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The ultrastructural mechanisms of aposporous embryo sac initial cell (AIC) appearance and its developmental process have been studied in gametophytic apomicts of guinea grass (Panicum maximum Jacq.), using ovary clearing treatment and Nomarski differential interference-contrast microscopy (DIC), and ultra-thin section and the transmission electron microscopy (TEM). For the observation of AIC appearance by DIC, AIC appears while megaspore degenerated in apomictic accessions, and most of the ovules contain several AICs, and the number of AICs increased as the ovary grew before anthesis. That is, several AICs in the same ovule did not differentiate synchronously, but instead, they seemed following a continuous course and appeared one by one during the period from after megasporogenesis to the first AIC-derived embryo sac maturity. It was also found that the higher the frequency of apospory was, the greater the number of AICs was, and the longer the duration of AICs appearance should be. For the mechanism of seed-forming embryo development in polyembryonic ovules in apomictic accessions observed by DIC, the first AIC is located dominantly in micropylar end, and the percentage of mature embryo sacs in micropylar end was higher than that in the other ends. The rates of the ovules contained developed embryo sacs in micropylar end at 4 days after anthesis were 33~91% comparable to 0~2% of that in the other ends. The embryo of the developed sac in micropylar end, in final, became a seed-forming embryo, and in contrast, the other sacs, were crowed out to chalazal end and degenerated at 10 or more days after anthesis. Form the results described here, in can be concluded that the AIC-derived embryo sac in micropylar end, in most of

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Ultrastructural Mechanisms of Aposporous Embryo Sac Initial Cell Appearance and Its Developmental Process in Gametophytic Apomicts of Guinea Grass (Panicum maximum)