Abstract:Mecodium paniculiflorum (Presl) Copel. and M. osmundoides (v. d. B.) Ching from Sichuan Province have been examined cytologically. They have the chromosome numbers n = 26 and n=28 respectively, which are recorded for the first time. Their spores produced after normal meiosis are seemly available. Both the species are sexual diploid. The chromosome number n=26 of M. paniculiflorum shows that it is distinct from M. polyanthos (Sw.) Copel. with the chromosome number n=28. However, M. osmundoides has the same chromosorrie number with M. polyanthos. Therefore, it may be an Asian member of M. polyanthos group. The occurrence of the basic number 26 in the genus Mecodium has given a strong evidence of the close relationship between Mecodium and Hymenophyllum, Meringium. It has been known that in Hymenophyllaceae, they are the only three genera with more than four base numbers. They have shared the base numbers 21 and 28. Now, they have the third common base number 26 (or 13) which is a spicific basic number in the family. In addition, all they have bivalved involucres and other similar characters, so it is reasonable that they are treated as subgenera or section under the same genus Hymenophyllum by C. V. Morton[13] and K. lwatsuki[7,8]. During the sporogenesis of M. paniculiflorum and M. osmundoides, the initial archesporiaI cell in a developing sporangium usually divides successively five times by mitosis resulting 32-spore mother cells, and then meiosis occurs regularly, giving rise to 128 spores finally. In a few cases, 64-SMC or 256-spore sporangia are also produced in M, osmundoides. It is suggested that besides forming 64-spore sporangium, forming 128-spore and 256-spore sporangia should be conscidered as normal sporogenesis lines in sexually-reproducing ferns, especially in Hyme- nophyllaceae and other ancient group of leptosporangiate ferns. The voucher specimens are deposited in the Herbarium of our Institute (PE).