AN ULTRASTRUCTURAL STUDY ON THE DEVELOPMENT OF CENTRAL CELLS IN RICE EMBRYO SAC
水稻中央细胞发育期间超微结构变化的观察

An ultrastructural study on the development of the central cell was carried out on rice (Oryza sativa L.). The results showed that following the formation of the eight nuclei in the embryo sac, cellularization took place resulting in the formation of seven cells. One of the cells situated in the centre of the embryo sac was the central cell. The central cell contained a large vacuole, two polar nuclei (located in micropylar part and chalzal end, respectively) and a large number of cytoplasmic organella. Further development of the central cell mainly involved changes in the orientation of the polar nuclei and the distribution of the cytoplasm. Changes in the orientation of the polar nuclei involved the following stages of developments a. The two polar nuclei increased in size and became elliptical in shape. On both sides of the polar nuclei the cytoplasm showed an asymmetrical pattern of distribution. b. The two polar nuclei moved to the central region of embryo sac. At this stage the two polar nuclei were close to each other and they were lying ("one on top of the other") in parallel to the long axis of the embryo sac. When the polar nuclei moved to the central region of the embryo sac, a thick strand of cytoplasm (forming a cytoplasmic "bridge" ) became associated with the polar nuclei. The cytoplasm "bridge" ran from one end of the embryo sac to the other. c. The two polar nuclei began to move towards the egg cell and finally "sat" themselves on the top of the egg cell. The two polar nuclei had by now changed from a parallel (one on top of the other) to perpendicular (side by side) orientation (with respect to the long axis of the embryo sac). The side of the nuclear envelope of the two polar nuclei facing each other produced a lot of protrusions and fusion bridges throughout the different stages of development and reorientation of the polar nuclei. The cytoplasm surrounding the polar nuclei also went through some changes e. g. starch grains disappeared and smooth ER increased. UP till stage c the size of the embryo sac did not change much. But immediately after the completion of stage c the embryo sac greatly increased in size. The embryo sac then reached full maturity and apparently did not undergo any further changes.