78 CHAP. X. THE ELECTRIC RESPONSE TO LIGHT 



I give a record (fig. 21) of the electric response thus 

 obtained ; the duration of exposure was one minute, and 

 a very large down-response occurred, indicating galvano- 



metric posilivity of the ex- 

 posed leaves. On cessation 

 of light, the electric response 

 disappeared during recovery. 

 The record here given shows 

 the predominant A effect. The 

 responses of Musa and of very 

 actively assimilating Hydrilla 

 under light are seen to exhibit 

 characteristic differences on 

 account of the relative pre- 

 dominance of the D or A 

 effect. In Musa D is pre- 

 dominant, A being exhibited 

 either as a positive after- 

 effect or by the overshooting 

 of response in the positive 

 direction. In an exceptionally 

 active Hydrilla plant, on the 

 other hand, A is predominant 

 In less vigorous Hydrilla the 

 positive becomes masked by the negative, when the resultant 

 response appears to be similar to that of Musa. 



We will next observe the characteristic changes in the 

 positive response of Hydrilla under external variations. 



Fig. 21. The Positive Electric 

 Response of Hydrilla 



and the response is positive. 



Effect of Variation of Temperature 



All physiological activity is arrested at a sufficiently low 

 temperature ; it is increased with the rise of temperature, 

 till we obtain the strongest action at the optimum ; at a 

 still higher temperature the activity undergoes a decline. 



These characteristic effects are manifested in the positive 

 electric response of Hydrilla. The temperature was first 



