Studies on the development of endosperm in rice : 1. Process of endosperm tissue formation

Fertilization of polar nuclei with a male nucleus completed within 5 hours after anthesis at temperature of 28°C to 31°C. The endosperm primary nucleus thus formed began the first division without rest stage, while the zygote took about 10 hours' rest. By the 3rd day, a multi-nucleate layer of endosperm was formed in the, periphery of the embryosac cavity (Figs. 1, 9 and 10). On the 4th day, cell membrane formation occurred at first from the embryonic end and extended to the entire of the peripheral tissue. Coincidently, all the cells divided inwards, thus forming two-celled layers' endosperm (Fig. 11). From the 5th day onward, the development of the endosperm tissue was observed by the increment of cell layers and their cell numbers in the cross section in the middle part and in the longitudinal section of a caryopsis. A possible explanation of this development process was tried to be given by following the GORDON's theory that "the development of cereal endosperm can be attained exclusively by the division of the cells in the outermost cell layer". Ranging over the 4th to the 5th day, cells of the outermost layer (4-1) might have repeated periclinal divisions, thus forming 3 cell layers (5-1, -2, and -3), and at the same time, half the number of the layer (4-1) divided in a circumferential direction, . and the remaining half in a longitudinal direction. The cell layers which have been formed by The 4th day (4-2 and -3) were squeezed into the central portion of the embryosac cavity and folded. As a result, the folded cell layers came to be arranged in numerous concentric circular cell layers (5-4, -5…-11) surrounding the respective central point (Fig. 16). About 32, 000 cells were counted throughout the entire tissue of the 5th day's endosperm. Ranging, over the 5th to 6th day, the outermost layer (5-1) might have divided inward 3 times and added 3 layers (6-1, -2, and -3) to the outside of the 5th day's tissue, and on the other hand, the layer (5-1) divided once in a circumferential direction, but not in a longitudinal direction. Ranging oyer the 6th to 7th day, it showed the increment of 2 cell layers (7-1 and -2) in a radius direction, and half the number of the layer (6-1) might have divided in a circumferential direction. Total number of the cells reached 95, 400 (Fig. 14). Extending over the 7th to 8th day, 2 cell layers increased in a radius direction, and the half of the layer (7-1) divided in a longitudinal direction. Extending over the 8th to 9th day, the last division of the layer (8-1) occurred in a radius direction. Thus 19 to 20 cell layers were formed in the dorsal radius, 15 to 16 cell layers in the ventral radius and 13 to 14 cell layers in the lateral radius (or central point to side surface of endosperm). (Fig. 15). The endosperm cells formed amounted approximately to 180, 000 in the aggregate. From the 10th day on, no increase was found in the number of cells. Increment in the number of endosperm cells has continued after the stage where the embryosac cavity had been filled up with endosperm tissue. Nay, it was clarified that the increase in the number of cells was rather markedly in the later; stages (Table 2). It was observed that the development of endosperm tissue formation was actuated, from the incipient stage of nucleus, in the dorsal side somewhat earlier than in the ventral side (Figs. 9, 10, and 11). This fact may be related to the observation that the antipodal tissue developed for the 3 days after fertilization and then degenerated (Figs. 4, 5, 6, 7 and 10).