UNIT 6 SEED AND FRUIT 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).