Functional Anatomy of the Ovule in Broad Bean, Vicia faba L. II. Ultrastructural Development up to Early Embryogenesis

Abstract

Ovules of broad beans (Vicia faba) have been studied to analyze ultrastructural features for nutrient transport to the embryo sac at various ontogenetic stages up to 10 d after pollination. In unpollinated flowers a notable homogeneous or fibrillar material is deposited in the endostome, between the two integuments and on the nucellus. Osmiophilic globules accumulate at the plasmalemma and in the walls at the micropylar end of the inner integument. These globules increase in number after fertilization and appear also in other cells near the embryo sac. The central cell, which has some wall ingrowths typical for transfer cells, shows intrusive growth between cells of the nucellar cap. After fertilization wall thickenings occur in cells close to the embryo sac. At 10 d after pollination the inner integument has degenerated entirely. Also the nucellus, including the nucellar cap, is digested. In the endosperm free-nuclear divisions start and the cytoplasm increases in amount. Wall ingrowths are formed along the whole embryo sac boundary. The suspensor consists of two pairs of multinucleate cells: the pair adjacent to the embryo proper have rounded cells; the other pair have elongated ones. The suspensor cells that are attached to the embryo sac boundary become transfer cells. Their plastids have prolamellar bodies, and these structures are not seen anywhere else in the ovule. Our study confirms that transfer cells are common at junctions between the different generations in the ovules, that the transport to the embryo sac is apoplastic, and that symplastic transport is possible between endosperm and embryo and further between suspensor and embryo proper.