5/6 PLANT RESPONSE 



rest of the plant will also constantly increase. The occur- 

 rence of oscillatory action, under these balanced and opposed 

 forces, is what might be expected. Moreover, in the re- 

 sponding organ itself, we see the action of opposed forces 

 to be induced ; for while the direct effect of local stimulus is 

 to cause contraction, the absorbed stimulus is meanwhile 

 increasing the internal energy, the result of which is the 

 opposite expression in expansion. Thus, from what has 

 been said, it would appear that in all these — namely, the 

 hydrostatic action and reaction between the excited organ 

 and the rest of the plant, the opposed effects of external 

 stimulus and internal energy, and the presence of an excess 

 of latent energy, with periodic variations of excitability — -we 

 have so many factors, which would all contribute to a 

 common result, in the oscillatory character of the responsive 

 expression. 



But when we come to the case of anisotropic or dorsi- 

 ventral organs, we find an additional element making for 

 alternation of effects, for when such an organ is diffusely 

 stimulated, we obtain a differential response. But as the 

 constitutions of the two anisotropic halves are different, the 

 fatigue produced on opposite sides will not be simultaneous 

 but alternate. Similar effects will also appear under uni- 

 lateral stimulation of a radial organ. For here, too, the 

 organ, owing to the relatively greater fatigue of one side, 

 becomes molecularly anisotropic, and the stimulus by its long- 

 continued action becomes internally diffused. 



Similarity of responsive reaction under light and other 

 forms of stimulation. — It has thus been fully demonstrated 

 that the fundamental response of the plant to the stimulus of 

 light takes place, like that to all other forms of stimulation, 

 by contraction, leading, in the case of growing organs, to 

 retardation of growth. Hence the unilateral stimulus of 

 light may be expected to induce curvatures similar to those 

 observed under other forms of stimulation, that is to 

 say : 



I. The direct effect of moderate unilateral stimulus of 



