FUNCTIONAL-ANATOMY OF THE OVULE IN BROAD BEAN (VICIA-FABA L) - ULTRASTRUCTURAL SEED DEVELOPMENT AND NUTRIENT PATHWAYS

Ovules of broad bean (Vicia faba L.) were studied to disclose ultrastr uctural features, which can facilitate nutrient transport to the embry o sac from 10 d after pollination (DAP) to the mature seed. Fertilizat ion occurs during the first 24 h after pollination. The endosperm is a coenocyte, which is eventually consumed by the embryo. By 10 DAP the inner integument is degraded and the outer integument adjoins the embr yo sac boundary. The heart-shaped embryo approaches the embryo sac bou ndary al two sites, which here are named contact zones. Small integume nt cells in the neighbourhood of the first formed contact zones become separated by prominent intercellular spaces. A heterogenous scatterin g material, probably representing secretion products accumulates in th ese spaces. By 14-16 DAP the integument exudate disappears, and the su spensor degenerates. As the contact zones increase in size, wall ingro wths form a bridging network in the narrow space between the embryo sa c boundary and the extraembryonic part of the endosperm wall. The epid ermal cells of the embryo separate adjacent to these zones, and develo p conspicuous wall ingrowths. At 20 DAP vacuoles showing various stage s in formation of protein bodies appear in the cells of the embryo.