ULTRASTRUCTURAL STUDIES OF EMBRYOGENESIS IN FAGOPYRUM-ESCULENTUM

Patterns of Fagopyrum esculentum embryo, endosperm, and ovule developm ent were studied from two-celled embryo to torpedo-shaped embryo stage s, using ultrathin sections of ovules 1-5 d after pollination (DAP) wi th a transmission electron microscope. At 1 DAP, the zygote had divide d and formed a two- to eight-celled embryo, which was separated from t he free nuclear endosperm by a thin wall developed from plasm membrane in the chalazal end of the egg cell. At this early stage, the cytopla sm contains extensive RER, active dictyosomes, polymorphic mitochondri a, and plastids. The globular-shaped embryo, 3 DAP, consists of a comp letely developed vacuolated suspensor and an embryo proper that contai ns cells with denser cytoplasm than the endosperm. Wall ingrowths are completely formed on the parietal cell layer of the endosperm and cove r the whole embryo. After this stage, the endosperm become cellularize d and the embryo is changed from globular to heart-shaped at 4 DAP. Ax is and cotyledon elongation as well as enlargement occur at 5 DAP. By this time the embryo appears torpedo-shaped and the suspensor begins t o degenerate. The nucellus in the micropylar region still consist of a single cell layer of cells containing abundant cytoplasm. However, in the chalazal end the outer cell layers of the nucellus undergo repeat ed cell divisions to form Several cell layers, which appear to provide a nutrient source for the developing embryo.