Embryo Sac Formation in Wheat

Abstract

Megagametogenesis of common wheat was examined embryologically paying attentions not only to the egg apparatus and polar nuclei but also to the antipodals whose activities were thought to contribute to the early development of the endosperm. The results of daily observations were summarised as follows: During 10∼8 days before flowering (for short "b. f." in the rest), embryo sac gave rise to 8 nucleate stage through 3 times of nuclear division comprising a micropylar and central quartet (Figs. 1-5). Then of 8 nuclei, 3 at the micropylar end gave rise to the egg apparatus; 3 at the chalazal side of the central quartet gave rise to antipodal cells; and remaining 2, one from each quartet migrated into the micropylar vacuole without cell membrane formation, and gathered to form the polar nuclei. 2 synergid cells started protrution in their charazal sides next day (7 days b. f.), and maturated by 2 days b. f., thus they became slightly senile on flowering day. The egg cell entered into growing at 6 days b. f. and continued its growth till flowering day. 2 polar nuclei getting closer mutually were located near the egg apparatus by 3∼2 days b.f. In mature embryo sac they were in close contact with each other, however, never fused prior to fertilization. Very interesting observations were obtained in the development of antipodal cells. Immediately after the formation of 3 antipodal cells, they entered into cell division to fill up the chalazal vacuole (Figs. 7∼10). These additional cell divisions proceeded 4 times and resulted in about 40 cells which occupied the chalazal half space of the embryo sac by 4∼3 days b. f. (Fig. 14). After the 2nd division, one of the antipodal cells which was located at the chalazal pole showed a singularity and divided to form a haustrium composed of some special shaped cells at 6∼5 days b. f. (Figs. 10∼14). The antipodal haustrium was short-lived and degenerated after the 4th or the last division of other antipodal cells and the contiguous antipodal cells also begun to degenerate continuously. However, antipodal cells in micropylar side increased in their size accompanying with rapid enlargement of the embryo sac that occurred from 2∼1 days b. f. (Fig. 16). on the flowering day, antipodal complex was situated in somewhat lateral side of the embryo sac, and consisted of 12∼17 large, mature cells (Fig. 17). The number of the antipodal cells had little relation with the varietal and environmental differences but varied with the age of the embryo sac remarkably (Table 2). Based upon these observations, the authors thin that the antipodal cells contribute to the growing of the embryo sac during before flowering. By 2 days b. f. the embryo sac came to obtain the ability to fertilize, however, its subsequent development was somewhat incomplete. On the day before flowering the embryo sac was able to give rise to normal seed if artificially pollinated, although its anthers were not yet mature.