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

32 Histopathology of Seed-Borne Infectionsexotegmic, mesotegmic, and endotegmic on the basis of the position of the mainmechanical layer in the outer epidermis, middle layers, or inner epidermis of theouter or the inner integument, respectively.2.9 SEED COAT DEVELOPMENT IN SELECTED GENERAThe seed coat development for some economically important plants, e.g., for trueseeds of Brassica (Brassicaceae), Hibiscus, Gossypium (Malvaceae), Crotalaria(Fabaceae), Lycopersicon (Solanaceae), Cucurbita, Sechium (Cucurbitaceae), andone-seeded indehiscent fruits of Lactuca (Asteraceae) and Triticum (Poaceae), isdescribed.2.9.1 BRASSICAThe ovule is bitegmic; initially each integument is two or three-layered. Both theinteguments show slight multiplication. The outer integument becomes four-layered(Figure 2.13A). The cells in the inner epidermis of the outer integument elongateradially, and those of the other layers stretch tangentially and lose contents. Thecells of the inner epidermis acquire U-shaped thickenings, forming the main mechanicallayer in the mature seed coat (Figure 2.13C, D).The inner integument becomes many-layered (Figure 2.13B), and its cellsenlarge and undergo absorption during seed development (Figure 2.13C). The innerepidermis persists and forms the endothelium. Its cells develop pigmented materialand some proteinaceous bodies (Vaughan, 1956, 1959; Rathore and Singh, 1968;Prasad, 1974). The mature seed coat consists of layers of the outer integument andthe endothelium (Figure 2.13D).2.9.2 CROTALARIAThe ovules are bitegmic (Figure 2.13E, F) but the inner integument disappears duringdevelopment (Figure 2.13G, H) and the outer integument alone forms the seed coat.The outer integument is multiplicative. The cells of the outer epidermis elongateradially and form the palisade layer of sclereids or macrosclereids (Figure 2.13G toI). This is the main mechanical layer in fabaceous seed.The cells of the subepidermal layer become columnar, develop unequal thickenings,and form the hourglass cells (Figure 2.13H). The remaining layers remainthin-walled and stretch tangentially, and are partly digested during development(Corner, 1976; Pandey and Jha, 1988).The differentiation in the hilar region in developing fabaceous seed and itsstructure at maturity are quite characteristic (Baker and Mebrahtu, 1990), The regionconsists of rim-aril (when present), counter palisade (differentiated in the funiculus),hilar fissure, palisade layer (macrosclereids), tracheid bar, stellate parenchyma, andaerenchyma (Figure 2.13I). The development of hilar fissure, counter palisade,tracheid bar, and stellate parenchyma begins quite early and is nearly completed bythe time 35% maximum seed size is attained in soybean (Baker and Mebrahtu, 1990).