According to our former observation, low doses of X-radiations had some stimulating effect on the growth of gametophytes of L. japonica. In order to verify this conclusion four different populations of gametophytes from different lines of descent were used in this investigation. The young sporophytes developed from the low-dose-X-irradiated gametophytes all grew significantly bigger than the control (Tables 1-2). The stimulating effect was seen to maintain up to the adult stage of sporophyte (Tables 3-4). It was previously found that the faster rate of growth was due to the faster rate of cell division occurred in sporophytes developed from low-dose-X-irradiated gametophytes.