RESPONSIVE GROWTH-CURVATURES IN PLANTS 739 



organisms. These swimming movements are brought about 

 by the rhythmic beats of the anisotropic cilia under unilateral 

 stimulus of light, either the up or down stroke of each such 

 beat being relatively quicker than the other. For reasons 

 which have been explained, moderate stimulation, initiating 

 these rhythmic responses, causes movement in one direction, 

 and stronger stimulation movement in the opposite direction ; 

 or, as in the case of Desmodium leaflets, there may be 

 recurrent reversals, causing alternate progressions or retro- 

 gressions to and fro. Similar forms of response with similar 

 variations are brought about by forms of stimulation other 

 than light ; there are thus thermotactic, galvanotactic, and 

 chemotactic swimming movements. 



Nyctitropic movement. — The nyctitropic movement has 

 been shown to be the result of heliotropic action, the fall of 

 the leaf of Mimosa at evening being due not to the action of 

 on-coming darkness, but to the cumulative stimulus of the 

 whole day's illumination. Taking this plant as the type, it 

 was shown that the diurnal movement was caused by the 

 action of two different periodic factors, namely: (1) the 

 differential effect of light on the pulvinus itself during the 

 day, alternating with the cessation of stimulus at night ; and 

 (2) a periodic inflow and outflow of water, which takes place 

 in the plant as a whole by the recurrent action of light and 

 darkness. By the first of these factors the leaf is progres- 

 sively depressed during the day, the reverse process taking 

 place during the night, as a result of natural recovery, aided 

 by the conserved internal energy which gives an impulse 

 opposite to that of external stimulus. These two periodic 

 factors, of the effect on the pulvinus itself, and of that on the 

 plant as a whole, act concordantly, and give rise to periodic 

 movements of the leaf which are of large amplitude. Such 

 forced diurnal vibrations, by long repetition, give rise to 

 periodic after-effects which persist for a time, even on the 

 cessation of the periodically exciting cause. 



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