Page No.
6.1 Introduction Objective 6.2 Seed 6.2.1 Parts of a Seed 6.2.2 How Seed Develops 6.3 Seed Appendages 6.3.1 Aril 6.3.2 Caruncle 6.3.3 Operculum 6.3.4 Wings and Hairs 6.4 Stored Metabolites 6.5 Development of Fruit 6.5.1 Follicle 6.5.2 Caryopsis 6.5.3 Legume 6.5.4 Berry 6.5.5 Drupe 6.6 Dispersal of Seeds 6.6.1 Autochory 6.6.2 Anemochory 6.6.3 Hydrochory 6.6.4 Zoochory 6.7 Parthenocarpy 6.8 Vivipary 6.9 Summary 6.1 0 Terminal Questions 6.11 Answers
6.1 INTRODUCTION
Sexual reproduction in flowering plants requires two processes-pollination and fertilization, about which you have studied in Unit 3. Following double fertilization, the zygote develops to form the embryo. The Primary endosperm nucleus divides and the resulting nuclei further proliferate to eventually give rise to the nutritive tissue- endosperm (Unit 4). Meanwhile the ovule undergoes a series of changes that transform it into a protective container of the young sporophyte (Unit 5). In this unit you will study various aspects of the development of seeds and fruits. A seed may be defined as the unit of reproduction which has the embryo and commonly food-laden endosperm enveloped by a seed coat derived from the integumentb) of the ovule. Beginning with pollination, the ovary is activated to form the fruit, which encloses the developing seeds. The fruit and the seed not only protect and nurture the young sporophyte, but also serve the function of dispersal. Seeds of many plants remain viable for long periods in the soil. They may even have a certain period of dormancy to ensure germination only when conditions (temperature and moisture in particular) become suitable. Incidentally, the food stored in the fruit wall and seed are also the main source of nutrition for man, wild and domestic animals, bacteria and fungi. Objectives After studying this unit ,you should be able to: explain the structure and development of the seed; describe the various modifications in seed structure for effective dispersal; know the nature of reserve materials in the seed for the nutrition of the young sporophyte during seed germination and seedling establishment; trace the changes that occur as the ovary develops into a fruit; classify the various types of fruits; Seed and Fruit elucidate how some fruits such as banana develop without having seeds; describe the relatively rare but interesting phenomenon of vivipaq, which involves in situ germination of the seed while still enclosed and attached to the parent plant.
6t2- SEED
A seed is a mature ovule enclosing an embryonic plant, stored food material (in endosperm, persistent nucellus or embryo itself) and a seed coat formed by one or two integuments. In h broad sense the term seed is also applied to small one-seeded, dry fruits (e.g., grains of wheat or barley which are in fact made up by fusion of fruit wall and seed coat) or other disseminules (fruits with attached bracts, inflorescences or even vegetative structures such as tubers and bulbils). The size, shape, colour and surface of the seed show innumerable variations. Most orchids have minute seeds like dust particles. Seeds of a majority of flowering plants are a few millimetres in diameter (e.g., mustard, guava and poppy) or extend to a length of about a centimetre (e.g., castor, cucumber and groundnut). Some tropical trees and lianas have fruits with very large seeds. The double coconut, Lodoicea maldivica has bilobed seeds as large as 10 cm weighing nearly 6 kg.
The seed surface may be smooth, wrinkled, striated, ribbed, furrowed or it may have a variety of patterns on it. l'hk surface may be glossy (as in linseed and castor), fleshy or pulpy (in Magnolia) or covered with hair (in cotton).
Parts of a Seed Seed is attached to the fruit by a stalk, the funiculus (funicle). The prolongation of the funiculus running along the seed and terminating at the chalaza is called raphe (Fig. 6.1). The funicular vascular supply is responsible for the flow of food reserves. When the seed is separated from the funiculus a scar is left at the point of attachment which is termed mum.
COTYLEDONS,
SEED COAT
RADICLE
SHOOT
Fig. 6.1: Various parts commonly present in the seeds How Seed Develops? Corresponding with the development of embryo and endospenn the ovule, the integument(s) and the nucellus also embark on certain changes which eventually result in the formation of mature seed. The usual alterations are described with the help of a few examples. Nucellus: In a large majority of flowering plants the nucellus is gradually utilized by the endosperm or embryo, In leguminous seeds, for example, the nucellus degenerates completely. Sometimes, as observed in Euphorbia spp., nucellar cells near the micropyle (termed epistase) and chalaza (hypostase) survive longer and may even persist in the mature seed. In the black pepper fruit the bulk of the volume is occupied by the persistent nucellus (Fig. 6.2), which is also the chief food storage tissue (endosperm is relatively little). Such persisting nucellus in the seed is designated perispenn. In Daphniphyllum himalayense the seed has copious endospenn surrounded by perisperm, which is characterized by the presence of oil droplets and even protein crystals (Fig. 6.3). Plant Developmeml-I
. ,, ENDOSPERM
Fig. 6.2: Black pepper-A fruiting branch of Peper nigrum. B-a pendulous dense spike. C - whole and split fruits D -Fruits with seed cut in longitudinal section.
PROEMBRYO
Fig. 6.3: Endosperm and seed coat of DaphniphyUum. A. L.S. of seed at globular proembryo ,stage. B. Magnified to show scelerification of tegmen and presence of crystalline protein reserves in perisperm
Integuments The ovule has usually one or two integuments. After fertilization, there may be initially a proliferation of cell layers in one or both the integuments. An integument in which the cell layers increase is described as multiplicative. If the number of cell layers in the integument remains the same as in the mature ovule then the integument is regarded non-multiplicative. Alternatively, a process of disorganisation of cells may begin at an early stage. In either case, as the seed matures, most of the cell layers degenerate and get compressed. At the same time, some particular cell layers in one or both the integuments persist and may become hard to form a protective sheath. Cells of the protective layer often enldge in the anticlinal (perpendicular to the seed surface) plane, and their walls become lignified and even cutinized. The characteristic layer, if present, is described d sclerotic, mechanical, palisade or Malpighian layer. Some seed biologists prefer to call the palisade-like cells as a layer of macrosclereids. In a seed developing from a bitegrnic ovule, the persisting outer integument is termed testa and the inner integument tegmen. In seeds originating from unitegmic ovules the seed coat is loosely termed testa. Seeds with characteristic testa are called testal Seed and those with prominent tegmen are described as tegmic. Seeds in which outer part of the outer integument constitutes the mechanical layer are designated exotestal, and those having hardened inner portion are endotestal. Similarly, seeds with outer part of the inner integument modified as sclerotic zone are exotegmic and those with inner layers fonning the protective sheath are endotegmic. In some plants which have a stony or tough fruit wall, the seed coat may be thin and soft (coconut and almond). The histological changes that lead to the formation of the seed coat may be studied with the help of examples of cotton, melon, mustard and bean. In cotton (Gossypium spp.) the ovule has two integuments (Fig. 6.4) and both participate in formation of the seed coat. At mature embrvo sac stage the outer integument is 4-6
Fig. 6.4: Seed coat development in Gossypium newaceurn . A L.S. of ovule having mature embryo sac. B. Portion of integuments from ovule after 2-3 days of pollination. cells thick and inner is 8-15 cells thick. The inner integument is multiplicative. Six days after pollination the outer integument can be distinguished into three zones (Fig. 6.5): (i) outer epidermis; (ii) outer pigmented zone of 2-5 layers having some tannin and starch-filled cells; (iii) inner epidermis. In the inner integument, cells of the outer epidermis start elongating radially. These epidermal cells enlarge many times their original size and their walls become thick (Fig. 6.5.). This layer forms the sclerotic layer of the mature seed coat. In the mature seed the inner integument has four zones: outer palisade layer; a pigmented zone of 4 or 5 layers, inner colourless zone of 9 or 10 layers; inner epidermis. Thus, the mature seed coat has seven distinct zones made up by both the integuments.
OUTER PIGMENTED
OUTER COLOURLESS
PALISADE LAYER
INNER PIGMENT
INNER COLOURLE
FRINGE LAYER
Fig. 65: Structure of integuments of Gossypium herbaceurn. A-Mature seed coat. B-A lint hair C--A fuzz hair. During development of seed coat in cotton, some of the outer epidermal cells of the outer integument enlarge and then elongate outward to form hairs. These hairs, the cotton of commerce, are single celled, thin-walled and attain a size of up to 40 mm. Lint hairs are longer with characteristic twists, whereas fuzz hairs are short and without twists. In the gourd family, cucurbitaceae the ovules are bitegmic but the outer integument alone forms the seed coat. In LuRa spp. at mature embryo sac stage the outer integument is 10-15 layers thick and inner is 2 or 3 layered (Fig, 6.6). During seed development the inner integument degenerates. The outer epidermal cells of the outer integument elongate radially and develop rod-like thickenings on their radial walls (Fig. 6.6). A few layers of small pitted cells lie beneath the epidermis. The innermost layer of this zone comprises radially elongated cells. A single layer of large, radially elongated and somewhat bone-shaped cells occur next, constituting the palisade or mechanical layer. Inside the palisade layer is a region of spongy parenchyma having thin walls and intercellular air spaces.
Fig. 6.6: Seed-coat development in Luff;. (iii), inner integument; oi, outer integument; I,, epidermis, l,, hypodermls; I,, sclerenchymatous layer; 1, ,aerenchyma). . A. Portion of longisection of integuments before fertilization in L hermphr&. B. Portion of longisection of mature seed-coat in L graveolens; chlorenchymatous zone is not drawn. In mustard, Brassica campestris the outer integument has 2-5 layers of cells and the inner has up to 10 layers. Cells of the outer epidermis of outer integument become large and filled with mucilage (Fig. 6.7). The subepidermal layer has tangentially elongated cells which are gradually crushed. Inner epidermis of the outer integument forms the sclerotic layer. The inner.integument gets obliterated, except for its inner epidermis which forms a pigmented layer.
Fig. 6.7: T.S. of testa of Brassica sp. ewpidermis, sep-sub epidermis, Pal-palisade; pil-pigment layer; al-4eurone layer of endosperm.
In leguminous seeds, such as those of Phaseolus lunarusi also the seed coat is derived from the outer integument whereas the inner integument degenerates. The outer epidermis of the testa forms the palisade layer Fig. 6.8). Its cells have a Sad and Mt characteristic light line running tangentially along the middle or close to the outer walls. The light line is the result of intense refraction in the particular region of ' epidermal walls. Orientation of microfibrils that constitute the wall thickening is responsible for the varying refraction. The subepidermal layer of cells differentiates into hour-glass or funnel-shaped cells. Below this is the region of thin-walled parenchymatous tissue with vascular bundles. The outer part of this tissue has well- develo~edintercellular spaces. The region of hilum in leguminous seed has an extraordinary organisation. The attachment of funicle forms a disc-shaped structure which fits into the depression of the hilum. The outer layer of cells of the head of the funicle also forms a palisade layer (termed as counter-palisade), which is attached to the palisade layer of the testa (Fig. 6.8). Both palisade and counter-palisade are interrupted in the centre by a narrow groove which serves as an air passage in the ripening seed. The groove also leads to a group of tracheids in the seed called the tracheid bar. On either side of the bar is aerenchyma. Since the testa is impermeable to water, the tracheid bar also serves as a 'hygroscopic valve' for absorbing moisture during seed ripening and germination.
Fig. 6.8: Median L.S. of seed coat through hilum of Phaseolus aureus. A-photographic representation of the region of hilum. B. Diagrammatic representation of funicle region.
The seed coat of certain plants is fleshy or juicy. In the pomegranate, Punica granatum, for example, the outer epidermal cells of the testa enlarge and become filled with a sweet sap under considerable turgor pressure. This layer forms the juicy edible part of the seed (Fig. 6.9). The inner part of the testa is hard and the tegmen is membranous. In Magnolia spp. the inner integument forms the hard shell, whereas the outer integument is fleshy and brightly-coloured. Pulpy and fleshy testa can be termed sarcotesta. Fig. 6.9: L.S. of seed (~ia*ammatic representation) of Punica gmnututn. Outer epidermis of testa has radially elongated cells. Thesecells form fleshy part of seed.Solid black portion is inner portion of outer integument and is made up of sclerenchyma. SAQ 1 State whether the following statements are true or false. Write either T or F in the boxes provided. a) Seeds of flowering plants are always enclosed in a fruit. 1 I b) Micropyle of the ovule is represented in the seed as hilum. [ 1 c) A true seed is formed from a ovule and it encloses embryo formed with or without fertilization. I 1 d) In the majority of seeds the nucellus constitutes the nutritive tissue. 1 e) Malpighian layer of seed coat has radially elongated cells with thick wall impregnated with lignin or cutin. [ 1 f) The seed of cotton is exotegmic. [ 1 g) Leguminous seed is endotestal. [ 1
6.3 SEED APPENDAGES
Seeds of certain plants have specialized outgrowths or envelopes. These structures develop following fertilization from parts of the ovule or funicle. Here you will study the origin and structure of some seed appendages. In a later part of this unit you will learn more about how these seed appendages help in effective seed dispersal. Aril. It is an outgrowth that arises from the funicle or the testa near the raphe and covers the seed partially or completely. It is often referred to as the third integument. It is often fleshy or brightly coloured. The edible part of litchi fruit is aril, which envelops the hard brown seed coat. In Myristica fragrans the hard seed (nutmeg of commerce) is covered by a thin, irregular and bright-orange aril (that gives us the precious spice mace). Seeds of Pithecellobium dulce, a leguminous tree, have a fleshy red aril that partially surrounds the seed. In Crossosoma californicum a fimbriate aril covers the seed on the sides (Fig. 6.10). Cells of the aril contain oils, starch, sugars, pigments and aroma containing compounds. The appendage is mostly an attraction for birds, which consume the aril and scatter the seeds. Seeds of white water lily, Nymphaea alba have a spongy aril that provides buoyancy for dispersal of seed by water. Seed and FI
Fig. 6.10: hature seed of Crossosoma calijornieum- which is surrounded by imbriate aril. Caruncle. This is a white, collar-like structure borne on the micropylar end of the seed in many members of the Euphorbiaceae, such as castor, Ricinus communis. The soft outgrowth capping the hard seed is formed by proliferation of the cells at the tip of the outer integument. It is rich in starch and sugars. The violent opening of the fruit (this is termed gun-shot mechanism) causes seeds to be thrown away a few feet. It is believed that ants consume the caruncle and in the process carry the seed further away. The caruncle is also considered to be hygroscopic and it may be helpful in absorbing moisture for seed germination. Operculum. The term operculum is applied to a plug-like structure formed in the . micropylar portion of the seed by proliferation of cells at the tip of the inner integument or the nucellus. An operculum has been observed in seeds of many monocotyledonous families, such as the Commelinaceae, Musaceae, Lemnaceae and Zingiberaceae, and a few dicotyledonous families like Bignoniaceae and Nymphaeaceae. In Lempaucicostata, cells at the tip of the inner integument undergo a remarkable expansion after fertilization and form a dome-shaped, stopper- like operculum (Fig. 6.1 1). Cells of.the operculum are thick-walled and contain an orange-red substance. During germination of the embryo the operculum becomes detached from =st of the seed and facilitates emergence of the embryo.
Fig. 6.11: L.S. of mature seed of hnapaucicostakr. A prominent operculum can be seen at the micropylar end. (After Maheshwari and Kapil, 1964). 4. Wigs and hairs. Seeds of certain plants have epidermal outgrowths or the integuments themselves may form folds and projections that present a wing-like appearance. In the tree Oroxylon sp. a thin, transparently white, circular or oval, disc-like wing spreads all around the two-lobed seed (Fig. 6.12). The seed of cucurbit Zanonia macrocarpa has wings measuring up to 10 cm. In the drumstick plant Moringo oleifera the seed has three equidistant wings. Seed wings provide large surface area, with optimum strength, combined with a minimum of material. A surprisingly large number of winged seeds can be arranged wmpactly in a fruit. Wings help the seed to propel, sail or spin some distance away when
Fig. 6.12: Wiged seed of Oroxylon. Have you observed some tiny structures with lustrous white hairs floating around in the air, specially in early summer? These are seeds which travel long distances with the help of hairs. Seeds of milkweed, Calotropis procera have a tuft of hairs at one pole (Fig. 6.13). Those of Adenium sp. of the Apocyanaceae have hairs at both ends. In cotton and poplars the seeds have hair all over the seed coat surface. Hairs provide the seed a large surface area without a corresponding increase in weight, thus helping in dispersal by air. In some aquatic plants such as Nymphoides spp. the seed hairs are filled with air and provide buoyancy to the seed to float and become distributed over a large area.
Fig. 6.13: Mature and dehisced fruit of Calatropis, showing hairy seeds at one end. SAQ 2 Given below are some inwmplete statements regarding seed appendages. Fill in the blanks using appropriate words: a) The sweet, edible part we relish in a litchi fruit is ...... b) Caruncle is involved in seed dispersal by ...... c) Aril is also known as the ...... d) In the drumstick fruit the seed has ...... wings. e) Operculum is produced by proliferation of cells at the tip of ...... or ...... f) ...... and ...... are two families in which the members have seeds hairs. g) In Oroxylon the seed has a ...... for dispersal. h) Several members of the family ....! ...... : ...... bear caiunculate seeds. I Seed and 6.4 STORED METABOLITES
In a large majority of seeds food is stored in the cells of the endosperm. In coconut, wheat and castor bean for example, it is tlie endosperm which stores the bulk of the food reserves. Food stored in endosperm is utilized by the embryo during development and seed germination. In Unit 4 you have already learnt that endosperm is usually a triploid tissue derived from the fusion product of a male gamete (brought by pollen tube) and two polar nuclei in the central cell of the embryo sac. The endosperm surrounds the embryo all around and is ideal for nurturing the embryo till the seedling begins to photosynthesize and becomes antotrophic. A second sear/& of nutritive tissue in some seeds is the perisperm, which represents the persisting nucellus. Nucellus is observed in some monocot families, such as the Zingiberaceae (to which turmeric and ginger belong), and a few dicotyledonous families, including the Piperaceae (e.g., black pepper) and the Nymphaeaceae (e.g., lotus). In Canna the chalazal cell of the ovule divides repeatedly to form a starch-containing tissue called chalazosperm. Mature seeds containing persistent endosperm or perisperm are I called albuminous and those lacking them exalbuminous.. Mature leguminous seeds lack endosperm. In pea, gram (chick pea) groundnut and a whole range of pulses the large cotyledons of the embryo take up the function of storage of food. In cotton seed also the large, lobed and folded cotyledons (Fig. 6.14) are the main repository of nutrients.
HYPOCOTYL
COTYLEDON
RESIN DUCT
ENDOSPERM
Fig. 6.14: L.S. of cotton seed kernel showing embryo witb intricate folds of the cotyledons dark dots are glands. The embryo in a germinating seed has critical requirements to mobilize food reserves. It needs a source of carbon skeletal precursors, and a source of energy to assemble the precursors for building complex compounds. Energy is not only provided by carbohydrates but also by the stored lipids. Their oil content varies$om 30% of dry weight in sunflower to 50% in castor and groundnut and even%i&er in coconut and oil palm. Carbohydrates may be stored in the seed by way of thickened walls of the endosperm (as in the date palm and coffee beans) or cotyledons (in balsam and garden nasturtium). More often, cells of endosperm and cotyledons contain starch grains. Cereal grains such as wheat and rice contain 70430% starch, mainly in the endospenn. In beans, cotyledonary cells may have as much as 50% starch.
Seeds are often described as starch type (cereals) or oil type (castor, linseed). However, nearly all seeds have in addition protein reserves for supplying nitrogenous compounds to the young seedling till it becomes capable of absorbing nitrogen from the soil witb the help of roots. Protein reserves are also needed for rapid synthesis of enzymes Plant Development-I required for digestion of starch at the time of germination. Storage proteins occur in the form of discrete protein bodies, often termed aleurone grains. These grains also contain some minerals. Protein bodies occur in the endosperm and embryo in most plants. Soybean seeds are particularly rich in protein content. However, in some others such as the cereal grains these are concentrated in the specialized outer layer of the endosperm called the aleurone layer. The aleurone protein bodies become active during seed germination, triggered by the gibberellins released by the embryo. Tbis results in the production of the enzyme a-amylase which digests starch present in rest of the -'endosperm. Grain legumes are rich in reserve proteins. For example, groundnut contains about 25% and soybean nearly 40% protein. SAQ 3
State whether the following statements are me or false by indicating T or F in tbe box provided.
a) In a majority of seeds the endosperm is the main reservoir of nutrients. [ 1 b) Leguminous seeds have well-developed perisperm. [ 1 c) Castor seeds are exalbuminous. [ 1 d) Food reserves in the seed are meant for tiding over the period of dormancy. 11 e) Proteins occur in only those seeds which lack starch and lipids. [ 1 f) In wheat barley grains proteins occur mainly m the cells of the outermost layer of the endosperm called the aleurone layer. 11 g) During seed gemination, enzymes responsible for digestion of starch are produced in the seed itself. 11
6.5 DEVELOPMENT OF FRUIT
Concment with the development of the seed(s), the ovary is transformed into a fruit. The fruit protects the seeds and allows their release or germination. In primitive families such as the Magnoliaceae the fruit opens while still on the plant and the seed itself is the unit of dispersal. However, in most of the flowering plants the function of dispersal is at least partly transferred to the fruit.
SEPAL BUNDLE PETAL BUNDLE
THE DORSAL AND VENTRAL BUNDLES
Fig. 6.15: Cross-section of the fruit of Pyrus malus (diagrammatic representation).
A true fruit develops from the carpel, specifically from its ovary. However, in many so-called fruits, organs or tissues in addition to those of the ovary participate in protection and dispersal of seed. Examples of accessory tissues or organs contributing to fruit formation are many. In strawberry, Fragaria spp. the floral receptacle extends to form the fleshy edible part of the fruit. In apple, Pyrus malus the floral tube formed by the floral organs and the receptacle around the inferior ovary, together constitute the bulk of the fruit (Fig.6.15). In both these in$inces the edible fruit is product of carpellary and accessory tissues. On the other hand, in jackfruit, Artocarpus integrifolia I the perianth and in pineapple, Ananas comosus the bracts surrounding the flowers in an Seed and inflorescence proliferate to contribute to formation of the fruit. Where organs other than gynoecium participate in forming a fruit, the fruit is termed a false fruit or pseudocarp. The wall of a true fruit is termed pericarp. The mature pericarp is often made up of three distinct regions. In mango, for instance, the outer skin or peel represents the exocarp or epicarp. The fleshy and juicy middle portion is the mesocarp. The inner shell or stone is formed by the endocarp. Fruits of different plants display a rich diversity in size, shape, structure, and hardness. Chemical constituents and dispetsal mechanisms. From the morphological standpoint they are classified into a few types based on two criteria. The main criterion is the degree of hardness of the fruit wall or pericarp whether it is dry and hard or soft and fleshy. The second criterion is the ability of the fruit to dehisce or remain intact after ripening. Based on these criteria some important types of fruits are classified in Table 6.1. Table 6.1: Types of Fruits
Dry fruits
Indebiscent fruits Developing from a single carpel
1. Achene. Contains only one seed which is loose inside the fruit (buttercup). 2. Caryopsis. It is like an achene but the pericarp and the testa of the single seed become fused (wheat, corn). 3. Samara. A winged one-seeded fruit (maple). Developing from a compound gynoecium with several carpels 4. Nut. A single-seeded fruit that develops from an ovary that originally contains several carpels of which all but one degenerate. Mature nut has one carpel and one seed (walnut).
Dehiscent Fruits Developing from a single carpel 5. Follicle. A pod-like fruit that splits open on the ventral side (larkspur). 6. Legume. The pod splits open on both ventral and dorsal sides (pea, beans). Developing from a syncarpous ovary with two or more carpels. I 7. Capsule. The dehiscence is along fusion lines of carpels (Hypericum), along dorsal bundles or lines'of dehiscence in each carpel (Iris), by splitting transversely into top and bottom portions (Primula) or by small pores which develop in the pericarp (below the persistent but dried stigma poppy). 8. Siliqua. A pod-like fruit of two carpels with a false septum dividing the locule. When the fruit ripens the two valves separate and the seeds remain attached to the septum (mustard).
Fleshy Fruits
9. Berry. Exocarp, mesocarp and endocarp are distinguishable but soft or fleshy. The fleshy pericarp encloses one or many seeds (grapes, tomato). 10. Drupe. Like a berry except that endocarp is thick and hard (peach, mango).
11. Pepo. Like a berry, but the exocarp is hard forming a rind (pumpkin, squash).
Schizocarpic Fruits There are fruits that develop from multilocular ovaries that separate when ripe into individual achenes, each representing a carpel (Malva, Abutilon). Plant Development-I Development of fruit can be studied by selecting a few representative examples of follicle, caryopsis, legume, capsule, berry and drupe.
1. Follicle. In larkspur, Delphinium sp. the exocarp develops from the outer epidermis and sometime the hypodermis of the ovary wall. It consists of thick-walled cells (Fig. 6.16). Mesocarp is parenchymatous. The endocarp, formed from inner epidermis, consists of thick-walled cells. At maturity the pericarp dries up and the follicle opens along the line of fusion of the carpels.
OUTER EPlDERMlS nf-I-<
Fig. 6.16: T.S. of the follicle wall of Delphinium.
2. Caryopsis. In cereals each carpel has one ovule and therefore the mature fruit has, just one seed. During maturation, very little or no cell divisions are necessary in the ovary wall. The pericarp and the remains of the integuments of the seed get completely fused. In wheat caryopsis three main regions can be distinguished: (i) the caryopsis wall which includes the pericarp, seed coat and remains of nucellus; (ii) endosperm; (iii) embryo (Fig. 6.17). The pericarp can be distinguished into five layers (Fig. 6.18): (i) epidermis; (ii) hypodermis; (iii) zone of thin-walled cells; (iv) cross cells; (v) tube cells. The outer epidermis and hypodermis together form the exocarp, having thick-walled compressed cells. Inside the exocarp are one or a few layers of thin-walled parenchymatous cells. These are followed by the cross cells that have thick walls with characteristic pits elongated transversely to the cell. The tube cells constitute the inner epidermis of the pericarp. These cells have thinner walls than cross cells but these too are pitted. In the mature caryopsis testa is destroyed but tegmen is discernible along with one or two layers of nucellus.
Fig. 6.17: A L.S. of Grain of Triticum B. L.S. of embryo of Trificum.
3. Legume. Outer epidermis of the ovary usually forms the exocarp of the leguminous pod. Next few cell layers constitute the mesocarp with thick-walled parenchyma. The endocarp consists of sclerenchyma on the inside of which is epidermis or a few layers of parenchyma and epidermis. In Astragalus macrocarpus, for example, inside the sclerenchymatous part of endocarp there is a thin-walled hypodermis and an epidermis (Fig.6.19). Vascular bundles are located in the mesocarp accompanied by some sclerenchymatous cells. The two valves of the dried legume usually twist, resulting in their opening and scaaering of seeds. The twisting and opening of valves is brought about by shrinkage of the sclerenchymatous cell layers which are obliquely oriented.
Flg. 6.18: Portion of CS. through the caryopsis of Wumat mature stage of development R'EPIDERMIS
Fig. 6.19: AstrugcJIu macrocarpus (oblique dn).
4. Berry. In a berry much of the @carp is fleshy or juicy. In tomato. Lycopersicon esculentwn even the placenta on which the seeds are borne is fleshy (Fig. 6.20). The exocarp consists of an epidermis and 3 or 4 layers of collenchymatouc cells. The epidermis is covered by a cuticle. Mesocarp consists of large, thin-wailed cells 1 with abundant intercellular spaces. There is a large increase in cell volume in this zone of @carp. With the development of ovules, aMpollination, the I parenchymatous tissue of the placenta grows around the funiculi. This r parenchymatous tissue continues to pmlifemw furha and envelops the seeds . cOrnPle@ly. FLESHY, PARENCHYMATOUS TISSUE
Fig. 6.20: Diagrammatic, ~epresentationof T.S. of Lycospersion esculcnhmi~after fertilization. 5. Drupe. In the drupe of peach, Prunus persica most of the cell divisions occur in the ovary wall before fertilization or soon after it. Further growth of the fruit is accomplished mainly by cell enlargement. To begin with, cell enlargement occurs in all directions, but later expansion is mainly in radial direction. This is more so in the inner portion of the mesocarp. At maturity the outer epidermis has thick cuticle and unicellular hairs. Mesocarp has loose parenchyma. Endocarp has sclereids and forms the stone of the fruit. SAQ 4 State whether the following statements are me or false by indicating T or F in the box against each. a) A me seed develops within a carpel. [ 1 b) Apple is called a false fruit because it has no fertile seeds. [ 1 c) In mango the entire pericarp is fleshy and the seed coat is hard. [ 1 d) In the caryopsis of wheat the pericarp and seed coat are fused together. [ 1 e) A follicle opens along the line of marginal fusion of the carpel . [ 1 f) In a tomato fruit only the endocarp is edible. [ 1
6.6 DISPERSAL OF SEEDS
A plant usually bears many fruits and innumerable seeds. If all the seeds produced by a plant were to germinate in the immediate vicinity, this will have several disadvantages. The resulting seedlings would compete intensely among themselves for space, light, water and minerals. They will be more vulnerable to atrack by pests and pathogens. Moreover, there will be greater chances for backcrossing, in the resulting progeny, rendering them genetically inferior. To overcome these problems most plants have evolved one or another mechanism for dispersal of seeds over a wide area. Fruits of some plants have built-in mechanism for dispersing their seeds to considerable distances (autochory). Other plants depend on external agencies such as air (anemochory), water (hydrochory) and animals (&hory) to disseminate their fruits or seeds. 1. Autochory. This mechanism of self-dispersal is based on forceful expulsion of the seed from the fruit because of desiccation or turgidity of the cells of the pericarp. In balsam, Impatiens spp., for example, the fruit is a cylindrical capsule fonned by the, fusion of five carpels (Fig. 6.21 A). The fruit wall comprises three regions of which the middle is made up of radially elongated cells with high turgor pressure (Fig. 6.21 B). This zone is termed expansion zone. In the dry, ripe capsule the cells of the expansion zone are in a state of high tension. However, the inner portion of the pericarp consists of 2 or 3 layers of collenchyma which offer resistance. At this stage even a mild touch or jerk results in separation of the carpels at the base (Fig. 6.21 C). The five carpels instantly curl inward throwing the seeds at a distance of about 2 metres. In Arcenthobium sp, the fruit is a pseudoberry enclosing a single seed. The mesocarp is formed by viscin cells. When the fruit is detached from its stalk the cells of the viscid Seed and Fruit layer generate a high pressure and the pericarp contracts to hurl the seed out with great force (Fig. 6.22).
\ 1 B OUTER EPIDERMIS
RESISTANCE EXPANSION TISSUE TISSUE
Fig. 6.21: A L.S. of closed fruit of Impatiens. L.S. through pericarp showing elongated ceUs of middle region with high turgor pressure. A fruit the valves of which have curled inwards and thus have ejected the seeds.
Fig. 6.22: Diagrammatic representation of explosive seed discharge in Arcenthobium.
2. Anemochory. Seeds that are dispersed by air currents are usually light or are provided with special structures to help them remain air-borne for long periods. Since a large proponion of seeds are wasted, anemochorus plants usually produce a profuse quantity of seeds per plant. Some of the orchids, for example, produce as much as seven hundred million seeds per plant and the seeds are so tiny that these are blown away like dust particles. The orchid seeds have an undifferentiated embryo and lack endospenn (Fig. 6.23).
You have studied earlier in this unit about seeds and fruits of several species with hairs or wings that propel them even in mild wind. Winged fruits and seeds are a characteristic feature of some tall trees. In maple the wings represent expansions of the pericarp.
Fig. 6.23: A seed of Qpripeb. Embryci qn be seen without endoepcrm through transparent seed-coat. , 3: Hydrochory. Plants that grow in or along the bank of water bodies often utilise water as an agency for dissemination of fruits and seeds. Coconut, Cocos nucifcra is an excellent example of a fruit which has spread to different continents because of its ability to float over hundreds or even thousands of kilometres. The fruit has a smooth, waterproof exocarp, followed by a fibrous and air-filled mesocarp and a hard endocarp. The seed with a thin seed coat retains its viability for more than three months. During this period the fruit can travel as long as 4300 km. Seeds of lotus and some cypresses have air-filled cortical tissues.
4. Zoochory. Some fruits are eaten by animals and the seeds are passed out with the excreta (endozoochory). Plums, Lantana, grapes, figs and guava are examples of some of the fruits eaten by birds. Fleshy part of the fruit is digested in the gut of the bird and the seeds are expelled along with the droppings. Using the excreta as manure, many of such seeds germinate. It is believed that the seeds of Ficus species germinate only after they have passed through the gut and have been subjected to the scarification in the gullet by small pebbles and digestive fluids'in the alimentary canal. In plants like neem, maulsari etc., the seeds are too large. The birds eat the pulp and discard the seeds below their perch. Fruits and seeds of many other plants are carried by animals externally (exozoochory), sticking to their body or mouth parts. Seeds of mistletoe, (Viscum album) are dispersed widely because they adhere to the beaks of birds feeding on its fruits. Fruits of many members of the Asteraceae have spines (e.g., Xanthium) or hooks (BidCm) with the help of which they stick to the bodies of animals and get distributed over a large area.
Mammals like squirrels, monkeys and even goats (these are used by fanners to encourage germination of seed species of Acacia nitotica (feeding pods) play an important role in dispersal. Humans serve as active agents of deliberate distribution of seeds to raise plants useful to them.
6.7 PARTHENOCARPY -
It is generally observed that the fruit develops after fertilization and it has fertile seeds inside it. However, this is not always so. Fruits of certain varieties of plants, such as edible banana (Musa sp.), tomato (Lycopersicon esculentum), orange (Citrus sp.) and grapes (Vifis vinifera), develop without seeds. In the seed-bearing bananas the three Seed' adFndt ldesof the beny are occupied by large seeds, whereas in the parthenocarpic varities, the ovules degenerate and the cells of the pericarp and septae proliferate to form the ,I pulp regions (Fig. 6.24). The phenomenon of formation of fruit without fertilization is I known as parthenocarpy. Parthenocarpic development may require pollination (stimulative parthenocarpy), or it may occur even without pollination (vegetative p-enocarpy ). n
Fig. 6.24: T.S. Fruits of developing parthenocarpic and seed beating varieties of banana. (a) ~akbenocarpicfruit at the time of emergence of the inflorescence. (b) Parthenocarpic fruit 8 weeks after emergence, showing the pulp (p) invading the ovarian cavity (oc) or locule. (c) Seeded Mtat the time of emergence (d) Seed,ed fruit 8 weeks after emergence. Very little pulp is present around the lo&, which are occupied by the enlarging seeds. 0, ovuk; s. seed, vb, vascular bundle. Three types of parthenocarpy are generally recognised: (i) genetical (ii) environmental, (iii) chemically-induced. Genetical parthenocarpy is obsmed in many species cultivated for their fruit It arises due to mutations or hybridization. Generally parthenocarpic varieties have sterile pollen so that the pollination stimulus is available, but fertilization Qes not take place. As a result, a fruit is produced which has no seeds. The famous navel orange has arisen hm a normal seeded Citrus variety through mutation in an axillary bud, which formed a branch bearing seedless fruits. Several cultivated varieties of grapes and cucumber have also resulted hmbud mutations. Environmental parthenocarpy is the result of some environmental conditions such as hst or low temperature which interferes with the normal reproductive process. In tomato, for instance, cultivation under low tempture and high light intensity can .. induce parthenocarpy. Under these conditions pollination is so poor that seeds are not produced but ovary is activated to form the fruit
Induced parthenocarpy involves lreatment of the flowers with certain plant growth regulators. Auxins and gibberellins at low concentrations (about lC7 - 104 M) have been successfully utilised for induction of parthenocarpy in a large numbex of plants which otherwise bear seeded fruits. Seedless guava, tomato, and straw- fruits have been obtained by this method.
Paabenocarpy is important in horticulm because seedless fruits are more convenient to consume and are particularly suitable for the industry manufacturing jams, jellies and fruit juices. Gibberellins also cause fruit enlargement (in grapes which are considered commercially beneficial for packaging and marketing, and also cause looseness of b19ches). 6.8 VIVIPARY
In flowering plants the seed or fruit generally is dispersed and germination occurs when the conditions are congenial for growth. However, in some plants growing along the sea shores or in mangrove areas the soil is too saline or*salty for seed germination. Moreover, in such places the seed and the young seedlings are likely to get washed away by the tide. Mangrove plants (e.g., Rhizophora spp.) have, therefore, evolved a unique adaptation for ensuring seed germination and nurturing of the young sporophyte. The young seedling grows out of the intact fruit and hangs with its pointed radicular end facing downward. Once the seedling attains a large size it drops down and penetrates the soil. Roots are produced rapidly by the radicle and the seedling is fixed firmly in the soil. By this time the seedling would have developed an extensive photosynthetic tissue to ensure its establishment and has been compared to the similar state observed in mammals. Certain bamboos and forest trees also practice vivipary. This mechanism of rearing the young one by the parent is termed vivipary. It is a specialized characteristic evolved by mangrove plants as a strategy for survival. SAQ 5 Fill in the blanks in the statements given below wlth appropriate words. a) Dispersal of seeds by means of wind is termed ...... b) ...... involves built-in mechanism in the fruit for expulsion of seed. c) Seeds of the banyan tree are mostly dispersed by ...... d) Edible banana is a ...... fruit. e) ...... and ...... are plant growth regulators employed for induction of parthenocarpy. f) Vivipary is observed in ...... plants.
6.9 SUMMARY
In this unit we have studied the development of fruits and seeds. You have seen that the fruit and seed svucture display immense diversity. We have examined the nature of reserve materials and the sites of their storage in the seed. The interesting aspects of the range of seed dispersed agencies have been discussed. The formation of seedless fruits (parthenocarpy)and their importance in horticulture have been explained. The rather rare phenomenon of vivipary in plants has been highlighted as an adaplive feature by the mangroves. What we have learnt can be summarized as follows: A true seed is a fertilized ovule. It contains an embryo and generally the endosperm. These are enclosed by the seed coat derived from the integuments. Seed protects the young sporophyte and serves as an efficient propagule. One or both the integuments form the seed coat. A layer called palisade or sclerotic layer is usually differentiated which fonns the protective shell. In some plants Perisperm (persistant nucellus) forms an additional nutritive tissue. In many seeds, principally leguminous seeds, the cotyledons perform the function of storing reserve food. Seeds may have appendages such as aril, caruncle, operculum or wings and hairs which help in their dispersal. carbohydrates (in the form of starch and cell wall materials), lipids and proteins Seed and Fruit are the chief sources of nutrition for the germinating embryo and young seedling. A true frbit develops from the gynoecium as a result of stimulus provided by pollination and/or fertilization. Fruits are classified on the basis of their water content (fleshy or dry), number of carpels and locules, number of seeds and importantly on the criterion of their ability to dehisce or not. Fruits and seeds are variously adapted for self-dispersal (autochory). or dispersal by agencies such as wind (anemochory), water (hydrochory) and animals (zoochory). Parthenocarpy involves formation of fruits without fertilization so that no fertile seeds are produced. Varieties of horticultural plants can evolve by mutationsnr hybridization to bear seedless fruits. Parthenocarpy can also be induced by environmental factors and application of growth hormones regulators. \ Vivipary is a unique phenomenon observed chietly in mangrove plants. It involves nurturing and germination of the.young sporophyte while the fruit is still attached to the parent plant.
The study of fruits and seeds becomes fascinating by a visit to a garden. crop field, open land or a forest. You will need observant eyes, probably a knife and of necessity a magnifying glass. The types of fruits and seeds you have studied in this Unit can be found in your immediate surroundings, some even in a vegetable market or kitchen garden! The spectacle of seed dispersal can be observed in nature at all places and at all time.
6.10 TERMINAL QUESTIONS
1) What are the chief advantages of the seed habit? 2) Which are the food storage tissues in the seed? In what form is the food stored? 3) Name some seed appendages and describe how they help in dispersal or germination of seed. 4) What is a true fruit? Why are apple and Jack fruit considered false fruits? 5) What are the chief criteria for the classification' of fruits? 6) What parts of the seed or fruit is edibleluseful in the following plants: i) banana ii) tomato iii) coconut iv) groundnut
vi) litchi vii) cotton viii) castor ix) pineapple x) strawberry xi) pomegranate xii) mustard
7) How are the fruitslseeds of the following plants dispersed? What type of adaptations have been developed by these plants? Plant Development-I i) coconut ii) mistletoe iii) banyan iv) balsam v) milkweed
8) What is the commercial importance of parthenocarpic fruits? How can seedless fruits be induced artificially?
9) What is vivipary? How does ,it help the mangrove species to survive in their saline/ estuarine habitat?
6.11 ANSWERS
Self Assessment Questions True False True
False , True True :False aril b) ants . C) third integument d) three e) inner integument or nucellus f) Apocyanaeae and Asclepiadaceae , g) wing h) Euphorbiaceae 3. a) True b) False c) False d) False e) False f) True g) True 4. a) True b) False C) False d) True e) True f) False 5. a) anemochory b) autochory c) birds d) parthenocarpic e) auxins and gibberellins Terminal Questions
1. The seed not only protects and nurtures the young sporophyte, but also serves as a propagule for wider distribution. It may remain viable for long periods, or stay dormant till conditions are suitable for germination and growth of the seedling.
2. Endosperm, cotyledons, perisperm and rarely the chalazosperm are the food storage tissues of the seed. Carbohydrates (in the form of starch and wall materials), lipids and proteins are the chief stored food reserves.
3. Aril is consumed by birds and the animals and the discarded seed gets dispersed widely. The caruncle is sought after by ants, which carry the seeds far from where they have fallen after dehiscence of the fruit. In some aquatic plants the spongy aril provides buoyancy to the seed to stay afloat and travel long distances. During seed germination, the operculum (a lid-like structure at the micropylar part of seed) gets detached and facilitates the emergence of embryo to form the seedling. Wings and hairs also help in dispersal of seed by wind.
4. A true fruit develops from a carpel. Apple is regarded a false fruit because the floral tube and the receptacle around the inferior ovary also contribute to the fruit wall. In the fruit of the jack tree the perianth of the flowers in the inflorescence also proliferate and ! contribute to the edible portion. t 5. The chief criteria for classification of fruit are: degree of hardness, number of carpels and locules; number of seeds per locule; ability of the fruit to dehisce or stay intact.
6. i) Mesocarp, endocarp and placentae
ii) The whole fruit (sometimes including seed) minus the persistant calyx. iii) The whole seed.
iv) Whole seed (sometimes without seed coat, and sometimes only cotyledons).
v) Hypanthium (basal portions of perianth and stamens) and the fleshy receptacle vi) Aril
vii) Lint hairs yield textile fibre fuzz hair which are used for high class cellulose acetatelnitrate and the cotyledons give oil. The oil cake is used as manure or a medium for fungi~bacteriain industry or as source of gossypol.
viii) Endosperm
ix) Bracts
x) Receptacle of the fruit (flower) plus individual fruits developed fiom all the carpels (including seeds) xi) Testa xii) The whole seed is crushed for oil.
7. i) by sea water; impervious exocarp, buoyant mesocarp and hard endocarp
ii) by birds; seeds are sticky iii) mostly by birds; the figs (Syconia) consumed by birds, and the tiny fruits bearing hard seed (one each) emerge ready for germination
iv) autochory; high turgor pressure in the mesocarp
v) by wind; seed hairs
8. Seedless fruits are more convenient to consume. These are particularly suitable for fruit preservation industries. Seedless fruits can be evolved by mutations or hybridization and environmental conditions such as low temperature. Application of growth regulators can induce parthenocarpy in some plants. 9) Vivipary involves germination of seed and nurturing of the young sporophyte in the fruit while it is still attached to the parent plant. It is an adaptation to tbe pP;culiar environmental conditions that prevail in mangroves ecosystems. If a seed of mangrove is liberated it is likely to be carried away by the tidal waters. Moreover, germination of seed and growth of young seedling may be seriously impaired by the high salt content. As an adaptation the seedling in mangrove plants separate from the parent only when it is sufficiently large and capable of establishment in saline watea as an autotroph. GLOSSARY androecim : male reproductive organs of a plant; stamens taken collectively archesporium : .a cell or mass of cells, dividing to form microspore mother cells. anticlinal : line of division of cells at right angles to the surface of apex of a growing point. apomixis : a reproductive process without fertilization in plants, akin to parthenogenesis but including development from cells other than ovules, as apogarny and apospory. cleistogamy : the condition of having flowers which never open, and are self pollinated. dehiience : the spontaneous opening .of an organ or structure along certain lines or in a definite direction. embryogeny : the process by which the embryo is formed. embryo sac : the megaspore in angiosperms, containing the female gemetophyte. endosperm : the nutritive tissue of most seeds. endothecium : one of the wall layers of anther that helps in the dehiscence of the anther. endothelium : synonymous-integumentary tapetum; specialized cells having nutritive role, these develop from the inner-most layer of the integument and closely surround the nucellus. generative cell : the smaller of the two cells of the pollen grain, divides and forms the sperms. haustorim : an outgrowth of embryo sac which extends to the nutritive tissue to draw nutrition. - - indehiscent :'fruits which do not open to release seeds, but the whole fruit is shed hm. the plant. massula : a group of pollen grains that occur in a mass. megaspore : it develops to form the female garnetophyte, or the embryo sac. microsporangium : pollen sac; a sporangium containing a large number of microspores. microspore : the cell from which pollen grain - the male gametophyte, develops. parthenogenesis : reproduction without fertilization by male gamete. periclinal : division parallel to the surface of the cell, or the apex of the growing point. pollen tube : a tubular structure developed from pollen grain after pollination, it grows towards the ovule, carrying male gametes to the embryo sac. polyembryony : formation of several embryos in one ovule. proembryo : an embryonic structure preceding true embryo. suspensor : a chain of cells, developed from hypobasal segment of angiosperm zygote; it attaches embryo to the embryo sac. tapetum : nutritive layer investing the sporogenous tissue in a sporangium; it supplies nutrition to the developing microspores. tetrad : 4 spores formed after the 1st and 2nd meiotic division of the microspore mother cell. FURTHER READING
1. Bhojwani, S.S. & Bhatnag~S.P. 1993. The Embryology of Angiosperms. Vikas Publishing House Pvt. Ltd., New Delhi.
2. Maheshwari, P. 1950. An Introduction to the Embryology of Angiospem. Tata- McGraw Hill Publishing Company Ltd., New Delhi.
3. Shivanna, K.R. & Johri, B.M. 1985. The Angiosperm Pollen: Structure and Fwrczion. Wiley, New Delhi.,