=172. Light as a stimulus.= In nature, light stimuli are determined by intensity and not by quality. A single exception is afforded by those aquatic habitats where the depth of water is great, and in consequence of which certain rays disappear by absorption more quickly than others. In forests and thickets, where the leaves transmit only the green and yellow rays, it would appear that the light which reaches the herbaceous layers is deficient in red and violet rays. The amount of light transmitted by an ordinary sun leaf is so small, however, that it has no appreciable effect upon the quality of the light beneath the facies, which is diffuse white light that has passed between the leaves. Indeed, it is only in the densest forests that distinct sunflecks do not appear. Coniferous forests, with a light value less than .005, which suffices only for mosses, lichens, and a few flowering plants, show frequent sunflecks. This is convincing evidence that the light of such habitats is normal in quality. It warrants the conclusion that in all habitats with an intensity capable of supporting vascular plants the light, no matter how diffuse, is white light. The direction of the light ray is of slight importance in the field, apart from the difference in intensity which may result from it. In habitats with diffuse light, the latter comes normally and constantly from above. Likewise, in sunny situations, direction can have little influence, since both the direction and the angle of the incident rays change continually throughout the day, and the position of the leaf itself is more or less constantly changed by the wind. The influence of duration upon the character of light stimuli is difficult to determine. There can be no question that the time during which a stimulus acts has a profound bearing upon the response that is made to it. In nature the problem is complicated by the fact that light stimuli are both continuous and periodic. The duration of sunlight is determined by the periodic return of night as well as by the irregular occurrence of clouds. Since one is a regular, and the other at least a normal happening, it is necessary to consider duration only with respect to the time of actual sunlight on sunny days, except in the case of formations belonging to regions widely different in the amount of normal sunshine, i. e., the number of cloudy days. In consequence, duration is really a question of the intensities which succeed each other during the day. The differences between these have already been shown to fall within the efficient difference for light, and for this reason the ratio between the light intensity of a meadow and of a forest is essentially the ratio between the sums of light intensity for the two habitats, i. e., the duration. The latter is of importance only where there is a daily alternation between sunshine and shadow, as at the edge of forest and thicket, in open woodland, etc. In such places duration determines