Life history of a floating liverwort, Ricciocarpos natans (Ricciaceae, Hepatics), growing at irrigated rice fileds, was observed twice a week from May 8th, 2014 to July 28th, 2014 in Shiga Prefecture, Central Japan. Positions, numbers, and developing stages of antheridia, archegonia, and sporophytes of R. natans were recorded for every five thalli in a total of 23 observations. Antheridia were first found in 10 days after irrigation in the rice field, and production of antheridia continued by the end of July. On the other hand, archegonia were first found in late May. Fertilization was confirmed in the beginning of June and so on. Embryos rapidly grew and matured spores in black color were observed during the middle of June and the middle of July. It was confirmed that the dorsal grooves of thalli begin spreading with the development of reproductive organs, as well as sporophytes. Normal growth of sporophytes was significantly disturbed by shortage of water supply caused by artificial midsummer drainage in late June. We confirmed that sporophytes of R. natans could be produced even in autumn, but it was only under the accidental, exceptionally favorable conditions for production of sexual organs and fertilization.
