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

Reproductive Structures and Seed Formation 272.8.2 EMBRYOThe zygote cytoplasm becomes homogeneous, the vacuole disappears, and the cellorganelles show distinct polarization. It develops the cell wall all around. It usuallydivides after the primary endosperm nucleus has undergone several divisions. Thedivisions in the zygote follow a schematic order, and the pattern is characteristic ofa species. During the development of the embryo, two main stages are identified:the formation of the proembryo, and the formation of the embryo proper. The modeof development in dicotyledons and monocotyledons has been shown in Viola tricolorand Najas lacerata, respectively (Figure 2.9E to M). The development of theproembryo is similar, maintaining bilateral symmetry in two groups. Main embryotypes are recognized depending on the plane of divisions in cells of the two-celledproembryo and the contribution made by cells of the four-celled proembryo in theformation of parts of the embryo proper (Souéges, 1932; Johansen, 1950; Maheshwari,1950; Crété, 1963). The proembryo may have a conspicuous or an inconspicuoussuspensor. For detailed information on embryo development in angiosperms,the reader should refer to Plant Embryology by Johansen (1950).During the development of the embryo proper and the differentiation of organs,bilateral symmetry is maintained throughout in dicotyledons (Figure 2.9E to K),while in monocotyledons, the globular proembryo acquires unilaterality. The differentiationof structures takes place along one face of the developing embryo; theother remains smooth and barren (Figure 2.9L, M). The mature embryo comprisesan embryonal axis made up of the plumule (epicotyl, shoot apex), the hypocotyl —radicle axis — and one or two cotyledons. The shape and size of the embryo andthe cotyledons show considerable variation in dicots and monocots. The cotyledonsare thick or thin, straight or curved, with margins smooth or curled. The shoot apexlies between the two cotyledons in dicots (Figure 2.10A) and by the side of thesingle cotyledon in monocots (Figure 2.10B).Generally, the embryo in seed is fully differentiated into the radicle, the plumule(epicotyl), and the cotyledons, but in some plants, it is reduced and lacks differentiation.The coiled embryo of Cuscuta is devoid of cotyledons and radicle. Theembryo in Orobanchaceae (Tiagi, 1951, 1963), Orchidaceae (Poddubnaja-Arnoldi,1967), and several other families, mostly of parasitic and saprophytic floweringplants, is without the differentiation of organs or even tissue.The structure of the embryo is unique in Poaceae. The single cotyledon is shieldshapedand called the scutellum. The embryo has a few additional organs, viz., thecoleoptile, the coleorhiza, and the epiblast (Figure 2.10C). The epiblast is absent inZea and Sorghum embryos.The basal part of the embryo that does not participate in the formation ofembryo proper is known as the suspensor. It is inconspicuous in most angiosperms,but in Fabaceae (Maheshwari, 1950; Johri, 1984), Brassicaceae (Maheshwari,1950), Orchidaceae (Poddubnaja-Arnoldi, 1967), Crassulaceae, and Rubiaceae(Bhatnagar and Johri, 1972), the suspensors are well developed and probablyhaustorial. The ultrastructure of the developing embryo of soybean shows wallinvaginations (Figure 2.11A to C) in cells of the suspensor (Dute et al., 1989). Wallingrowths in suspensor cells have been observed in several species (Schulz and