Author: Chen Zu-keng and Wang Fu-hsiung
J Integr Plant Biol 1980, 22 (3): -.
Material of Fokienia hodginsii was collected in 1964 from Fengyangshan (alt. 1000–1400 M) in Lungchuan county, Chekiang province. This paper deals with the fertilization in Fokienia. It includs the structure of male and female gametes as wed1 as the process of fusion of their nuclei and cytoplasm respectively. The division of the spermatogenous cell of Fokienia occurred by the end of June (1964) and two sperms similar in shape and size were formed when pollen tube reached the top of archegonia. Two equalsperms look like two hemispherical bodies conjoined togather. The sperm possesses cell wall and is about 65 μ in diameter. Its nucleus is rather large and about 45–50 μ in diameter. There is a nucleolus in the nucleus. Outside the nucleus the dense cytoplasm forms the deep colored zone, some 10 μ in thickness. This zone is separated from the nucleus by a narrow perinuclear zone, and from the plasmalemma by a marginal zone. The perinuclear zone is about 2 μ thick, and the mariginal zone is from 3 to 4 μ thick. Both zones have transparent cytoplasm. When the archegonium is formed, the central cell has a small nucleus which is located below the neck ceils. At the middle of June (1964), the central cell divides to form the ventral nucleus and the egg nucleus. The egg nucleus sites primarily at the upper part of archegoninm and has only one nucleolus. Then the egg nucleus increases gradually in sim and moves to the central part of the archegoninm. In mature archegonium there are usually 4–5, rarely 6–7 nucleoli in the egg nucleus, each of them is about 15 μ in diameter. The egg cell in Fokienia hodginsii is about 500 in length. The female nucleus is larger than the male one. After egg cell matures, its cytoplasm increases gradually, while the central vacuole decreases gradually and almost disappears completely after fertilization. It is interesting to note that there are 1–2 dense cytoplasm masses at the upper or lower part of egg nucleus. The shape of the mass is similar to that of the egg nucleus but no membrane is formed. These cytoplasm masses are about 50–70 μ in diameter in some cases. The fertilization of Fokienia took place at the end of June when the growing tip of pollen tube had reached the top of the archegoninm. Then the neck cells become disorganized and degenerated. It is possible that all the cytoplasmic contents of pollen tubes are released into the archegoninm. Before fertilization, the cytoplasm around the sperms and sterile cell and tube nucleus are in front of these two sperms. Then the sperms separate from each other and come down into the cytoplasm of the egg. When the mede nucleus contacts with the egg nucleus, both become flattened along their contact surface. Then the nuclear membranes of both sperm and egg nuclei become ultimately disintegrated. Thus the fusion process is complete. However, it is nia, though the opposite is the case in an exceptional example. When the sperm nucleus passes into the cytoplasm of egg cell, its cytopasm is released inside the archegonium along with it. During the course of fusion of the male and female nuclei, tile fertilized nucleus is surrounded by both female and male cytoplasm. Thus the male cytoplasm along with the peripheral cytoplasm of the egg cell invests the two nuclei lying in contact and forms a dense neocytoplasm. When the zygote divides, the neoeytoplasm is full of the starch grains and a dense cytoplasm sheath is formed. After fertilization, the fused nucleus moves toward the base of the egg cell. It seems that the movement of the fused nucleus is not a simple mechanical movement but turned over repeatedly toward the base of the arehegonium. Sometimes the position of the sperm and egg nuclei makes a turn of 180. At the same time the track of the fertilized egg nucleus with vacuoles in the archegonium may be traced. After zygote moves into basal part of the archegonium, first intranuclear mitosis occurs. The nuclear envelop of zygote disappears gradually at the telophase of the first mitosis. Then division of the free nuclei of proembryo follows. From fertilization to the stage of proembryo formation, the second sperm may sometimes enter into the cytoplasm of the egg cell. Mitosis of the second sperm nucleus may take place in the upper part of the archegonium. In addition, there are often several supernmnerary nuclei (as many as 7–8 in number) in the same egg cell. These nuclei are also surrounded by dense cytoplasm. They may persist for some time and be recognizable at somewhat later stages of the proembryo or even after the elongated suspensors are formed. In some cases, there are some cell groups above the upper tier of proembryo. These cell groups are also surrounded by dense cytoplasm. Either the supernumerary nuclei or cells are surrounded by the dense cytoplasm. Probably they are derived from the mitosis or amitosis of the second sperm. Investigations on submicroscopic structures of sperm and egg in relation to the fertilization of Cupressaceae have been carried out extensively during the last decade. The fate of male cytoplasm has been debated for a long time and this problem attracted attention again in the nineteen seventies. At last the concept of neocytoplasm has been established soundly based upon the information from observation of electron microphotographs. The neocytoplasm is also visible under the light microscope though the components are not recognizable. The sperms of Fokienia are similar to those of Cupressus funebris, Juniperus communis, Sabina virginiana, Tetraclinis articulata, Chamaecyparis pisifera as well as the genus Thujopsis and others. Two sperms are all effective in fertilization and this is the common phenomenon of the family Gupressaceae.