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

Reproductive Structures and Seed Formation 23been observed in Capsella (Schulz and Jensen, 1972) and Helianthus (Newcomb,1973).The embryo sac wall lacks plasmodesmatal connections with the surroundingtissue (Orel and Shmaraev, 1987; Folsom and Cass, 1988; Johansson and Walles,1993). Its wall shows the formation of protuberances on the internal surface andmicroinvagination of plasmalemma (Figure 2.12A; see p. 30). These formations aresimilar to those of the transfer cells involved in short-distance transport of metabolites.2.7 FERTILIZATIONDouble fertilization is characteristic of angiosperms. The pollen tube is porogamous— entering the ovule through the micropyle (Figure 2.8A); chalazogamous —entering through the chalaza (Figure 2.8B); and, very rarely, mesogamous —penetrating laterally through integuments between the micropyle and chalaza. Inalmost all the economically important plants, the pollen tube enters the ovule throughthe micropyle and is porogamous. Under all the above conditions, the entry of thepollen tube into the embryo sac is at the micropylar end. The pollen tube entersthrough one of the synergids (Figure 2.8C), and the sperms are released in thecytoplasm of the synergid through a subterminal or terminal pore (Cass and Jensen,1970; Jensen, 1973). One sperm comes in contact with the egg plasma membraneand the other sperm come in contact with the plasma membrane of the central cell(Figure 2.8D, E). An opening in the fused plasma membranes of egg and spermallows the entry of the sperm nucleus into the egg. Similarly, the other sperm nucleusenters the central cell. There is no evidence of the entry of the pollen tube or spermcytoplasm in the egg or in the central cell.The sperm nucleus fuses with the egg nucleus. Three types of karyogamy —premitotic, postmitotic, and intermediate — are involved in their fusion (Gerassimova-Navashina,1960). The phenomenon is known as syngamy. The other malenucleus, which comes to lie close to the polar nuclei, or their fusion product, thesecondary nucleus, fuses with them to achieve triple fusion, forming the triploidprimary endosperm nucleus.2.8 SEED DEVELOPMENTPollination and fertilization provide the stimulus for the development of fruit andseed. The fertilized ovule undergoes changes in all parts during seed formation. Theovule enlarges, the zygote forms the embryo, and the primary endosperm cell withthe triploid nucleus forms the endosperm, which is required to provide nutrition tothe developing embryo. The nucellus, chalaza, and the integuments undergo varieddegrees of structural change. The development of seed is relatively constant in agroup. A general account of the development of the endosperm, embryo, and otherparts of the ovule is given here.