70 RESPONSE IN THE LIVING AND NON-LIVING 



are able to judge that a plant has died, only after- 

 various indirect effects of death, such as withering, have 

 begun to appear. But in the electric response we have 

 an immediate indication of the arrest of vitality, and we 

 are thereby enabled to determine the death-point, which 

 it is impossible to do by any other means. 



It may be mentioned here that the explanation 

 suggested by Kunkel, of the response being due to 

 movement of water in the plant, is inadequate. For 

 in that case we should expect a definite stimulation to 

 be under all conditions followed by a definite elec- 

 tric response, whose intensity and sign should remain 

 invariable. But we find, instead, the response to be 

 profoundly modified by any influence which affects the 

 vitality of the plant. For instance, the response is at 

 its maximum at an optimum temperature, a rise of a 

 few degrees producing a profound depression ; the 

 response disappears at the maximum and minimum 

 temperatures, and is revived when brought back to 

 the optimum. Anaesthetics and poisons abolish the 

 response. Again, we have the response undergoing an 

 actual reversal when the tissue is stale. All these 

 facts show that mere movement of water could not be 

 the effective cause of plant response. 



