Prefix Symbol Multiple Prefix Symbol 10–1 deci- d 101 deca- da 10–2 centi- c 102 hecto- h 10–3 milli- m 103 kilo- k 10–6 micro- m 106 mega- M 10–9 nano- n 109 giga- G 10–12 pico- p 1012 tera- T 10–15 femto- f 1015 peta- P 10–18 atto- a 1018 exa- E 10–21 zepto- z 1021 zetta- Z 10–24 yocto- y 1024 yotta- Y

248 Appendix II

Webpages The following all have useful information:

American Society of Plant Biologists www.aspb.org/education/

Botanical Society of America www.botany.org

European Initiative for Biotechnology www.rdg.ac.uk/EIBE/ Education

Internet Directory for Botany www.botany.net/IDB/info.html

Missouri www.mobot.org/

National Museum of Natural History nmnhwww.si.edu/departments/ botany.html

New York Botanical Garden www.nybg.org

Photosynthesis and the Web photoscience.la.asu.edu/photosyn /photoweb/default.html

Royal Botanic Gardens, Kew www.rbgkew.org.uk/

University of California Botanical Garden www.mip.berkeley.edu/garden/

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