| Abstract: | In the cultured young ovaries of rice, the processes of megagametophyte develop- ment could be switched to the formation of various abnormally organized embryo sacs and then to the initiation of synergid apogamy. The main pathway leading to apogamy was found to go via a linearly oriented 4-nucleate embryo sac to the formation of a li- nearly oriented egg apparatus, from which it was usually the chalazal synergid giving rise to an apogamous proembryo, and the micropylar synergid degenerated. The proembryo thus produced was located at the base of a vacuolated egg cell (Plate Ⅰ, 1–7). The second pathway went through a nonlinearly oriented 4-nucleate embryo sac to the formation of an egg apparatus in which the two synergids were located at one side of the egg and oriented longitudinally. In this case it was often the chalazal synergid that could be triggered to apogamy, resulting in a hook-shaped proembryo embracing the egg cel1 from one side (Plate Ⅰ, 8–11). When ovaries with nearly matured embryo sac were cultured, in a few cases where apogamy was induced, the proembryos observed were all situated at one side of the egg and were hook-shaped (Plate Ⅰ, 12). All these pathways are summarized in a diagram (Fig. 23). Some interesting changes were observed in the synergid and the egg cell of the cultured ovaries by PAS reaction and mercuric bromphenol blue staining. The egg cells, in contrast to in vivo condition, often contained abundant starch grains,. The synergids and synergid proembryos were rich of cytoplasmic protein (Plate Ⅱ, 13, 14). We supposed that the egg may supply some nutrients as well as stimulants to the developing synergid in the course of apogamy. The distribution of starch and protein in apogamous embryoids during subsequent development was also described in this paper (Plate, 15–22). |
