The cell cycle of a protonema of Adiantum capillus-veneris L. cultured under continous red light is controlled by two photoreceptors: phytochrome and a blue and near-ultraviolet light-absorbing pigment (Pb−nuv). Irradiation with blue light, which is absorbed by Pb−nuv located in the nuclear region, shortens the duration of the G1 phase, but does not affect other phases. On the other hand, phytochrome has two roles in cell cycle regulation. Irradiation at the beginning of the G1 phase with far-red light, which is absorbed by phytochrome located throughout the entire protonema, lengthens the duration of the G2 phase. Phytochrome also arrests the cell cycle at the G0 phase under continuous red light, and, after red light irradiation, returns to the G0 phase any cell cycle which has already been started by dark incubation, unless the cell cycle has progressed to a ‘point of no return’ which coincides with the beginning of the S phase. Photoregulation of the cell cycle is summarised in Figure 4.