The ovule of Gasteria verrucosa at receptivity of the stigma: an ultrastructural study

An ultrastructural study on the embryo sac and the micropylar part of the nucellus, at the time of stigma receptivity, was carried out to reveal possible changes due to pollination. At the time of stigma receptivity, the development of the egg cell, central cell, antipodals and the micropylar nucellar cells of Gasteria have been completed. Only the synergids and the filiform apparatus are still differentiating. Frequently, anomalies in the morphology of the unfertilized embryo sac are observed. Comparative observations before and 2-5 h after cross-pollination did not show ultrastructural changes in the embryo sac and in the micropylar nucellar cells. The egg cell showed polarity. The nucleus and most of the cytoplasm lay in the micropylar half, while vacuoles filled most of the chalazal half. The two synergids did not differ from each other and a clear degeneration had not taken place. Based on the observed ultrastructural differences of the filiform apparatus and the synergid cytoplasm among the investigated ovules, and data of previous studies, a sequence of differentiation of the very large filiform apparatus was defined. The nucleus and most of the cytoplasm of the central cell were positioned near the degenerating antipodals. The ultrastructure of the micropylar nucellar cells indicates a secretory function of these cells and an involvement in the process of pollen tube acceptance.