NYCTITROPIC MOVEMENTS 687 



it has been shown that the fall of the leaf in the evening is 

 not due to the action of on-coming darkness, but to the 

 cumulative stimulation of the day's illumination — that is to 

 say, to the action of light as a constant force. 



It is possible to trace out the gradual evolution of this 

 periodic diurnal movement, taking as the simplest type that 

 of the plagiotropic stem. Under the action of stimulus, 

 which is internally or externally diffused, the lower and more 

 excitable side of such a stem undergoes progressive con- 

 cavity, the lowest position being attained in the evening. 

 At night, however, on removal of the stimulus of light, re- 

 covery takes place by erection of the stem. A diurnal up- 

 and-down movement is thus induced. 



A similar effect is observed in the petiole of Biophytum. 



In the leaf of Mimosa the action is precisely the same. 

 Here, owing to the direct action, and the positive after- 

 effect of light, the leaf is depressed progressively till evening. 

 At night, however, recovery takes place by an erectile move- 

 ment. This is not due to a passive recovery merely, but is 

 aided by the negative after-effect, consequent on the storage 

 of internal energy by the day's illumination. By this active 

 internal impulse the leaf attains its highest position some 

 time before dawn. The alternate impulses acting periodi- 

 cally on the leaf are thus : (1) the direct effect of continuous 

 stimulation of light, and (2) the opposite impulse due to 

 internal energy. 



In addition to these alternate impulses the plant as a 

 whole contributes periodic impulses, which are concordant 

 with the periodic impulses in the leaf. The external tissues 

 of the plant, acted on by light stimulus, contract and drive the 

 water inwards, into the central reservoir. At night a reverse 

 movement of water takes place. As a result of this alter- 

 nation of external stimulus during the day, and internal 

 stimulus during the night, there is a periodic inflow and out- 

 flow, a diminution and increase of tension, and these varia- 

 tions of tension are indicated by the periodic depression and 

 erection of all motile leaves synchronously. 



