Microsporogenesis of Cycas elongata and its systematic implication
越南篦齿苏铁小孢子发生及其系统学意义

Microsporogenesis and pollen ontogeny in Cycas elongata (Leandri) D. Y. Wang were studied to amass additional information on the reproductive biology of cycads and to provide some data for further studies on the factors causing the endangerment of this species. This species tends to initiate the male cones in middle or late May. The microsporophylls bear their microsporangia on the abaxial surface, and 3-5 microsporangia seem to be arranged radially into a sorus with a stalk. The microsporangium wall comprises one epidermis, four to five middle layers, and a tapetum. The tapetum may be derived from the outer layer of mature sporogenous tissue and conforms to secretory type. In the middle of June the microspore mother cells are produced and immediately enter into meiosis. The cytokinesis in the later stage of meiosis is both simultaneous and successive, a phenomenon observed even in the same microsporophyll. Following meiosis, the nuclei of microspore mother cells divide into four to form tetrahedral, isobilateral, tetragonal and occasionally linear tetrads in late June. Subsequently the microspores are released from tetrads. With the development of pollen, the microspores then undergo two asymmetric mitosis to form mature pollen consisting of three cells, i.e., a tube cell, a generative cell and a prothallial cell. Pollen further develops and is dispersed through the dehiscence of the sporangium in late July. In addition, both starch grains and callose have a regular dynamic distribution during the course of microsporogenesis and pollen ontogeny. The phenomenon that the cytokinesis at the later stage of meiosis in microsporogenesis is both simultaneous and successive has not been previously reported in any other cycads, even in other gymnosperms, and thus may have systematic significance and developmental implications.