THE EFFECTS OF DAYLENGTH ON THE MOVEMENTS OF PULVINATE LEAVES

S ummary Various illumination cycles, of the sort that are known to have effects in photoperiodism, were tested for effect on the course of the diurnal leaf movements of Bauhinia monandra . Under 12 hour: 12 hour alternations of light and darkness, the leaves open abruptly on illumination and begin to close again rapidly just before the end of the light period. In continued illumination, the evening closure is completed but the leaves reopen at once and further movements are indistinct and irregular. It was concluded that the effects of previous photoperiodic treatment are all but exhausted 12 hours after beginning illumination. Attention was therefore concentrated upon the effects of previous treatments on the timing and amplitude of the evening closure. Moderate lengthening or shortening of the preceding dark period had small, if any, effect on the amplitude, but advanced or retarded (respectively) the time of closure. After very short dark periods (less than 6 hours) closure practically fails altogether. Beginning the illumination period with weak light also delays closure. Light breaks in the dark period advance closure, dark breaks in the daytime retard it, but neither of these effects is evident unless the break is of some hours’ duration. The observations are discussed in relation to Biinning's Tagesrhythmik theory and it is pointed out that the periodicity of Bauhinia leaf movements has been shown not to be constant, whereas, on the other hand, their amplitude is generally more stable. The leaf movements of several other plants have been compared with those of Bauhinia and a classification into long‐ and short‐day types of behaviour as suggested by Bunning has been attempted. This could not, however, be made to agree with their actual photoperiodic sensitivities for flower initiation.