74 



CHAP. X. THE ELECTRIC RESPONSE TO LIGHT 



During the period of rest there is a recovery of the 

 normal condition of the tissue from the effect of stimulus : 

 the pulvinus expands, the leaf becomes re-erected, and there 

 is a restoration of the original turgor ; the induced electric 

 negativity also disappears. 



The A effect is, as previously stated, detected by an 

 electric response of opposite sign, namely of galvanometric 

 posilivity. 



Electric Detection of A and D Effects under Light 



The electric detection of the A and D effects under light 

 will now be described. The difficulty of detection of the 



Fig. i 8. Arrangement for obtaining Electric Response to Light 

 The two figures to the right show the photo-electric cell. 



A effect arises from the fact that the concomitant positive 

 electric response is relatively feeble, while the negative D is 

 far more pronounced and therefore masks the positive A. 

 It is only under favourable circumstances that the posi- 

 tive electric response indicative of the A effect becomes 

 sufficiently large for external manifestation. This we shall 

 find in an organ which has a special capacity for vigorous 

 photosynthesis. 



I will first describe the method for obtaining the electric 

 response of leaves under the action of light. A broad leaf 



