Glossary

A of on other roots, buds developing on leaves or roots abaxial Directed away from the axis. Opposite of instead of in leaf axils on shoots. adaxial. With regard to a leaf, the lower, or “dorsal,” aerenchyma Parenchyma tissue containing particu- surface. larly large intercellular spaces of schizogenous, lysige- accessory bud A bud located above or on either side nous, or rhexigenous origin. of the main axillary bud. aggregate ray In secondary vascular tissues; a group accessory cell See subsidiary cell. of small rays arranged so as to appear to be one large acicular crystal Needle-shaped crystal. ray. acropetal development (or differentiation) Pro- albuminous cell See Strasburger cell. duced or becoming differentiated in a succession toward aleurone Granules of protein (aleurone grains) the apex of an organ. The opposite of basipetal but present in seeds, usually restricted to the outermost means the same as basifugal. layer, the aleurone layer of the endosperm. (Protein actin fi lament A helical protein fi lament, 5 to 7 nano- bodies is the preferred term for aleurone grains.) meters (nm) thick, composed of globular actin mole- aleurone layer Outermost layer of endosperm in cules; a major constituent of all eukaryotic cells. Also cereals and many other taxa that contains protein bodies called microfi lament. and enzymes concerned with endosperm digestion. actinocytic stoma Stoma surrounded by a circle of aliform paratracheal parenchyma In secondary radiating cells. xylem; vasicentric groups of axial parenchyma cells adaxial Directed toward the axis. Opposite of having tangential wing-like extensions as seen in trans- abaxial. With regard to a leaf, the upper, or “ventral,” verse section. See also paratracheal parenchyma and surface. vasicentric paratracheal parenchyma. adventitious Refers to structures arising not at their alternate pitting In tracheary elements; pits in diago- usual sites, as roots originating on stems or leaves instead nal rows.

Esau’s Plant Anatomy, Third Edition, By Ray F. Evert. Copyright © 2006 John Wiley & Sons, Inc.

521 522 | Glossary amyloplast A colorless plastid (leucoplast) that forms precursors of the primary tissues of root or shoot; may starch grains. be vegetative, initiating vegetative tissues and organs, anastomosis Refers to cells or strands of cells that are or reproductive, initiating reproductive tissues and interconnected with one another as, for example, the organs. veins in a leaf. apoplast Cell wall continuum and intercellular spaces analogy Means having the same function as but a dif- of a plant or plant organ; the movement of substances ferent phylogenetic origin than another entity. via the cell walls is called apoplastic movement or anatomy The study of the internal structure of transport. organisms; morphology is the study of their external apoptosis Programmed cell death in animal cells structure. mediated by a group of protein-degrading enzymes angiosperm A group of plants whose seeds are borne called caspases; involves a programmed series of events within a mature ovary (fruit). that leads to dismantling of the cell contents. angstrom (originally ångström) A unit of length apotracheal parenchyma In secondary xylem; axial equal to one-tenth of a nanometer (nm). Symbol A or parenchyma typically independent of the vessels (pores). Å. Includes diffuse and diffuse-in-aggregates. angular collenchyma A form of collenchyma in apposition Growth of cell wall by successive deposi- which the primary wall thickening is most prominent tion of wall material, layer upon layer. Opposite of in the angles where several cells are joined. intussusception. anisocytic stoma A stomatal complex in which three articulated laticifer Laticifer composed of more than subsidiary cells, one distinctly smaller than the other one cell with common walls intact or partly or entirely two, surround the stoma. removed; anastomosing or nonanastomosing; a com- anisotropic Having different properties along differ- pound laticifer. ent axes; optical anisotropy causes polarization and aspirated pit In gymnosperm wood; bordered pit in double refraction of light. which the pit membrane is laterally displaced and the annual ring In secondary xylem; growth ring formed torus blocks the aperture. during one season. The term is deprecated because astrosclereid A branched, or ramifi ed, type of more than one growth ring may be formed during a sclereid. single year. axial organ Root, stem, infl orescence, or fl ower axis annular cell wall thickening In tracheary elements without its appendages. of the xylem; secondary wall deposited in the form of axial parenchyma Parenchyma cells in the axial rings. system of secondary vascular tissues; as contrasted with anomalous secondary growth A term of conve- ray parenchyma cells. nience referring to types of secondary growth that axial system All secondary vascular cells derived differ from the more familiar ones. from the fusiform cambial initials and oriented with anomocytic stoma A stoma without subsidiary their longest diameter parallel with the main axis of cells. stem or root. Other terms: vertical system and longitu- anther Pollen-bearing part of the stamen. dinal system. anthocyanin A water-soluble blue, purple, or red fl a- axial tracheid Tracheid in the axial system of second- vonoid pigment occurring in the vacuolar cell sap. ary xylem; as contrasted with ray tracheid. Anthophyta The phylum of angiosperms, or fl ower- axil Upper angle between a stem and a twig or a ing plants. leaf. anticlinal Commonly refers to orientation of cell wall axillary bud Bud in the axil of a leaf. or plane of cell division; perpendicular to the nearest axillary meristem Meristem located in the axil of a surface. Opposite of periclinal. leaf and giving rise to an axillary bud. apex (pl. apices), or summit Tip, topmost part, pointed end of anything. In shoot or root the tip con- B taining the apical meristem. banded parenchyma In secondary xylem; axial apical cell Single cell that occupies the distal position parenchyma in concentric bands as seen in transverse in an apical meristem of root or shoot and is usually section, mainly independent (apotracheal) of vessels interpreted as the initial cell in the apical meristem; (pores). typical of seedless vascular plants. bark A nontechnical term applied to all tissues outside apical dominance Infl uence exerted by a terminal the vascular cambium or the xylem; in older trees bud in suppressing the growth of lateral, or axillary, may be divided into dead outer bark and living inner buds. bark, which consists of secondary phloem. See also apical meristem A group of meristematic cells at the rhytidome. apex of root or shoot that by cell division produces the bars of Sanio See crassulae. Glossary | 523 basifugal development See acropetal development. callus A tissue composed of large thin-walled cells basipetal development (or differentiation) Pro- developing as a result of injury, as in wound healing duced or becoming differentiated in a succession toward or grafting, and in tissue culture. (The use of callus the base of an organ. The opposite of acropetal and for accumulations of callose on sieve areas is basifugal. deprecated.) bast fi ber Originally phloem fi ber, now any extraxyl- callus tissue See callus. ary fi ber. calyptrogen In root apex; meristem giving rise to the bicollateral vascular bundle A bundle having rootcap independently of the initials of cortex and phloem on two sides of the xylem. central cylinder. biseriate ray A ray in secondary vascular tissue, two cambial initials Cells so localized in the vascular cells in width. cambium or phellogen that their periclinal divisions can blind pit A pit without a complementary pit in an contribute cells either to the outside or to the inside of adjacent wall, which may face a lumen of a cell or an the axis; in vascular cambium, classifi ed into fusiform intercellular space. initials (source of axial cells of xylem and phloem) and bordered pit A pit in which the secondary wall over- ray initials (source of the ray cells). arches the pit membrane. cambium A meristem with products of periclinal bordered pit-pair An intercellular pairing of bor- divisions commonly contributed in two directions and dered pits. arranged in radial fi les. Term preferably applied only to boundary parenchyma See marginal bands. the two lateral meristems, the vascular cambium and brachysclereid A short, roughly isodiametric scler- the cork cambium, or phellogen. eid, resembling a parenchyma cell in shape; a stone Casparian strip, or band A band-like wall formation cell. within primary walls that contains suberin and lignin; branch gap In the nodal region of a stem; a region of typical of endodermal cells in roots, in which it occurs parenchyma in the vascular cylinder of the stem located in radial and transverse anticlinal walls. where the branch traces are bent toward the branch. cell Structural and physiological unit of a living organ- Usually confl uent with the gap of the leaf subtending ism. The plant cell consists of protoplast and cell wall; the branch. in nonliving state, of cell wall only, or cell wall and some branch root See lateral root. nonliving inclusions. branch traces Vascular bundles connecting the vas- cell plate A partition appearing at telophase between cular tissue of the branch and that of the main stem. the two nuclei formed during mitosis (and some meioses) They are leaf traces of the fi rst leaves (prophylls) on the and indicating the early stage of the division of a cell branch. (cytokinesis) by means of a new cell wall; is formed in branched pit See ramiform pit. the phragmoplast. bulliform cell An enlarged epidermal cell present, cell wall More or less rigid outermost layer of plant with other similar cells, in longitudinal rows in leaves cells, which encloses the protoplast. In higher plants, of grasses. Also called motor cell because of its pre- composed of cellulose and other organic and inorganic sumed participation in the mechanism of rolling and substances. unrolling of leaves. cellulose A polysaccharide, β-1,4 glucan—the main bundle cap Sclerenchyma or collenchymatous paren- component of cell walls in most plants; consists of long chyma appearing like a cap on the xylem and/or phloem chain-like molecules whose basic units are anhydrous side of a vascular bundle as seen in transverse glucose residues of the formula C6H10O5. section. central cylinder A term of convenience applied to bundle sheath Layer or layers of cells enclosing a the vascular tissues and associated ground tissue in stem vascular bundle in a leaf; may consist of parenchyma or and root. Refers to the same part of stem and root that sclerenchyma. is designated stele. bundle sheath extension A plate of ground tissue central mother cells Rather large vacuolated cells in extending from a bundle sheath to the epidermis in a subsurface position in apical meristem of shoot in leaf; may be present on one or on both sides of gymnosperms. the bundle and may consist of parenchyma or centrifugal development Produced or developing sclerenchyma. successively farther away from the center. centripetal development Produced or developing C successively closer to the center. callose A polysaccharide, β-1,3 glucan, yielding glucose chimera A shoot apical meristem composed of cells on hydrolysis. Common wall constituent in the sieve of different genotypes. In periclinal chimeras, cells of areas of sieve elements; also develops rapidly in reaction different genetic composition are arranged in periclinal to injury in sieve elements and parenchyma cells. layers. 524 | Glossary chlorenchyma Parenchyma tissue containing tracheal parenchyma and aliform paratracheal chloroplasts; leaf mesophyll and other green parenchyma. parenchyma. contact cell A paratracheal parenchyma cell or a ray chloroplast A chlorophyll-containing plastid with parenchyma cell in direct contact with the vessels and thylakoids organized into grana and intergrana (or physiologically associated with them. Analogous to stroma) thylakoids, and embedded in a stroma. companion cell in the phloem. chromatolysis Nuclear degeneration involving the coordinated growth Growth of cells in a manner loss of stainable contents (chromatin and nucleoli) and that involves no separation of walls, as opposed to eventual rupture of the nuclear envelope. intrusive growth. chromoplast A plastid containing pigments other cork See phellem. than chlorophyll, usually yellow and orange carotenoid cork cambium See phellogen. pigments. cork cell A phellem cell derived from the phellogen, circular bordered pit A bordered pit with circular nonliving at maturity, and having suberized walls; pro- aperture. tective in function because the walls are highly impervi- cisterna (pl. cisternae) A fl attened, saclike membra- ous to water. nous compartment as in endoplasmic reticulum, Golgi corpus The core in an apical meristem covered by the body, or thylakoid. tunica and showing volume growth by divisions of cells collateral vascular bundle A bundle having phloem in various planes. only on one side of the xylem, usually the abaxial cortex Primary ground tissue region between the vas- side. cular system and the epidermis in stem and root. Term collenchyma A supporting tissue composed of more also used with reference to peripheral region of a cell or less elongated living cells with unevenly thickened, protoplast. nonlignifi ed primary walls. Common in regions of cotyledon Seed leaf; generally absorbs food in mono- primary growth in stems and leaves. cotyledons and stores food in other angiosperms. colleter A multicellular appendage (emergence) crassulae (sing. crassula) Thickenings of intercellu- formed from both epidermal and subepidermal tissues. lar material and primary wall along the upper and lower They produce sticky secretions, and are common on margins of a pit-pair in the tracheids of gymnosperms. bud scales and young leaves. Also called bars of Sanio. columella Central part of a rootcap in which the cells cristae (sing. crista) Crest-like infoldings of the inner are arranged in longitudinal fi les. membrane in a mitochondrion. companion cell A specialized parenchyma cell asso- cross-fi eld A term of convenience for the rectangle ciated with a sieve-tube element in angiosperm phloem formed by the walls of a ray cell against an axial tra- and arising from the same mother cell as the sieve-tube cheid; as seen in radial section of the secondary xylem element. of conifers. complementary tissue See fi lling tissue. crystal sand A mass of very fi ne free crystals. complex tissue A tissue consisting of two or more crystalloid Protein crystal that is less angular than a cell types; epidermis, periderm, xylem, and phloem are mineral crystal and swells in water. complex tissues. cuticle Waxy or fatty layer on outer wall of epidermal compound laticifer See articulated laticifer. cells, formed of cutin and wax. compound middle lamella A collective term applied cuticularization Process of formation of the cuticle. to two primary walls and middle lamella; usually used cutin A complex fatty substance considerably imper- when the true middle lamella is not distinguishable vious to water; present in plants as an impregnation of from the primary walls. May also include the earliest epidermal walls and as a separate layer, the cuticle, on secondary wall layers. the outer surface of the epidermis. compound sieve plate A sieve plate composed of cutinization Process of impregnation with cutin. several sieve areas in either scalariform or reticulate cyclocytic stoma Stoma surrounded by one or two arrangement. narrow rings of subsidiary cells, numbering four or compression wood Reaction wood i n con i fer s, wh ich more. Also called encyclocytic. is formed on the lower sides of branches and leaning or cyclosis Streaming of cytoplasm in a cell. crooked stems and characterized by dense structure, cystolith A concretion of calcium carbonate on an out- strong lignifi cation and certain other features. growth of a cell wall. Occurs in a cell called lithocyst. conducting tissue See vascular tissue. cytochimera A chimera having combinations of cell confl uent paratracheal parenchyma In secondary layers with diploid and polyploid nuclei. See also xylem; coalesced aliform groups of axial parenchyma chimera. cells forming irregular tangential or diagonal bands, cytohistological zonation Presence of regions in the as seen in transverse section. See also para- apical meristem having distinctive cytological charac- Glossary | 525 teristics. The term is meant to imply that a cytological dicotyledons Obsolete term used to refer to all angio- zonation results in a subdivision into distinguishable sperms other than monocotyledons; characterized by tissue regions. having two cotyledons. See also eudicotyledons and cytokinesis The process of division of a cell as distin- magnoliids. guished from the division of the nucleus, or karyokine- dictyosome See Golgi body. sis (mitosis). differentiation A physiological and morphological cytological zonation See cytohistological zonation. change occurring in a cell, a tissue, an organ, or a plant cytology The science dealing with the cell. during development from a meristematic, or juvenile, cytoplasm Living matter of a cell, exclusive of the stage to a mature, or adult, stage. Usually associated nucleus. with an increase in specialization. cytoplasmic ground substance See cytosol. diffuse apotracheal parenchyma Axial parenchyma cytoskeleton Flexible, three-dimensional network of in secondary xylem occurring as single cells or as microtubules and actin fi laments (microfi laments) strands distributed irregularly among the fi bers, within cells. as seen in transverse section. See also apotracheal cytosol Cytoplasmic matrix of the cytoplasm in which parenchyma. the nucleus, various organelles, and membrane systems diffuse-porous wood Secondary xylem in which the are embedded. Also referred to as cytoplasmic ground pores (vessels) are distributed fairly uniformly through- substance and hyaloplasm. out a growth layer or change in size gradually from ear- lywood to latewood. D dilatation Growth of parenchyma by cell division decussate Arrangement of leaves in pairs that alter- in pith, rays, or axial system in vascular tissues; causes nate with one another at right angles. the increase in circumference of bark in stem and dedifferentiation A reversal in differentiation of a root. cell or tissue that is presumed to occur when a more or distal Farthest from the point of origin or attachment. less completely differentiated cell resumes meristematic Opposite of proximal. activity. distichous Arrangement of leaves in two vertical derivative A cell produced by division of a meriste- rows; two-ranked arrangement. matic cell in such a way that it enters the path of differ- dorsal Equivalent to abaxial in botanical usage. entiation into a body cell; its sister cell may remain in druse A globular, compound, calcium-oxalate the meristem. crystal with numerous crystals projecting from its dermal issue See dermal tissue system. surface. dermal tissue system Outer covering tissue of a duct An elongated space formed by separation of cells plant; epidermis or periderm. from one another (schizogenous origin), by dissolution dermatogen Meristem forming the epidermis and of cells (lysigenous origin), or by a combination of the arising from independent initials in the apical meristem. two processes (schizolysigenous origin); usually con- One of the three histogens, plerome, periblem, and cerned with secretion. dermatogen, according to Hanstein. desmotubule The tubule traversing a plasmodesma- E tal canal and uniting the endoplasmic reticulum of the earlywood Wood formed in fi rst part of a growth two adjacent cells. layer and characterized by a lower density and detached meristem A meristem, with a potential to larger cells than the latewood. Term replaces spring give rise to an axillary bud, appearing detached from wood. the apical meristem because of the vacuolation of inter- eccrine secretion Secretion leaves the cell as indi- vening cells. vidual molecules passing through the plasma membrane determinate growth Growth of limited duration, and cell wall. Compare with granulocrine secretion. characteristic of fl oral meristems and leaves. ectodesma See teichode. development Change in form and complexity of an elaioplast A leucoplast type of plastid forming and organism or part of an organism from its beginning to storing oil. maturity; combined with growth. embryogenesis (or embryogeny) Formation of diacytic stoma A stomatal complex in which one embryo. pair of subsidiary cells, with their common walls at right embryoid An embryo, often indistinguishable from a angles to the long axis of the guard cells, surrounds the normal one, developing not from an egg but from a stoma. somatic cell, often in tissue culture. diaphragms in pith Transverse layers (diaphragms) encyclocytic See cyclocytic. of fi rm-walled cells alternating with regions of soft- endocytosis Uptake of material into cells by means walled cells that may collapse with age. of invagination of the plasma membrane; if solid 526 | Glossary material is involved, the process is called phagocytosis; and transported to the cell surface; there, the mem- if dissolved material is involved it is called pinocytosis. brane of the vesicle fuses with the plasma membrane, endodermis Layer of ground tissue forming a sheath expelling the vesicle’s contents to the outside. around the vascular region and having the Casparian exodermis Outer layer, one or more cells in depth, of strip in its anticlinal walls; may have secondary walls the cortex in some roots; a type of hypodermis, the later. It is the innermost layer of the cortex in roots and walls of which may be suberized and/or lignifi ed. stems of seed plants. exogenous Arising in superfi cial tissue, as an axillary endodermoid Resembling the endodermis. bud. endogenous Arising from a deep-seated tissue, as a expansins A novel class of proteins involved with the lateral root. loosening of cell wall structure. endomembrane system Collectively, the cellular external phloem Primary phloem located externally membranes that form a continuum (plasma membrane, to the primary xylem. tonoplast, endoplasmic reticulum, Golgi bodies, and extrafl oral nectary Nectary occurring on a plant nuclear envelope). part other than a fl ower. See also nectary. endoplasmic reticulum (usually abbreviated to ER) extraxylary fi bers Fibers in various tissue regions A system of membranes forming cisternoid or tubular other than the xylem. compartments that permeate the cytosol. The cisternae appear like paired membranes in sectional profi les. The F membranes may be coated with ribosomes (rough ER) false annual ring One of more than one growth or be free of ribosomes (smooth ER). layers formed in the secondary xylem during one growth endoreduplication (endoreplication) A DNA repli- season, as seen in transverse section. cation cycle in which no mitosis-like structural changes fascicle A bundle. take place; during endoreduplication, polytene chromo- fascicular cambium Vascular cambium originating somes are formed. from procambium within a vascular bundle, or enucleate Lacking a nucleus. fascicle. epiblem Term used sometimes for the epidermis of festucoid Pertaining to the Festucoideae a subfamily the root. See also rhizodermis. of grasses. epicotyl Upper part of the axis of an embryo or seed- fi ber An elongated, usually tapering sclerenchyma ling, above the cotyledons (seed leaves) and below the cell with a lignifi ed or nonlignifi ed secondary wall; may next leaf or leaves. See also plumule. or may not have a living protoplast at maturity. epidermis The outer layer of cells in the plant body, fi ber-sclereid A sclerenchyma cell with characteris- primary in origin. If it is multiseriate (multiple epider- tics intermediate between those of a fi ber and a mis), only the outermost layer differentiates as a typical sclereid. epidermis. fi ber-tracheid A fi ber-like tracheid in the secondary epithelium A compact layer of cells, often secretory xylem; commonly thick walled, with pointed ends and in function, covering a free surface or lining a cavity. bordered pits that have lenticular to slit-like apertures. epithem Mesophyll of a hydathode concerned with fi bril Submicroscopic threads composed of cellulose secretion of water. molecules that constitute the form in which cellulose ergastic substances Passive products of protoplast occurs in the wall. such as starch grains, fat globules, crystals, and fl uids; fi le meristem See rib meristem. occur in cytoplasm, organelles, vacuoles, and cell fi liform Thread-like. walls. fi liform sclereid A much elongated, slender sclereid eudicotyledons One of two major classes of angio- resembling a fi ber. sperms. Eudicotyledones, formerly grouped with the fi lling tissue Loose tissue formed by the lenticel phel- magnoliids, a diverse group of archaic fl owering plants, logen toward the outside; may or may not be suberized. as “dicots”; abbreviated as eudicot. Also called complementary tissue. eukaryotic (also eucaryotic) Refers to organisms fl ank meristem A misnomer used with reference to having membrane-bound nuclei, genetic material orga- the peripheral region of an apical meristem. The use of nized into chromosomes, and membrane-bound cyto- the word fl ank implies that the entity is two-sided. The plasmic organelles. Opposite of prokaryotic. term should be replaced with peripheral meristem. eumeristem Meristem composed of relatively small fl oral nectary See nectary. cells, approximately isodiametric in shape, compactly fl origen A hypothetical hormone presumed to be arranged, and having thin walls, a dense cytoplasm, and concerned with the induction of fl owering. large nuclei; word means “true meristem.” founder cells Group of cells in the peripheral zone exocytosis A cellular process in which particulate of the apical meristem involved with the initiation of a matter or dissolved substances are enclosed in a vesicle leaf primordium. Glossary | 527 fundamental tissue See ground tissue. ground tissue Tissues other than the vascular tissues, fundamental tissue system See ground tissue the epidermis, and the periderm. Also called funda- system. mental tissue. fusiform cell An elongated cell tapering at the ground tissue system The total complex of ground ends. tissues of the plant. fusiform initial In vascular cambium; an elongated growth Irreversible increase in size by cell division cell with approximately wedge-shaped ends that gives and/or cell enlargement. rise to the elements of the axial system in the secondary growth layer A layer of secondary xylem or second- vascular tissues. ary phloem produced during a single growth period, which may extend through one season (annual ring) G or part of one season ( false annual ring) if more than gelatinous fi ber A fi ber with a so-called gelatinous one layer is formed in one season. Also called growth layer (G-layer), an innermost secondary wall layer that increment. can be distinguished from the outer secondary wall growth ring A growth layer of secondary xylem or layer(s) by its high cellulose content and lack of secondary phloem as seen in transverse section of stem lignin. or root; may be an annual ring or a false annual genome Totality of genetic information contained in ring. the nucleus, plastid, or mitochondrion. guard cells A pair of cells fl anking the stomatal pore genomics Field of genetics that studies the content, and causing the opening and closing of the pore by organization, and function of genetic information in change in turgor. whole genomes. gum A nontechnical term applied to material result- genotype Genetic constitution of an organism; con- ing from breakdown of plant cells, mainly of their trasted with phenotype. carbohydrates. germination Resumption of growth by the embryo gum duct A duct that contains gum. in a seed; also beginning of growth of a spore, pollen gummosis A symptom of a disease characterized by grain, bud, or other structure. the formation of gum, which may accumulate in cavities gland A multicellular secretory structure. or ducts or appear on the surface of the plant. glandular hair A trichome having a unicellular or guttation Exudation from leaves of water derived multicellular head composed of secretory cells; usually from the xylem; caused by root pressure. borne on a stalk of nonglandular cells. gymnosperm A seed plant with seeds not enclosed glyoxysome A peroxisome containing enzymes nec- in an ovary; the conifers are the most familiar group. essary for the conversion of fats into carbohydrates. Golgi apparatus A term used to refer collectively to H all the Golgi bodies of a given cell. Also called Golgi hadrom (or hadrome) The tracheary elements and complex. the associated parenchymatous cells of the xylem tissue; Golgi body A group of fl at, disk-shaped sacs, or cis- the specifi cally supporting cells (fi bers and sclereids) ternae, that are often branched into tubules at their are excluded. See also leptom. margins; serve as collecting and packaging centers for half-bordered pit-pair A pit-pair consisting of a bor- the cell and concerned with secretory activities. Also dered and a simple pit. called dictyosomes. haplocheilic stoma Stomatal type in gymnosperms; grana (sing. granum) Subunits of chloroplasts seen subsidiary cells are not related to the guard cells as green granules with the light microscope and as ontogenetically. stacks of disk-shaped cisternae, the thylakoids, with the hardwood A name commonly applied to the wood of electron microscope; the grana contain the chlorophylls a magnoliid or eudicot tree. and carotenoids and are the sites of the light reactions heartwood Inner layers of secondary xylem that have in photosynthesis. ceased to function in storage and conduction and in granulocrine secretion Secretion passes an inner which reserve materials have been removed or con- cytoplasmic membrane, usually that of a vesicle, and is verted into heartwood substances; generally darker extruded from the cell after the vesicle fuses with the colored than the functioning sapwood. plasma membrane and releases its contents to the helical cell wall thickening In tracheary elements outside. Compare with eccrine secretion. of the xylem; secondary wall deposited on the primary gravitropism Growth in which the direction is deter- or secondary wall as a continuous helix. Also referred mined by gravity. to as spiral cell wall thickening. ground meristem A primary meristem, or meriste- hemicellulose A general term for a heterogeneous matic tissue, derived from the apical meristem and group of noncrystalline glycans that are tightly bound giving rise to the ground tissues. in the cell wall. 528 | Glossary heterocellular ray A ray in secondary vascular tissues hypertrophy Refers most commonly to abnormal composed of cells of more than one form; in angio- enlargement. Hypertrophy of a cell or its parts involves sperms, of procumbent and square or upright cells; in no cell division. Hypertrophy of an organ may involve conifers, of parenchyma cells and ray tracheids. both enlargement of cells and abnormal cell multiplica- heterogeneous ray tissue system Rays in secondary tion (hyperplasia). vascular tissues all heterocellular or combinations of hypocotyl Axial part of embryo or seedling homocellular and heterocellular rays. Term not used for located between the cotyledon or cotyledons and the conifers. radicle. hilum (1) The central part of a starch grain around hypocotyl-root axis Axial part of embryo or seed- which the layers of starch are arranged concentrically; ling comprising the hypocotyl and the root meristem or (2) the scar left by the detached funiculus on a seed. the radicle, if one is present. histogen Hanstein’s term for a meristem in shoot or hypodermis A layer or layers of cells beneath the root tip that forms a defi nite tissue system in the plant epidermis distinct from the underlying ground tissue body. Three histogens were recognized: dermatogen, cells. periblem, and plerome. See defi nitions of these terms. hypophysis The uppermost cell of suspensor from histogen concept Hanstein’s concept stating that which part of the root and rootcap in the embryo of the three primary tissue systems in the plant—the epi- angiosperms are derived. dermis, the cortex, and the vascular system with the associated ground tissue—originate from distinct meri- I stems, the histogens, in the apical meristems. See idioblast A cell in a tissue that markedly differs in histogen. form, size, or contents from other cells in the same histogenesis The formation of tissues (hence, histo- tissue. genetic) having to do with origin or formation of included phloem Secondary phloem included in the tissues. secondary xylem of certain eudicots. Term replaces histogenetic See histogenesis. interxylary phloem. homocellular ray A ray in secondary vascular tissues increment In growth, an addition to the plant body composed of cells of one form only: in angiosperms, of by the activity of a meristem. procumbent, or square, or upright cells; in conifers, of indeterminate growth Unrestricted or unlimited parenchyma cells only. growth, as with a vegetative apical meristem that homogeneous ray tissue system Rays in secondary produces an unrestricted number of lateral organs vascular tissues all homocellular, composed of procum- indefi nitely. bent cells only. Term not used for conifers. initial (1) Cell in a meristem that by division gives rise homology A condition indicative of the same phylo- to two cells, one of which remains in the meristem, the genetic, or evolutionary, origin, but not necessarily the other is added to the plant body; (2) sometimes used to same in present structure and/or function. designate a cell in its earliest stage of specialization. horizontal parenchyma See ray parenchyma. More appropriate term for (2), primordium. horizontal system See ray system. initial parenchyma See marginal bands. hormone An organic substance produced usually in inner bark In older trees, the living part of the bark; minute amounts in one part of an organism, from which the bark inside the innermost periderm. See also it is transported to another part of that organism on bark. which it has a specifi c effect; hormones function as intercalary growth Growth by cell division that highly specifi c chemical signals between cells. occurs some distance from the meristem in which the hyaloplasm See cytosol. cells originated. hydathode A structural modifi cation of vascular and intercalary meristem Meristematic tissue derived ground tissues, usually in a leaf, that permits the release from the apical meristem and continuing meristematic of water through a pore in the epidermis; may be secre- activity some distance from that meristem; may be tory in function. See epithem. intercalated between tissues that are no longer hydromorphic Refers to the structural features of meristematic. hydrophytes. intercellular space A space between two or more hydrophyte A plant that requires a large supply of cells in a tissue; may have schizogenous, lysigenous, water and may grow partly or entirely submerged in schizolysigenous, or rhexigenous origin. water. intercellular substance See middle lamella. hygromorphic Synonym of hydromorphic. interfascicular cambium Vascular cambium arising hyperplasia Refers most commonly to an excessive between vascular bundles (fascicles) in the interfascicu- multiplication of cells. lar parenchyma. Glossary | 529 interfascicular region Tissue region located between lamina of leaf Expanded part of the leaf. Also called the vascular bundles (fascicles) in a stem. Also called blade of the leaf. medullary or pith ray. latewood The secondary xylem formed in the later intermediary cell Especially large companion cells part of a growth layer; denser and composed of smaller with fi elds of highly branched plasmodesmata leading cells than the earlywood. Term replaces summer wood. into them from the bundle-sheath cells; their presence lateral meristem A meristem located parallel with is correlated with the transport of large quantities of the sides of the axis; refers to the vascular cambium raffi nose and stachyose. and phellogen, or cork cambium. internal phloem The primary phloem located inter- lateral root A root arising from another, older root; nally from the primary xylem. Term replaces intraxyl- also called branch root, or secondary root, if the older ary phloem. root is the primary root, or taproot. internode Region between successive nodes of a latex (pl. latices) A fl uid, often milky, contained in stem. laticifers; consists of a variety of organic and inorganic interpositional growth See intrusive growth. substances, often including rubber. intervascular pitting Pitting between tracheary laticifer A cell or a cell series containing a character- elements. istic fl uid called latex. interxylary cork Cork that develops within the laticiferous cell A nonarticulated, or simple, xylem tissue. laticifer. interxylary phloem See included phloem. laticiferous vessel An articulated, or compound, intraxylary phloem See internal phloem. laticifer in which the cell walls between contiguous intrusive growth A type of growth in which a growing cells are partly or completely removed. cell intrudes between other cells that separate from each leaf buttress A lateral protrusion below the apical other along the middle lamella in front of the tip of the meristem constituting the initial stage in the develop- growing cell. Also called interpositional growth. ment of a leaf primordium. intussusception Growth of cell wall by interpolation leaf fi bers Technical designation of fi bers derived of new wall material within previously formed wall. from monocotyledons, chiefl y from their leaves. Opposite of apposition. leaf primordium A lateral outgrowth from the apical isodiametric Regular in form, with all diameters meristem that eventually will become a leaf. equally long. leaf sheath The lower part of a leaf that invests the isolation cells In secondary xylem, paratracheal stem more or less completely. parenchyma cells and ray cells that have no contact with leaf trace A vascular bundle in the stem extending the vessels; function as storage cells. between its connection with a leaf and that with another isotropic Having the same properties along all vascular unit in the stem; a leaf may have one or more axes. Optically isotropic material does not affect the leaf traces. light. leaf trace gap A region of parenchyma in the vascular cylinder of a stem located above the level where a leaf K trace diverges toward the leaf. Also called lacuna, an karyokinesis Division of a nucleus as distinguished interfascicular region; it involves no interruption of vas- from the division of the cell, or cytokinesis. Also called cular connections. mitosis. lenticel An isolated region in the periderm distin- guished from the phellem in having intercellular spaces; L the tissue may or may not be suberized. L1, L2, L3 layers The outer cell layers of angiosperm leptom (or leptome) The sieve elements and the asso- apical meristems with a tunica-corpus organization. ciated parenchymatous cells of the phloem tissue; the lacuna (pl. lacunae) Space. Usually air space between supporting cells (fi bers and sclereids) are excluded. See cells, which may be schizogenous, lysigenous, schizo- also hadrom. lysigenous, or rhexigenous in origin. Also used with leucoplast A colorless plastid. reference to the leaf gap. libriform fi ber A xylem fi ber commonly with thick lacunar collenchyma A collenchyma characterized walls and simple pits; usually the longest cell in the by intercellular spaces and cell wall thickenings facing tissue. the spaces. lignifi cation Impregnation with lignin. lamella A thin plate or layer. lignins Phenolic polymers deposited mainly in cell lamellar collenchyma A collenchyma in which cell walls of supporting and conducting tissues; formed from wall thickenings are deposited mainly on tangential the polymerization of three main monomeric units, the walls. monolignols p-coumaryl, coniferyl, and sinapyl alcohols. 530 | Glossary lithocyst A cell containing a cystolith. meiosis Two successive nuclear divisions in which longitudinal parenchyma See axial parenchyma. the chromosome number is reduced from diploid to longitudinal system In secondary vascular tissues. haploid and segregation of the genes occurs. See axial system. meristem Embryonic tissue region, primarily con- lumen Space bounded by (1) the plant cell wall; (2) cerned with formation of new cells. the thylakoid space in chloroplasts; (3) the narrow, meristematic cell A cel synthesizing protoplasm and transparent space of endoplasmic reticulum. producing new cells by division; varies in form, size, lutoids Vesicles, also called vacuoles, in laticifers wall thickness, and degree of vacuolation, but has only bounded by a single membrane and containing a spec- a primary cell wall. trum of hydrolytic enzymes capable of degrading most meristemoid A cell or a group of cells constituting an of the organic compounds in the cell. active locus of meristematic activity in a tissue com- lysigenous As applied to an intercellular space, origi- posed of somewhat older, differentiating cells. nating by a dissolution of cells. merophyte Immediate unicellular derivative of an lysis A process of disintegration or degradation. apical cell and the multicellular structural units derived lysosomal compartment A region in the cell proto- from them. plast or cell wall where acid hydrolases, capable of mesomorphic Refers to structural features of digesting cytoplasmic constituents and metabolites, are mesophytes. localized. Bounded by a single membrane in the proto- mesophyll Photosynthetic parenchyma of a leaf blade plast and usually constituting the vacuolar system. located between the two epidermal layers. Another term, lytic compartment. mesophyte A plant requiring an environment that is lysosome An organelle bounded by a single mem- neither too wet nor too dry. brane and containing acid hydrolytic enzymes capable mestome sheath An endodermoid sheath of a vascu- of breaking down proteins and other organic macro- lar bundle; the inner of two sheaths of leaves of Poaceae, molecules; in plants, represented by vacuoles. See also mainly those of the festucoid subfamily. lysosomal compartment. metacutisation Deposition of suberin lamellae in lytic compartment See lysosomal compartment. outer cells of root tips that cease to be active in growth and absorption at the end of seasonal growth. Late M suberization. maceration Artifi cial separation of cells of a tissue by metaphloem Part of the primary phloem that differ- causing a disintegration of the middle lamella. entiates after the protophloem and before the second- macrofi bril An aggregation of microfi brils in a cell ary phloem, if any of the latter is formed in a given wall visible with the light microscope. taxon. macrosclereid Elongated sclereid with unevenly dis- metaxylem Part of the primary xylem that diff-erenti- tributed secondary wall thickening; common in seed ates after the protoxylem and before the secondary epidermis of Fabaceae. xylem, if any of the latter is formed in a given taxon. magnoliids A clade, or evolutionary line, of angio- micelles Regions in cellulose microfi brils in which sperms leading to the eudicots. The leaves of most mag- the cellulose molecules are arranged parallel to noliids possess ester-containing oil cells. each other so that a crystalline lattice structure is major veins Larger leaf vascular bundles, which present. are associated with ribs; they are largely involved microbody See peroxisome. with the transport of substances into and out of the microfi bril A thread-like component of the cell wall leaf. consisting of cellulose molecules and visible only with mantle Outer layers of the kind of apical meristem the electron microscope. that shows a layered arrangement of cells. microfi lament See actin fi lament. marginal bands Parenchyma bands at the ends of micrometer One-thousandth millimeter; also called growth rings in secondary xylem; may be restricted to micron. Symbol μm. the end of a ring (terminal parenchyma) or to the micron See micrometer. beginning of one (initial parenchyma). microtubules Nonmembranous tubules about 25 mass meristem A meristematic tissue in which the nanometers (nm) in diameter and of indefi nite length. cells divide in various planes so that the tissue increases Located in the cytoplasm in a nondividing eukaryotic in volume. cell, usually near the cell wall, and form the meiotic or matrix Generally refers to a medium in which some- mitotic spindle and the phragmoplast in a dividing thing is embedded. cell. mechanical tissue See supporting tissue. middle lamella Layer of intercellular material, chiefl y medulla Synonym for pith. pectic substances, cementing together the primary medullary ray See interfascicular region. walls of contiguous cells. Glossary | 531 minor veins Small leaf vascular bundles, which are nonarticulated laticifer A simple laticifer consist- located in the mesophyll and enclosed by a bundle ing of a single, commonly multinucleate, cell; may be sheath; they are involved with the distribution of the branched or unbranched. transpiration stream and the uptake of the products of nonporous wood Secondary xylem having no photosynthesis. vessels. mitochondrion (pl. mitochondria) Double mem- nonstoried cambium Vascular cambium in which brane-bound cell organelle concerned with respiration; the fusiform initials and rays are not arranged in hori- carries enzymes and is the major source of ATP in non- zontal tiers on tangential surfaces. Also called nonstrati- photosynthetic cells. fi ed cambium. mitosis See karyokinesis. nonstoried wood Secondary xylem in which the monocotyledon A plant whose embryo has one coty- axial cells and rays are not arranged in horizontal tiers ledon; one of the two great classes of angiosperms, the on tangential surfaces. Also called nonstratifi ed wood. Monocotyledones; often abbreviated as monocot; the nonstratifi ed cambium See nonstoried cambium. other great class, the Eudicotyledones. nonstratifi ed wood See nonstoried wood. morphogenesis Development of form; the sum of nuclear envelope Double membrane enclosing the phenomena of development and differentiation of tissues nucleus of a cell. and organs. nucleoid A region of DNA in prokaryotic cells, mito- morphology Study of form and its development. chondria, and chloroplasts. mother cell See precursory cell. nucleolar organizer region A special area on a motor cell See bulliform cell. certain chromosome associated with the formation of mucilage cell Cell containing mucilages or gums or the nucleolus. similar carbohydrate material characterized by the prop- nucleolus (pl. nucleoli) A small, spherical body erty of swelling in water. found in the nucleus of eukaryotic cells, which is com- mucilage duct A duct containing mucilage or gum or posed of rRNA in the process of being transcribed from similar carbohydrate material. See also duct. copies of rRNA genes; the site of production of ribo- multiperforate perforation plate In vessel element somal subunits. of the xylem; a perforation plate that has more than one nucleoplasm Ground substance of the nucleus. perforation. nucleus In biology, organelle in a eukaryotic cell multiple epidermis A tissue two or more cell layers bounded by a double membrane and containing the deep derived from the protoderm; only the outermost chromosomes, nucleoli, and nucleoplasm. layer differentiates as a typical epidermis. multiseriate ray A ray in secondary vascular tissues O that is few to many cells wide. ontogeny Development of an organism, organ, tissue, myrosin cell Cell containing myrosinases, enzymes or cell from inception to maturity. that hydrolyze glucosinolates. Occur mainly in the opposite pitting Pits in tracheary elements disposed Brassicaceae. in horizontal pairs or in short horizontal rows. organ A distinct and visibly differentiated part of a N plant, such as root, stem, leaf, or part of a fl ower. nacré wall See nacreous wall. organelle A distinct body within the cytoplasm nacreous wall A nonlignifi ed wall thickening that is of a cell, specialized in function; specifi cally, often found in sieve elements and resembles a second- membrane-bound. ary wall when it attains considerable thickness; designa- organism Any individual living thing, either unicel- tion based on glistening appearance of the wall in fresh lular or multicellular. tissue. orthostichy A vertical line along which is attached a nanometer One millionth of a millimeter; symbol series of leaves or scales on an axis of a shoot or shoot- nm. Equal to 10 angstroms. like organ. Often incorrectly applied to a steep helix, or nectary A multicellular glandular structure secreting parastichy. a liquid containing organic substances including sugar. osteosclereid Bone-shaped sclereid having a colum- Occurs in fl owers ( fl oral nectary) and vegetative plant nar middle part and enlargements at both ends. parts (extrafl oral nectary). outer bark In older trees, the dead part of the bark; netted venation Veins in a leaf blade form an anasto- the innermost periderm and all tissues outside it; also mosing system, the whole resembling a net; also called called rhytidome. See also bark. reticulate venation. node That part of the stem at which one or P more leaves are attached; not sharply delimited paedomorphosis Delay in evolutionary advance in anatomically. some characteristics as compared with others resulting 532 | Glossary in a combination of juvenile and advanced characteris- pericyclic sclerenchyma See perivascular tics in the same cell, tissue, or organ. sclerenchyma. palisade parenchyma Leaf mesophyll parenchyma periderm Secondary protective tissue that replaces characterized by elongated form of cells and their the epidermis in stems and roots, rarely in other organs. arrangement with their long axes perpendicular to the Consists of phellem (cork), phellogen (cork cambium), surface of the leaf. and phelloderm. panicoid Pertaining to the Panicoideae, a subfamily perimedullary region or zone Peripheral region of of grasses. the pith (medulla). Also called medullary sheath. papilla (pl. papillae) A soft protuberance on an epi- perinuclear space Space between the two mem- dermal cell; a type of trichome. branes forming the nuclear envelope. paracytic stoma A stomatal complex in which one or perivascular fi ber A fi ber located along the outer more subsidiary cells fl ank the stoma parallel with the periphery of the vascular cylinder in the axis of a seed long axes of the guard cells. plant and not originating in the phloem. Alternate term, paradermal Parallel with the epidermis. Refers pericyclic fi ber. specifi cally to a section made parallel with the surface perivascular sclerenchyma Sclerenchyma located of a fl at organ such as a leaf; it is also a tangential along the outer periphery of the vascular cylinder and section. not originating in the phloem. Alternate term, pericyclic parallel venation Main veins in a leaf blade arranged sclerenchyma. approximately parallel to one another, although con- peroxisome A spherical, single membrane-bound verging at base and apex of leaf. organelle; some are involved in photorespiration and parastichy A helix along which is attached a series of others (called glyoxysomes) with the conversion of leaves or scales on an axis of a shoot or shoot-like organ. fats to sugars during seed germination. Also called See also orthostichy. microbody. paratracheal parenchyma Axial parenchyma in phellem (cork) Protective tissue composed of nonliv- secondary xylem associated with vessels and other ing cells with suberized walls and formed centrifugally tracheary elements. Includes aliform, confl uent, and by the phellogen (cork cambium) as part of the peri- vasicentric. derm. Replaces the epidermis in older stems and roots parenchyma Tissue composed of parenchyma cells. of many seed plants. parenchyma cell Typically a not distinctly special- phelloderm A tissue resembling cortical parenchyma ized cell with a nucleate protoplast concerned with one produced centripetally by the phellogen (cork cambium) or more of the various physiological and biochemical as part of the periderm of stems and roots in seed activities in plants. Varies in size, form, and wall plants. structure. phellogen (cork cambium) A lateral meristem parietal cytoplasm Cytoplasm located next to the forming the periderm, a secondary protective tissue cell wall. common in stems and roots of seed plants. Produces pectic substances A group of complex carbohydrates, phellem (cork) centrifugally, phelloderm centripetally derivatives of polygalacturonic acid, occurring in plant by periclinal divisions. cell walls; particularly abundant as a constituent of the phelloid cell A cell within the phellem (cork) but middle lamella. distinct from the cork cell in having no suberin in its peltate hair A trichome consisting of a discoid plate walls. May be a sclereid. of cells borne on a stalk or attached directly to the basal phenotype Physical appearance of an organism result- foot cell. ing from interaction between its genotype (genetic con- perforation plate Part of a wall of a vessel element stitution) and the environment. that is perforated. phlobaphenes Anhydrous derivatives of tannins. periblem The meristem forming the cortex. One of Amorphous yellow, red, or brown substances very con- the three histogens, plerome, periblem, and dermato- spicuous when present in cells. gen, according to Hanstein. phloem Principal food-conducting tissue of the vas- periclinal Commonly refers to orientation of cell wall cular plant composed mainly of sieve elements, various or plane of cell division; parallel with the circumference kinds of parenchyma cells, fi bers, and sclereids. or the nearest surface of an organ. Opposite of anticli- phloem elements Cells of the phloem tissue. nal. See also tangential. phloem initial A cambial cell on the phloem side of periclinal chimera See chimera. the cambial zone that is the source of one or more cells pericycle Part of ground tissue of the stele located arising by periclinal divisions and differentiating into between the phloem and the endodermis. In seed plants, phloem elements with or without additional divisions regularly present in roots, absent in most stems. in various planes. Sometimes called phloem mother pericyclic fi ber See perivascular fi ber. cell. Glossary | 533 phloem mother cell A cambial derivative that is the pit canal Passage from the cell lumen to the chamber source of certain elements of the phloem tissue, such as, of a bordered pit. (Simple pits in thick walls usually have a sieve-tube element and its companion cells or a strand canal-like cavities.) of phloem parenchyma cells. Used also in a wider sense pit cavity Entire space within a pit from pit mem- synonymously with phloem initial. brane to the cell lumen or to the outer pit aperture if a phloem parenchyma Parenchyma cells located in pit canal is present. the phloem. In secondary phloem refers to axial pit-fi eld See primary pit-fi eld. parenchyma. pit membrane Part of the intercellular layer and phloem ray That part of a vascular ray that is located primary cell wall that limits a pit cavity externally. in the secondary phloem. pit-pair Two complementary pits of two adjacent phloic procambium That part of procambium that cells. Essential components are two pit cavities and the differentiates into primary phloem. pit membrane. photoperiodism Response to duration and timing of pith Ground tissue in the center of a stem or root. day and night expressed in the character of growth, Homology of pith in root and stem is uncertain. development, and fl owering in plants. pith ray See interfascicular region. photorespiration Oxygenase activity of Rubisco plasma membrane Single membrane delimiting the combined with the salvage pathway, consuming O2 and cytoplasm next to the cell wall. A type of unit mem- releasing CO2; occurs when Rubisco binds O2 instead of brane. Also called plasmalemma. CO2. plasmalemma See plasma membrane. photosynthetic cell A chloroplast-containing cell plasmodesma (pl. plasmodesmata) A connection of engaged in photosynthesis. protoplasts of two contiguous cells through a channel in phragmoplast Fibrous structure (light microscope the cell wall. This plasma membrane-lined channel typi- view) that arises between the daughter nuclei at telo- cally is traversed by a tubular strand of tightly constricted phase and within which the initial partition (cell plate), endoplasmic reticulum called a desmotubule, which is dividing the mother cell in two (cytokinesis), is formed. continuous with the endoplasmic reticulum in the con- Appears at fi rst as a spindle connected to the two nuclei, tiguous cells. The region between the plasma membrane but later spreads laterally in the form of a ring. Consists and the desmotubule is called the cytoplasmic sleeve. of microtubules. plastid Organelle with a double membrane in the phragmosome Layer of cytoplasm formed across the cytoplasm of many eukaryotes. May be concerned with cell where the nucleus becomes located and divides. photosynthesis (chloroplast) or starch storage (amylo- The equatorial plane of the subsequently appearing plast), or contain yellow or orange pigments (chromo- phragmoplast coincides with the plane of the cytoplas- plast). See also leucoplast. mic layer. plastochron (or plastochrone) The time interval phyllochron Interval between the visible appear- between the inception of two successive repetitive ance or emergence of successive leaves in the intact events, as origin of leaf primordia, attainment of certain plant. stage of development of a leaf, etc. Variable in length as phyllotaxy (or phyllotaxis) Mode in which the measured in time units. leaves are arranged on the axis of a shoot. plastoglobule Globule in a plastid with lipid as the phylogeny Evolutionary relationships among organ- basic component. isms; the developmental history of a group of plate collenchyma See lamellar collenchyma. organisms. plate meristem A meristematic tissue consisting of phytomeres Units, or modules, repetitively produced parallel layers of cells dividing only anticlinally with by the vegetative shoot apex. Each phytomere consists reference to the wide surface of the tissue. Characteris- of a node, with its attached leaf, a subjacent internode, tic of ground meristem of plant parts that assume a fl at and a bud at the base of the internode. form as a leaf. pinocytosis See endocytosis. plerome The meristem forming the core of the axis pit A recess or cavity in the cell wall where the primary composed of the primary vascular tissues and associ- wall is not covered by secondary wall. Pit-like structures ated ground tissue such as pith and interfascicular in the primary wall are designated primordial pits, regions. One of the three histogens, plerome, periblem, primary pits, or primary pit-fi elds. A pit is usually a and dermatogen, according to Hanstein. member of a pit-pair. plumule Portion of the young shoot above the pit aperture Opening into the pit from the interior of cotyledon(s); the fi rst bud of an embryo. See also the cell. If a pit canal is present in a bordered pit, two epicotyl. apertures are recognized, the inner, from the cell lumen polyderm A type of protective tissue in which suber- into the canal, and the outer, from the canal into the pit ized cells alternate with nonsuberized parenchyma cells cavity. and both kinds of cell have living protoplasts. 534 | Glossary polymerization Chemical union of monomers, such primary phloem Phloem tissue differentiating from as glucose or nucleotides, resulting in the formation of procambium during primary growth and differentiation polymers, such as starch, cellulose, or nucleic acid. of a vascular plant. Commonly divided into the earlier polysaccharide A carbohydrate composed of many protophloem and the later metaphloem. Not differenti- monosaccharide units joined in a chain, for example, ated into axial and ray systems. starch, cellulose. primary phloem fi bers Fibers located on the outer polysome (or polyribosome) Aggregation of ribo- periphery of the vascular region and originating in the somes apparently concerned with protein synthesis as primary phloem, usually the protophloem. Often called a group. pericyclic fi bers. pore A term of convenience for the transverse section primary pit See primary pit-fi eld. of a vessel in the secondary xylem. primary pit-fi eld A thin area of the primary cell wall pore cluster See pore multiple. and middle lamella within the limits of which one or pore multiple In secondary xylem; a group of two or more pit-pairs develop if a secondary wall is formed. more pores (transverse sections of vessels) crowded Also called primordial pit and primary pit. together and fl attened along the surfaces of contact. primary plant body See primary body. Radial pore multiple, pores in radial fi le; pore cluster, primary root Taproot. Root developing in continua- irregular grouping. tion of the radicle of the embryo. porous wood Secondary xylem having vessels. primary thickening meristem A meristem derived P-protein Phloem protein; a proteinaceous substance from the apical meristem and responsible for the primary found in cells of angiosperm phloem, especially in sieve- increase in thickness of the shoot axis. May appear tube elements; formerly called slime. as a distinct mantle-like zone. Often found in precursory cell A cell giving rise to others by monocotyledons. division. primary tissues Tissues derived from the embryo preprophase band A ring-like band of microtubules, and the apical meristems. found just beneath the plasma membrane, that delimits primary vascular tissues Xylem and phloem differ- the equatorial plane of the future mitotic spindle of a entiating from procambium during primary growth and cell preparing to divide. differentiation of a vascular plant. primary body (of plant) Part of the plant, or entire primary wall See primary cell wall. plant if no secondary growth occurs, that arises from primary xylem Xylem tissue differentiating from the embryo and the apical meristems and their deriva- procambium during primary growth and differentiation tive meristematic tissues and is composed of primary of a vascular plant. Commonly divided into the earlier tissues. protoxylem and the later metaxylem. Not differentiated primary cell wall Version based on studies with the into axial and ray systems. light microscope: cell wall formed chiefl y while the cell primordial pit See primary pit-fi eld. is increasing in size. Version based on studies with the primordium (pl. primordia) An organ, a cell, or an electron microscope: cell wall in which the cellulose organized series of cells in their earliest stage of differ- microfi brils show various orientations—from random to entiation, for example, leaf primordium, sclereid pri- more or less parallel—that may change considerably mordium, vessel element primordium. during the increase in size of the cell. The two versions procambium Primary meristem or meristematic do not necessarily coincide in delimiting primary from tissue that differentiates into the primary vascular tissue. secondary wall. Also called provascular tissue. primary growth Growth of successively formed procumbent ray cell In secondary vascular tissues; roots and vegetative and reproductive shoots from the a ray cell having its longest axis in radial direction. time of their initiation by the apical meristems and until prodesmogen A meristem precursory to desmogen the completion of their expansion. Has its inception in ( procambium). The term has the same connotation as the apical meristems and continues in their derivative residual meristem. meristems, protoderm, ground meristem, and procam- programmed cell death The genetically controlled, bium, as well as in the partly differentiated primary or programmed, series of changes in a living cell or tissues. organism that leads to its death. primary meristem Often used for each of the three prokaryotic (also procaryotic) Refers to an organ- meristematic tissues derived from the apical meristem: ism, the cells of which lack a membrane-bound nucleus protoderm, ground meristem, and procambium. and membrane-bound organelles; Bacteria and primary metabolites Molecules that are found in all Archaea. plant cells and that are necessary for the life of the plant; prolamellar body Semicrystalline body found in examples are simple sugars, amino acids, proteins, and plastids arrested in development by the absence of nucleic acids. light. Glossary | 535 promeristem Initiating cells and their most recent radial system See ray system. derivatives in an apical meristem. Also called radicle Embryonic root. Forms the basal continuation protomeristem. of the hypocotyl in an embryo. prophyll First or one of two fi rst leaves on a lateral ramifi ed Branched. shoot. ramiform pit Pit that appears to be branched because proplastid A plastid in its earliest stages of it is formed by a coalescence of two or more simple development. pits during the increase in thickness of the secondary protoderm Primary meristem or meristematic tissue wall. giving rise to the epidermis; also epidermis in meriste- raphides Needle-shaped crystals commonly occur- matic state. May or may not arise from independent ini- ring in bundles. tials in the apical meristem. ray A panel of tissue variable in height and width, protomeristem See promeristem. formed by the ray initials in the vascular cambium and protophloem First-formed elements of the phloem in extending radially in the secondary xylem and second- a plant organ. First part of the primary phloem. ary phloem. protophloem poles Term of convenience for loci of ray initial A meristematic ray cell in the vascular phloem elements that are the fi rst to mature in the vas- cambium that gives rise to ray cells of the secondary cular system of a plant organ. Applied to views in trans- xylem and secondary phloem. verse sections. ray parenchyma Parenchyma cells of a ray in second- protoplasm Living substance. Inclusive term for all ary vascular tissues. Contrasted with axial living contents of a cell or an entire organism. parenchyma. protoplast Organized living unit of a single cell ray system Total of all rays in the secondary vascular including protoplasmic and nonprotoplasmic contents tissues. Also called horizontal system and radial of a cell but excluding the cell wall. system. protoxylem First-formed elements of the xylem in a ray tracheid Tracheid in a ray. Found in the second- plant organ. First part of the primary xylem. ary xylem of certain conifers. protoxylem lacuna Space surrounded by paren- reaction wood Wood with more or less distinctive chyma cells in the protoxylem of a vascular bundle. anatomical characteristics formed in parts of leaning or Appears in some plants after the tracheary elements of crooked stems and on lower (conifers) or upper (mag- protoxylem are stretched and torn. noliids and eudicots) sides of branches. See compres- protoxylem poles Term of convenience for loci of sion wood and tension wood. xylem elements that are the fi rst to mature in the vas- redifferentiation A reversal in differentiation in a cular system of a plant organ. Applied to views in trans- cell or tissue and subsequent differentiation into another verse sections. type of cell or tissue. provascular tissue See procambium. residual meristem Used in the sense of residuum of proximal Situated near the point of origin or attach- the least differentiated part of the apical meristem. A ment. Opposite of distal. tissue that is relatively more highly meristematic than pycnotic degeneration Nuclear degeneration during the associated differentiating tissues beneath the apical which the chromatin forms a very dense mass prior to meristem. Gives rise to procambium and to interfascicu- rupture of the nuclear envelope. lar ground tissue. resin canal See resin duct. Q resin duct A duct of schizogenous origin lined with quarter-sawed oak Oak wood sawed along a radial resin-secreting cells (epithelial cells) and containing plane so that the radial surface showing the wide rays resin. characteristic of this wood is exposed. reticulate cell wall thickening In tracheary ele- quiescent center Initial region in the apical meristem ments of the xylem; secondary cell wall deposited on that has reached a state of relative inactivity; common the primary so as to form a net-like pattern. in roots. reticulate perforation plate In vessel element of the xylem; a type of multiperforate plate in which R the bars delimiting the perforations form a net-like radial parenchyma See ray parenchyma. pattern. radial pore multiple See pore multiple. reticulate sieve plate A compound sieve plate with radial section A longitudinal section coinciding with sieve areas arranged so as to form a net-like pattern. a radius of a cylindrical body, such as stem. reticulate venation See netted venation. radial seriation Arrangement of units, such as cells, reticulum A net. in an orderly sequence in a radial direction. Character- retting Freeing fi ber bundles from other tissues istic of cambial derivatives. by utilizing the action of microorganisms causing, in a 536 | Glossary suitable moist environment, the disintegration of the scalariform sieve plate A compound sieve plate with thin-walled cells surrounding the fi bers. elongated sieve areas arranged parallel to one another rhexigenous As applied to an intercellular space, in a ladder-like pattern. originating by rupture of cells. scale bark A type of rhytidome in which the sequent rhizodermis Primary surface layer of the root. Use of periderms develop as restricted overlapping strata, each the term implies that this layer is not homologous with cutting out a scale-like mass of tissue. the epidermis of the shoot. See also epiblem. schizogenous As applied to an intercellular space, rhytidome A technical term for the outer bark, which originating by separation of cell walls along the middle consists of periderm and tissues isolated by it, namely lamella. cortical and phloem tissues. schizolysigenous As applied to an intercellular space, rib An elongate protrusion, as those along the large originating by a combination of two processes, separa- veins on the underside of a leaf. tion and degradation of cell walls. rib meristem A meristematic tissue in which the cells sclereid A sclerenchyma cell, varied in form, but typi- divide perpendicular to the longitudinal axis of an organ cally not much elongated, and having thick, lignifi ed and produce a complex of parallel, vertical fi les (“ribs”) secondary walls with many pits. of cells. Particularly common in ground meristem of sclerenchyma A tissue composed of sclerenchyma organs assuming a cylindrical form. Also called fi le cells. Also a collective term for sclerenchyma cells in meristem. the plant body or plant organ. Includes fi bers, fi ber- ribosome A cell component composed of protein sclereids, and sclereids. and RNA and concerned with protein synthesis. sclerenchyma cell Cell variable in form and size and Occurs in the cytosol, nucleus, plastids, and having more or less thick, often lignifi ed, secondary mitochondria. walls. Belongs to the category of supporting cells and ring bark A type of rhytidome resulting from the may or may not be devoid of protoplast at maturity. formation of successive periderms approximately con- sclerifi cation Act of becoming changed into scleren- centrically around the axis. chyma, that is, developing secondary walls, with or ring-porous wood Secondary xylem in which the without subsequent lignifi cation. pores (vessels) of the earlywood are distinctly larger sclerotic parenchyma cell A parenchyma cell that than those of the latewood and form a well-defi ned zone through deposition of a thick secondary wall becomes or ring in a transverse section of wood. changed into a sclereid. rootcap A thimble-like mass of cells covering the scutellum (pl. scutella) Cotyledon in Poaceae embryo apical meristem of the root. specialized for absorption of endosperm. root hair A type of trichome on root epidermis that secondary body Part of the plant body that is added is a simple extension of an epidermal cell and is con- to the primary body by the activity of the lateral meri- cerned with absorption of soil solution. stems, vascular cambium and phellogen. Consists of secondary vascular tissues and periderm. S secondary cell wall Version based on studies with sapwood Outer part of the wood of stem or root con- the light microscope: cell wall deposited in some cells taining living cells and reserves; it may or may not func- over the primary wall after the primary wall ceases to tion in the conduction of water. Generally lighter colored increase in surface. Version based on studies with the that the heartwood. electron microscope: cell wall in which the cellulose scalariform cell wall thickening In tracheary ele- microfi brils show a defi nite parallel orientation. The ments of the xylem; secondary wall deposited on the two versions do not necessarily coincide in delimiting primary so as to form a ladder-like pattern. Similar to a secondary from primary wall. helix of low pitch with the coils interconnected at secondary growth In gymnosperms, most magnoli- intervals. ids and eudicots, and some monocots. A type of growth scalariform perforation plate In vessel element of characterized by an increase in thickness of stem and the xylem; a type of multiperforate plate in which elon- root and resulting from formation of secondary vascular gated perforations are arranged parallel to one another tissues by the vascular cambium. Commonly supple- so that the cell wall bars between them form a ladder- mented by activity of the cork cambium (phellogen) like pattern. forming periderm. scalariform pitting In tracheary elements of the secondary metabolites Molecules that are restricted xylem; elongated pits arranged parallel to one another in their distribution, both within the plant and among so as to form a ladder-like pattern. different plants; important for the survival and propaga- scalariform-reticulate cell wall thickening In tra- tion of the plants that produce them; there are three cheary elements of the xylem; secondary thickening major classes—alkaloids, terpenoids, and phenolics. intermediate between scalariform and reticulate. Also called secondary products. Glossary | 537 secondary phloem Phloem tissue formed by the sieve cell A type of sieve element that has relatively vascular cambium during secondary growth in a undifferentiated sieve areas (with narrow pores), rather vascular plant. Differentiated into axial and ray uniform in structure on all walls; that is, there are no systems. sieve plates; found in phloem of gymnosperms. secondary phloem fi ber A fi ber located in the axial sieve element Cell in the phloem tissue concerned system of secondary phloem. with mainly longitudinal conduction of food materials. secondary plant body See secondary body. Classifi ed into gymnospermous sieve cell and angiosper- secondary root See branch root. mous sieve-tube element. secondary thickening Used for both deposition of sieve fi eld Old term for a relatively undifferentiated secondary cell wall material and secondary increase in sieve area found on wall parts other than the sieve thickness of stems and roots. plates. secondary tissues Tissues produced by vascular sieve pitting An arrangement of small pits in sieve- cambium and phellogen during secondary growth. like clusters. secondary vascular tissues Vascular tissues (both sieve plate Part of the wall of a sieve-tube element xylem and phloem) formed by the vascular cambium bearing one (simple sieve plate) or more (compound during secondary growth in a vascular plant. Differenti- sieve plate) highly differentiated sieve areas. ated into axial and ray systems. sieve tube A series of sieve-tube elements arranged secondary wall See secondary cell wall. end to end and interconnected through sieve plates. secondary xylem Xylem tissue formed by the vascu- sieve-tube element One of the series of cellular com- lar cambium during secondary growth in a vascular ponents of a sieve tube. It shows a more or less pro- plant. Differentiated into axial and ray systems. nounced differentiation between sieve plates (wide secretory cavity Commonly refers to a space lysige- pores) and lateral sieve areas (narrow pores). Also sieve- nous in origin and containing secretion derived tube member and the obsolete sieve-tube segment. from the cells that broke down in the formation of the sieve-tube member See sieve-tube element. cavity. silica cell Cell fi lled with silica, as in epidermis of secretory cell A living cell specialized with regard to grasses. secretion or excretion of one or more, often organic, simple laticifer Laticifer that is a single cell. A non- substances. articulated laticifer. secretory duct Commonly refers to a duct schizoge- simple perforation plate In vessel element of the nous in origin and containing a secretion derived from xylem; a perforation plate with a single perforation. the cells (epithelial cells) lining the duct. See simple pit A pit in which the cavity becomes wider, epithelium. remains of constant width, or only gradually becomes secretory hair See glandular hair. narrower during the growth in thickness of the second- secretory structure Any of a great variety of struc- ary wall, that is, toward the lumen of the cell. tures, simple or complex, external or internal, that pro- simple pit-pair An intercellular pairing of two simple duces a secretion. pits. seed coat Outer coat of the seed derived from the simple sieve plate Sieve plate composed of one sieve integument or integuments. Also called testa. area. septate fi ber A fi ber with thin transverse walls simple tissue A tissue composed of a single cell type; (septa), which are formed after the cell develops a parenchyma, collenchyma, and sclerenchyma are simple secondary wall thickening. tissues. septum (pl. septa) A partition. slime See P-protein. sheath A sheet-like structure enclosing or encircling slime body An aggregation of P-protein. another. Applied to tubular or enrolled part of an organ, slime plug An accumulation of P-protein on a sieve such as a leaf sheath, and to a tissue layer surrounding area, usually with extensions into the sieve-area pores. a complex of another tissue, as a bundle sheath enclos- softwood A name commonly applied to the wood of ing a vascular bundle. a conifer. shell zone In axillary bud primordia; a zone of paral- solitary pore A pore (transverse section of a vessel lel curving layers of cells, the entire complex shell-like in secondary xylem) surrounded by cells other than in form. A result of orderly cell division along the proxi- vessel elements. mal limits of the primordium. specialization Change in structure of a cell, a tissue, shoot Above-ground portions, such as the stem and plant organ, or entire plant associated with a restriction leaves, of a vascular plant. of functions, potentialities, or adaptability to varying sieve area A portion of the sieve-element wall con- conditions. May result in greater effi ciency with regard taining clusters of pores through which the protoplasts to certain specifi c functions. Some specializations are of adjacent sieve elements are interconnected. irreversible, others reversible. 538 | Glossary specialized Refers to (1) organisms having special rays may also be so arranged. (Rays alone may be adaptations to a particular habitat or mode of life; (2) storied.) Also called stratifi ed wood. cells or tissues having a characteristic function distin- Strasburger cell In gymnosperm phloem; certain ray guishing them from other cells or tissues, more general- and axial parenchyma cells spatially and functionally ized in their function. associated with the sieve cells, thus resembling the com- spindle fi bers Bundles of microtubules, some of panion cells of angiosperms but not originating from which extend from the kinetochores of the chromo- the same precursory cells as the sieve cells. Also called somes to the poles of the spindle. albuminous cells. spiral cell wall thickening See helical cell wall stratifi ed cambium See storied cambium. thickening. stratifi ed wood See storied wood. spongy parenchyma Leaf mesophyll parenchyma striate venation See parallel venation. with conspicuous intercellular spaces. stroma The ground substance of plastids. spring wood See earlywood. styloid An elongated crystal with pointed or square square ray cell In secondary vascular tissues, a ray ends. cell approximately square as seen in radial section. subapical initial A cell beneath the protoderm at the (Considered to be of the same morphological type as apex of a leaf primordium that appears to function as the upright ray cell.) an initial of the interior tissue of the leaf. Questionable starch An insoluble carbohydrate, the chief food concept. storage substance of plants, composed of anhydrous suberin Fatty substance in the cell wall of cork tissue glucose residues of the formula C6H10O5 into which it and in the Casparian strip of the endodermis. easily breaks down. suberization Impregnation of the cell wall with starch sheath Applied to the innermost region (one suberin or deposition of suberin lamellae on the wall. or more cell layers) of the cortex when this region is subsidiary cell An epidermal cell associated with a characterized by conspicuous and rather stable accumu- stoma and at least morphologically distinguishable from lation of starch. the epidermal cells composing the groundmass of the stele (column) Conceived by P. Van Tieghem as a tissue. Also called accessory cell. morphologic unit of the plant body comprising the vas- summer wood See latewood. cular system and the associated ground tissue (pericy- supernumerary cambium layer Vascular cambium cle, interfascicular regions, and pith). The central originating in phloem or pericycle outside the regularly cylinder of the axis (stem and root). formed vascular cambium. Characteristic of some plants stellate Star shaped. with anomalous type of secondary growth. stereom (or stereome) Collective term for supporting supporting cell See supporting tissue. tissue as contrasted with the conducting tissues hadrom supporting tissue Refers to tissue composed of cells and leptom. with more or less thickened walls, primary (collen- stoma (pl. stomata) An opening in the epidermis of chyma) or secondary (sclerenchyma) that adds strength leaves and stems bordered by two guard cells and to the plant body. Also called mechanical tissue. serving in gas exchange; also used to refer to the entire suspensor An extension at the base of the embryo stomatal apparatus—the guard cells plus their included that anchors the embryo in the embryo sac. pore. symplast Interconnected protoplasts and their stomatal complex Stoma and associated epidermal plasmodesmata; the movement of substances in the cells that may be ontogenetically and/or physiologically symplast is called symplastic movement, or symplastic related to the guard cells. Also called stomatal transport. apparatus. symplastic growth See coordinated growth. stomatal crypt A depression in the leaf, the epider- syndetocheilic Stomatal type in gymnosperms; sub- mis of which bears stomata. sidiary cells (or their precursors) are derived from the stone cell See brachysclereid. same protodermal cell as the guard-cell mother cell. storied cambium Vascular cambium in which the fusiform initials are arranged in horizontal tiers on tan- T gential surfaces; the rays may also be so arranged. Also tabular Having the form of a tablet or slab. called stratifi ed cambium. tangential In the direction of the tangent; at right storied cork Protective tissue found in the monocoty- angles to the radius. May coincide with periclinal. ledons. The suberized cells occur in radial fi les, each tangential section A longitudinal section cut at right consisting of several cells—all of which are derived angles to a radius. Applicable to cylindrical structures from one cell. such as stem or root, but used also for leaf blades when storied wood Wood in which the axial cells are the section is made parallel with the expanded surface. arranged in horizontal tiers on tangential surfaces; the Substitute term for leaf, paradermal. Glossary | 539 tannin General term for a heterogeneous group of transfer of solutes. Cells without wall ingrowths may phenol derivatives. Amorphous, strongly astringent sub- also function as transfer cells. stance widely distributed in plants, and used in tanning, transition zone With reference to an apical meri- dyeing, and preparation of ink. stem, a zone of orderly dividing cells disposed about the taproot First, or primary, root of a plant forming a inner limit of the promeristem or, more specifi cally, of direct continuation of the radicle of the embryo. the group of central mother cells. Is transitional between taxon (pl. taxa) Any one of the categories (species, the apical meristem and the subapical primary meriste- genus, family, etc.) into which living organisms are matic tissues. classifi ed. transverse division (of cell) With reference to cell, teichode A linear space in the outer epidermal division perpendicular to the longitudinal axis of the wall in which the fi brillar structure is more loose and cell. With reference to plant part, division of the cell open than elsewhere in the wall. Replaces the term perpendicular to the long axis of the plant part. ectodesma. transverse section A cross section. Section taken tension wood Reaction wood in angiosperms, formed perpendicular to the longitudinal axis of an entity. Also on the upper sides of branches and leaning or crooked called transection. stems and characterized by lack of lignifi cation and traumatic resin duct A resin duct developing in often by high content of gelatinous fi bers. response to injury. terminal parenchyma See marginal bands. trichoblast Commonly used for a cell in root epider- thylakoids Sac-like membranous structures (cister- mis that gives rise to a root hair. nae) in a chloroplast combined into stacks (grana) and trichome An outgrowth from the epidermis. Tri- present singly in the stroma (stroma thylakoids) as inter- chomes vary in size and complexity and include hairs, connections between grana. scales, and other structures and may be glandular. tissue Group of cells organized into a structural and trichosclereid A type of branched sclereid, usually functional unit. Component cells may be alike (simple with hair-like branches extending into intercellular tissue) or varied (complex tissue). spaces. tissue system A tissue or tissues in a plant or plant tropism Refers to movement or growth in response organ structurally and functionally organized into a to an external stimulus, the site of which determines unit. Commonly three tissue systems are recognized, the direction of the movement or growth. dermal, vascular, and fundamental (ground tissue tunica Peripheral layer or layers in an apical meristem system). of a shoot with cells that divide in the anticlinal plane tonoplast A single cytoplasmic membrane bordering and thus contribute to the growth in surface of the the vacuole. A kind of unit membrane. meristem. Forms a mantle over the corpus. torus (pl. tori) Central thickened part of the pit mem- tunica-corpus concept A concept of the organiza- brane in a bordered pit consisting mainly of middle tion of apical meristem of shoot according to which lamella and two primary walls. Typical of bordered pits this meristem is differentiated into two regions distin- in conifers and some other gymnosperms; also found in guished by their method of growth: the peripheral, several species of eudicots. tunica, one or more layers of cells showing surface totipotent Potential of a plant cell to develop into an growth (anticlinal divisions); the interior, corpus, a entire plant. mass of cells showing volume growth (divisions in trabecula (pl. trabeculae) A rod-like or spool-shaped various planes). part of a cell wall extending radially across the lumen tylose (pl. tyloses) In xylem, an outgrowth from a of a cell. In initials and derivatives of vascular cambium parenchyma cell (axial or one in a ray) through a pit in seed plants. cavity into a tracheary cell, partially or completely trachea Old term for xylem vessel, implying a resem- blocking the lumen of the latter. Growth typically is blance to an animal trachea. preceded by a deposition of a special wall layer on the tracheary element General term for a water- side of the parenchyma cell that forms the wall of the conducting cell, tracheid or vessel element. tylose. tracheid A tracheary element of the xylem that has tylosoid An outgrowth resembling a tylose. Examples no perforations, as contrasted with a vessel element. are outgrowths of parenchyma cells into sieve elements May occur in primary and in secondary xylem. May have in phloem and of epithelial cells into intercellular resin any kind of secondary wall thickening found in trache- ducts. ary elements. transection See transverse section. U transfer cell Parenchyma cell with wall ingrowths undifferentiated In ontogeny, still in a meristematic (or invaginations) that increase the surface of the plasma state or resembling meristematic structures. In a mature membrane. Appears to be specialized for short-distance state, relatively unspecialized. 540 | Glossary uniseriate ray In secondary vascular tissues, ray one velamen A multiple epidermis covering the aerial cell wide. roots of some tropical epiphytic orchids and aroids. unit membrane A historical concept of basic mem- Occurs in some terrestrial roots also. brane structure visualizing two layers of protein enclos- venation Arrangement of veins in the leaf blade. ing an inner layer of lipid, the three layers forming a vertical parenchyma See axial parenchyma. unit. The term continues to be useful for describing vertical system In secondary vascular tissues. See sectioned membranes (profi les), exhibiting two dark axial system. lines separated by a clear space, as seen with the elec- vessel A tube-like series of vessel elements, the tron microscope. common walls of which have perforations. upright ray cell In secondary vascular tissues, ray vessel element One of the cellular components of a cell oriented axially (vertically in the axis) with its vessel. Also vessel member and the obsolete vessel longest dimension. segment. vessel member See vessel element. V vestured pit Bordered pit with projections from the vacuolar membrane See tonoplast. overhanging secondary wall on the side facing the vacuolation Ontogenetically, the development of vac- cavity. uoles in a cell; in mature state, the presence of vacuoles in a cell. W vacuole Multifunctional organelles bounded by a wall See cell wall. single membrane, the tonoplast, or vacuolar mem- water vesicle A type of trichome. An enlarged, highly brane. Some vacuoles function primarily as storage vacuolated epidermal cell. organelles, others as lytic compartments. Involved in wood Usually secondary xylem of gymnosperms, uptake of water during germination and growth and magnoliids, and eudicots, but also applied to any other maintenance of water in the cell. xylem. vacuome Collective term for the total of all vacuoles wound cork See wound periderm. in a cell, tissue, or plant. wound gum Gum formed as a result of some injury. vascular Refers to plant tissue or region consisting of See gum. or giving rise to conducting tissue, xylem and/or wound periderm Periderm formed in response to phloem. wounding or other injury. vascular bundle A strand-like part of the vascular system composed of xylem and phloem. X vascular cambium Lateral meristem that forms the xeromorphic Refers to structural features typical of secondary vascular tissues, secondary phloem and sec- xerophytes. ondary xylem, in stem and root. Is located between xerophyte A plant adapted to a dry habitat. those two tissues and, by periclinal divisions, gives off xylem Principal water-conducting tissue in vascular cells toward both tissues. plants characterized by the presence of tracheary ele- vascular cylinder Vascular region of the axis. Term ments. The xylem may also serve as a supporting tissue, used synonymously with stele or central cylinder or in especially the secondary xylem (wood). a more restricted sense excluding the pith. xylem elements Cells composing the xylem tissue. vascular meristem General term applicable to pro- xylem fi ber A fi ber of the xylem tissue. Two types cambium and vascular cambium. are recognized in the secondary xylem, fi ber-tracheid vascular ray A ray in secondary xylem or secondary and libriform fi ber. phloem. xylem initial A cambial cell on the xylem side of the vascular system Total of the vascular tissues in their cambial zone that is the source of one or more cells specifi c arrangement in a plant or plant organ. arising by periclinal divisions and differentiating into vascular tissue A general term referring to either or xylem elements either with or without additional divi- both vascular tissues, xylem and phloem. sions in various planes. Sometimes called xylem mother vasicentric paratracheal parenchyma Axial cell. parenchyma in secondary xylem forming com- xylem mother cell A cambial derivative that is the plete sheaths around vessels. See paratracheal source of certain elements of the xylem, such as axial parenchyma. parenchyma cells forming a parenchyma strand. Used vein A strand of vascular tissue in a fl at organ, as a also in a wider sense synonymously with xylem leaf. Hence, leaf venation. initial. vein rib In a leaf, ridge of ground tissue occurring xylem ray That part of a vascular ray that is located along a larger (major) vein, usually on the lower side of in the secondary xylem. the leaf. xylotomy Anatomy of xylem.