PHOTOTROPISM 319 



stimulation remained more or less persistent. In certain 

 instances there was a partial recovery after a considerable 

 length of time ; in otiiers curvature was fixed by growth, 



PHENOMENON OF RECOVERY. 



On the cessation of stimulus of moderate intensity the 

 heliotropically curved organ straightens itself ; similar 

 eJSects are also found in other tropic curvatures. Thus 

 a tendril straightens itself after curvature induced by con- 

 tact of short duration. The theory of rectipitality has 

 been proposed to account for the recovery, which assumes 

 the action of an unknown regulating power by which the 

 organ is brought back to a straight line ; but beyond 

 the assumption of an unknown specific power, the 

 theory att'ords no explanation of the mechanism by 

 which this is brought about. 



The problem before us is to find out the means by 

 which the organ straightens itself after brie* stimulation^ 

 It will also be necessary to find out why there is no 

 recovery after prolonged stimulation. We have thus to 

 investigate the after-effect of stimulus of various intensities 

 on growth, and the Balanced Method of recording Growth 

 offers us an unique opportunity of studying the character- 

 istic after-effects. 



IMMEDIATE AND AFTER-EFFECT OF LIGHT ON GROWTH. 



As regards the effect of light I have already shown : 



(1) that a sub-minimal stimulus induces an accelera- 



tion of growth, but under long continued 

 action the acceleration is converted into 

 normal retardation (p. 22^>), 



(2) that a stimulus of moderate intensity induces the 



normal retardation of the rate of growth. 



