Sleep movements of leaves: In defense of Darwin's interpretation.

On the basis of extensive but indirect evidence, Darwin proposed that an important function of daily leaf movements is to minimize radiative heat loss to the night sky and thereby protect the leaves from nocturnal chilling, an interpretation which has currently fallen into disrepute. Measurements made on flowering bean seedlings demonstrate that consistent orientation-dependent differences in nighttime leaf temperature do indeed arise even on warm nights, as Darwin had concluded, but the observed differences tended to be quite small (usually less than 1°C). Nevertheless, experiments in which growth of bean seedlings was measured, while nighttime orientation of the primary leaves was mechanically constrained, demonstrate that leaf growth can be significantly retarded when nocturnal orientation of the leaves is horizontal. This reduction in growth rate, apparently mediated by very modest temperature effects, is consistent with Darwin's speculation about the "injurious" consequences of nocturnal leaf chilling under nonfreezing conditions.