FUNCTIONAL-ANATOMY OF THE OVULE IN BROAD BEAN (VICIA-FABA L) .1. HISTOGENESIS PRIOR TO AND AFTER POLLINATION

Structural adaptations promoting apoplastic transport to the embryo sa c were studied in broad beans (Vicia faba) during histogenesis from ov ule primordia to seed. Nucellus and the inner integument disappear at an early stage. The outer integument then adjoins the embryo sac bound ary. In the thin part distal to the funiculus the cells of the persist ing integument are smaller than in other parts and develop PAS-positiv e wall ingrowths opposite the embryo. At the late globular stage, the embryo establishes contact with the embryo sac boundary. In the contac t zone, embryo cells develop wall ingrowths. Wall ingrowths are furthe r formed on both sides of that part of the endosperm that is inserted between the outside of the cotyledons and the embryo sac boundary. The wall protiferations characterize all these cells as transfer cells. W e conclude that the embryo sac is supported with nutrients from digest ed maternal tissues (nucellus, chalaza, inner integument, and part of the outer integument). These tissues are generally rich in starch grai ns. After the embryo has established contact with the embryo sac bound ary, it is supported by transport of solutes from transfer cells in th e outer integument.