SUMMARYThe intensity of the light at the ground level in the rain forests of British Guiana was measured by means of a Bernheim photo‐electric cell. It was found that the shade light in different types of undisturbed forest lies between 0.8 and 0.2 per cent. of sunlight; and that the illumination of the ground, which is lighted by sun flecks as well as by the shade light, lies between 2 and 0.4 per cent. of sunlight.A proportionate reduction of 20 per cent. in the light a t the red end of the spectrum was found in the illumination of the forest floo