作 者 ：Chen Zu-keng and Wang Fu-xiong Wang Fu-hsiung
Keywords:Amentotaxus, Pollination, Fertilization,
Abstract:The present investigation was conducted during 1980–1982, and mater- ials collected from Jin-Fo shan (Golden Buddha Mountain), at a height of 1400-1600 m, Sichuan province, China. Pollination of Amentotaxus argotaenia began to proceed last week of May, and came into bloom the first week of June. The male strobiles were almost entirely wilting at June 12–15. Thus, florescence of Amentotaxus spread over a period of 3 weeks. While the pollen grains approaching to maturity, most of the microspores divide to form a larger tube cell and a smaller antheridial initial. In this case the mature pollen grains of Amentotaxus consist of two cells. Then pollen grains are attracted down into the pollen chamber in the apex of the nucellus after pollination. The pollen chamber of Amentotaxus in longitudinal section looks like a flask in shape and is very much similar to that of Ginkgo biloba. As pollen grains at pollen chamber begin to germinate, the antheridial initials divide again to give rise to a spermatogenous cell and a sterile cell. At first, the spermatogenous cell is of a size only 11–13 μ in diameter. When the pollen tube reaches the middle part of the nucellus, the spermatogeneous cell is of a size about 30 μ. In the middle of July, pollen tube approaches the top of the female gametophyte. In this time, the spermatogenous cell has already been mature enough and is of 58–85 μ in diameter. The nuclei of spermatogenous ceils, 30–36 μ in size, are usually lying in the lateral side of the cytoplasm at its micropylar end. From the middle to the end of July, spermatogenous cells divide to form two unequal sperms, one of which is larger than the other and is the functional one. The large sperm is almost round in shape and about 56 μ in diameter. The small sperm is elliptic in shape, non-functional, and about 33 μ in diameter. The nuclei of the large and small sperms are about 40 μ and 26 μ, respectively. In some cases there are lateral pollen tube and sperms in the ovules of Amentotaxus, or the pollen tube even grows toward the lower part of female gametophyte in the chalazal end and there are well developed sperms in such a case. In the middle of July, nucleus of the central cell divides to form a ventral canal nucleus and an egg nucleus. The former then breaks down quickly and the latter continues to develope and moves toward the central part of the egg cell gradually. It is interesting to note that there are a number of nucleolus-like grains in the cytoplasm of the egg cell in Amentotaxus. The large nueleolus-like grains contain a larger central vacuole with several smaller vacuoles surrounding it. These grains show a positive reaction and blue colour by PAS and aniline blue black or coomassie brilliant blue, respectively. The above facts show that the nucleolus-like grains contain not only po- lysaccharides, but also protein. Similar grains may also found in the developing pollen tube. This is a unique feature in Amentotaxus and even in Gymnosperms. Otherwise, there are often two groups of the dense cytoplasm under the egg nucleus in Amentotaxus. Fertilization of Amentotaxus took place around July 20–29 (1980–1982). Interval between pollination and fertilization was about two months. After male nucleus fuses entirely with the female nucleus, the zygote begins to divide by mitosis. During fertilization, in addition that the large sperm enters the egg cell and fuses with the egg nucleus, the small sperm, tube nucleus, and sterile cell are often delivered into the egg cell. But they are disintegrated gradual]y and eventually. It is worthy to note that the nucleolus-like grains and the starches in pollen tube are also released into the egg cell. Then enlargement, fusion, and budding in the nucleolus-like grains may be found within the cytoplasm of the egg cell after fertilization. The history of investigating Amentotaxus found in 1883 has been lasting a long period of 100 years. But researches in sex production has never been studied before. The present work has shown that fertilization in Araentotaxus is very much similar to that in Taxus, Pseudotaxus, and Torreya. In other words, they all belong to the same type, that is, mitosis of zygote taking place after fusion of the two sexual nuclei. This condition constitutes one of the features of Taxaceae. But fertilization in Cephalotaxaceae is different from that of Taxaceae in having mitosis taking place before fusion of the two sexual nuclei. Pollination of Amentotaxus is similar to that of Cephalotaxus with dual-cell pollen grains at shedding stage. On the other hand, interval between pollination and fertilization in Austrotaxus lasts for 13.5 months, and this is the longest one in Taxaceae, and it is similar to that of Cephalotaxus proceeding for 14 months. To sum up, from the point of view of pollination, fertilization, and embryogenesis, Amentotaxus could be considered a primitive type in Taxaceae. Perhaps an order of systematic position of the genera belonging to Taxaceae can be arranged thus: Amentotaxus, Austrotaxus, Taxus, Pseudotaxus, and Torreya. And Cephalotaxaceae may be related to Taxaceae by way of Amentotaxus.
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