The effect of irradiation with white or red light on the cell number of leaves of dark-grown plants has been investigated. Irradiation for 1 min or longer results in an increase in cell number over that of the dark-grown controls. This effect is greatest for plants 6 days old, which show the highest rate of cell division under dark conditions, and least or absent for plants 10 days old or older, in which divisions in darkness have stopped. The effect is shown to be mediated by the phytochrome system. Experiments in which far-red light is given at intervals after application of red radiation show phytochrome action to require more than 12 h to go to completion. Once action is completed, it is not possible to induce a second response. The response to light is dependent upon the presence of the cotyledons, and if these are removed cell number increase in control and irradiated plants is prevented. If removal of the cotyledons is delayed until 24 h after the light treatment the response is still reduced. The light stimulus can be perceived by leaves, or cotyledons, or both, but maximal response is found when leaves alone are irradiated. Detached leaves, and leaf disks do not show cell number increase when irradiated. It is concluded that the cotyledons supply materials to the leaves, and that this supply, without which divisions do not occur, is modified by short irradiation. Irradiation for 6 h or longer leads to much greater cell number increase, size of the response depending upon the duration of irradiation, at a fixed light intensity. The response is thought to involve photosynthesis. The growth of leaf cells during the period of active cell division is discussed, and it is concluded that division is dependent on growth of the cells, which in turn is controlled by metabolite supply, from the cotyledons, or through photosynthesis.