Histopathology of Seed-Borne Infections - Applied Research Center ...

Reproductive Structures and Seed Formation 37In Sechium edule, the cells of e, e≤ (wherever present), and e¢ get packed withstarch grains, but remain unlignified (Figure 2.15H). The remaining layers becomeaerenchymatous and acquire starch grains (Singh, 1965). The fruit in Sechium isone-seeded, and the seed shows many uncommon features, i.e., leathery testa, vivipary,and hypogeal germination.2.9.5 LYCOPERSICONThe single integument is multiplicative and differentiates into three zones — theouter epidermis, the mesophyll, and the inner epidermis, which forms the endothelium(Figure 2.16B to D). During the advanced stages of seed development, thecells of the outer epidermis undergo enormous radial elongation. Their inner tangentialand radial walls become thick, thickening and tapering from inside to outsideon radial walls. The outer wall remains thin and readily separates at maturity (Figure2.16E). The middle layers develop lysigenous cavities and are gradually absorbed.Only a few hypodermal layers persist. The cells of endothelium (Figure 2.16D, E)are flattened and accumulate pigmented contents (Saxena, 1970).A succulent envelope (arillode) surrounds the mature seed. The primordium forthe envelope originates in the placenta around the funiculus within 72 to 96 hoursof pollination (Figure 2.16A). It grows rapidly and gradually surrounds the seed.Finally, the seed is completely surrounded (Figure 2.16F, H). The cells of the arillodeare initially polygonal with prominent nuclei, but subsequently enlarge and acquirechloroplasts and starch grains (Figure 2.16 G). At maturity they lose their contentsand their cell walls gelatinize.2.9.6 LACTUCAThe cypsil, dry, one-seeded, indehiscent fruit, forms seed. The pericarp constitutesthe protective covering, and the seed coat is thin when present, or it may be completelyobliterated. Thus the integument and ovary wall both participate in formingthe seed cover.In Lactuca, the integument is six- or seven-layered, multiplicative, and differentiatesinto three and finally four zones (Figure 2.17A to C). The inner epidermisforms the endothelium, and three or four layers adjacent to it constitute the periendothelium(Figure 2.17C). All the layers except the epidermis and the hypodermisare absorbed. The cells of the periendothelium undergo gelatinization before absorption.The epidermal cells develop cellulose thickenings on the radial walls (Figure2.17E, F).The ovary wall comprises eight or nine layers, and its cells stretch tangentiallyduring development. The epidermal cells show pronounced gliding growth, formingspinescent structures. At maturity the pericarp is sinuate and in carinal (ridge)position bears a fibrovascular bundle (Figure 2.17D, E). Other components of thepericarp are the epidermis and one or two layers of narrow tangentially elongatedsclereids (Figure 2.17F, H). The size of the epidermal cells, the amount of tannin,and the ratio of their body and spinescent region (Figure 2.17G) vary in differentLactuca species (Kaul and Singh, 1982).