DIURNAL MOVEMENT OF W/yfOSA 587 



was obtained by a special contrivance of an oscillating 

 smoked glass plate, the up and down oscillation being 

 at intervals of 30 minutes, A detailed account of this 

 apparatus will, with its possibilities for meteorology, be 

 given in a future paper. I reproduce the recor 1 obtained 

 in my greenhouse on the r>th March (1919), which gives a 

 generahidea of the variation of the light from morning to 

 evening (Fig. 2V^). The record shows that the light began 

 to be perceptible at "5-30 a.m., and that the intensity 

 increased rapidly and continuously till it reached a climax 

 at noon, after which it began to decline slowly. The 

 decline of intensity of light was very abrupt after o p.m , 

 the effect being reduced to zero at G-30 p.m. 



THE EFFECT OF DIRECT LIGHT. 



Under natural conditions, the leaf of Mimosa is acted 

 on by light from above, and it is generally supposed that 

 the pulvinus is positively phototropic, that is to say, it 

 curves upwards till the leaf is placed at right angles to 

 the direction of light. My investigations show, however, 

 that the phototropic effects vary from positive to negative 

 through an intermediate stage of neutralisation, these de- 

 pending on the intensity and duration of exposure. When 

 light acts continuously on the upper half of the pulvinus, 

 there"' follows the 'following sequences of reaction : 



(1) The leaf is at first erected by the contraction of 

 the upper half of the pulvinus due to direct action of 

 light acting from above. 



(2) Under continuous stimulation of the upper half of the 

 pulvinus by light, the excitation is slowly conducted to the 

 lower half across the pulvinus. In consequence of this 

 transmitted excitation, the lower half begins to contract 

 and thus neutralises the first effect of erection. The upper 

 half of the pulvinus is less contractile than the lower 



