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 iminediate 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 madequate. 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. Ansesthetics 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. 
