596 PLANT RESPONSE 



responds under the briefest exposure to feeble candle- 

 light. This necessitates the making of observations on 

 heliotropic effects without the aid of light. 



The perceptive region for the stimulus of light in the 

 case of the terminal leaflet oi Desmodium is the pulvinus. 



There is no essential difference between the heliotropic 

 response of a growing and a pulvinated organ. 



On the cessation of stimulus the recovery of a pulvinated 

 organ is com.plete ; and this is more or less true also of the 

 recovery of a growing organ from response, if the stimulus 

 have not been excessive. 



For the explanation of this recovery in a growing organ, 

 it is not necessary to assume the existence of any specific 

 power such as recti-petality. The cessation of the difference 

 of hydrostatic pressure on the two sides — such difference 

 being only maintained during the action of stimulus — 

 together with the accelerated rate of growth on the proximal 

 side, which constitutes the negative after-effect, are quite 

 sufficient to explain the recovery from induced curvature. 



