[The importance of circadian leaf movements for the precision of day-length measurement].

1. In Glycine max, the circadian leaf movements are synchronized with the 24 hr period in 12:12 hrs light-dark cycles even with light intensities of only 0.6-0.8 lx. The normal phase-angle difference between light-dark cycle and the circadian rhythm is already nearly reached with intensities of 10 lx. 2. In Phaseolus multiflorus, light with the intensities of moonlight, offered during the dark-period of 12:12 hrs light-dark cycles, is able to shift the phases of the circadian leaf-movement rhythm. 3. In Phaseolus, the transition of the leaves to the night-position reduces the intensity of moonlight reaching the leaf's upper surface to values below the threshold values for phase shifts. 4. The epidermis of the upper leaf surface in Parochetus communis has "ocelli". The complicated transition of the leaves to their night-position results in a nearly complete darkening of these ocelli. 5. It is concluded that the circadian leaf movements prevent the interference of moonlight with the photoperiodic time measurement. Moreover, the leaf movements result in a very sharp increase or decrease of the light-intensity reaching the leaves' surface in the photoperiodically decisive range (between about 1 and 10 lx) during morning and evening twilight.