Most fern spore germination is light dependent. In a tetrahedral, non-chlorophyllous, dormant spore, the nucleus sits in one corner surrounded by three furrows. When spores are irradiated with red light after imbibition in the dark, they become round, and the nucleus, still in its corner position, divides, followed by cell division to produce large protonemal and small rhizoidal mother cells (Furuya et al., 1997). In A. capillus-veneris, Pteris vittata, and probably other species, red-light induced germination is inhibited by far-red light in a red/farred reversible manner, indicating the involvement of phytochrome (Sugai and Furuya, 1967;Furuya et al., 1997). The red light effect is inhibited by blue light, on exposure before or after the red light treatment (Sugai and Furuya, 1967; Furuya et al., 1997). The blue light inhibition effect, however, cannot be reversed nstantaneously by subsequent exposure to a pulse of red light, suggesting the involvement of a blue light receptor, but not a phytochrome system. Inhibition
can be prevented when the spores are kept in the dark for about a week (Sugai and Furuya, 1968;Furuya et al., 1997). The time period required for prevention of blue light inhibition is very much reduced if the spores are irradiated with red light. The red light effect can be reversed by far-red light, indicating phytochrome dependence (Sugai and Furuya, 1968;Furuya et al., 1997). The inhibitory effects of far-red and blue light could not be observed after the first mitosis in spores, suggesting that cell division is a crucial step for spore germination
(Furuya et al., 1997). Partial spore irradiation with red or blue microbeam lights showed that the blue light receptor is located in the nucleus, but the location of the red light hotoreceptor could not be identified (Furuya et al., 1997). The photoreceptors mediating spore germination (both phytochrome and blue light receptors) have not yet been identified, although several candidate genes have beenclonedandsequenced.