Abstract: Rice embryo sac consists of an egg cell, two synergids, a central cell with two polar nuclei, and a multicellular antipodal mass. The egg cell bears no wall at its chalazal end. Its nucleus is centrally located. The greater part of its cytoplasm surrounds the nucleus, while vacuoles of various sizes are distributed at the peripheral region. The mature egg cell appears to be an inactive cell in the light of its organelle state. The two synergids show no difference with each other one day before anthesis, but by the time just prior to anthesis, one of them has degenerated. The synergids bear no wall at the chalazal end and have a thick wall at the micropylar end, where a filiform apparatus is formed. The nucleus occupies a mid-lateral position with the majority of organelles distributed at the micropylar part and many vacuoles at the chalazal part. The central cell is occupied by a large vacuole which presses the cytoplasm to a thin peripheral layer. The polar nuclei, partially fused with each other, are located in proximity to the egg apparatus and surrounded by organelles. Long, parallel endoplasmic reticula are seen near the plasma membrane bordering the egg apparatus A thick, multilayered embryo sac wall which bears numerous finger-like ingrowths seperate the central cell from the nucellus. The antipodal cells keep dividing before and even after fertilization, proliferating into a nmlticellular mass. Amitosis accompanied with freely growing walls is a usual phenomenon in this proliferation process. The cells contain abundant, active organelles and have many ingrowths at the wall bordering the nucellus. Six hours after fertilization, double fertilization is finished, resulting in a zygote and several endosperm nuclei. The zygote has now formed a new wall at its chalazal end. Its cytoplasm shows no significant changes except that more polysomes are seen and the vacuoles become smaller in size. In contrast, the cytoplasm of endosperm becomes more abundant and active than before. The degenerated synergid elongates and embraces the chalazal part of the zygote. Besides formerly existing electron-dense materials many starch grains and spherical bodies are seen in the degenerated synergid as a result of pollen tube discharge. After fertilization, both synergids are characterized by newly formed walls at their chalazal part. To our knowledge, the present paper is the first report on ultrastructure of rice embryo sac. Based on the results obtained by this paper and the reports on other graminaceous species, a discussion is made around the following problems as polarity of the egg cell, degeneration of the synergid, changes of the boundary wall between egg apparatus and central cell, and division of antipodal cells.

Key words: Oryza sativa, Embryo sac, Ultrastructure