Abstract: Gametophyte development and oogenesis of the fern Lygodium japonicum were studied using microscopy. The spores are tetrahedral and trilete. The spore germination pattern is the Anemia-type. The gametophyte development showed various forms. When the filaments length reached 3-5 cells, they can produce plate by longitudinal division of the apical cell, and finally formed the heart-shaped prothallus, which can produce antheridia(an) and archegonium(ar). However, the gametophyte can also produce 10 cell-lengthed filaments. These filaments usually produce irregular prothallus, which can produce antheridia and do not produce archegonia. The development of the prothallus is the Adiantum-type. Sex organs are the Leptosporangiate-type. Sections observations showed that the archegonium develops from a superficial initial cell under the growth point. Two divisions of the initial cell result in a tier of three cells, i.e. the upper cell, primary cell and basal cell. The primary cell finally develops into a neck canal cell, a ventral canal cell and an egg by two unequal divisions. These three cells were closely associated with each other. Then the neck canal cell and the ventral canal degenerated gradually. A separation cavity was formed around the matured egg. Light microscope observation showed that no typical egg envelope was formed in the surface of the egg and no fertilization pore was observed in the matured egg.

Key words: Fern, Lygodium japonicum, gametophyte development, oogenesis