188 RESPONSE IN THE LIVING AND NON-LIVING 
Diphasic variation.—A diphasic variation is observed 
in nerve, if the wave of molecular disturbance does not 
reach the two contacts at the same moment, or if the 
rate of excitation is not the same at the two points. 
A similar diphasic variation is also observed in the 
responses of plants and metals (figs. 26, 68). 
Effect of temperature.—In animal tissues response 
becomes feeble at low temperatures. At an optimum 
temperature it reaches its greatest amplitude, and, again, 
beyond a maximum temperature it is very much 
reduced. 
We have observed the same phenomena in plants. 
In metals too, at high temperatures, the response is very 
much diminished (figs. 38, 65). 
Effect of chemical reagents.—Finally, just as the 
response of animal tissue is exalted by stimulants, 
lowered by depressants, and abolished by poisons, so 
also we have found the response in plants and metals 
undergoing similar exaltation, depression, or abolition. 
We have seen that the criterion by which vital 
response is differentiated is its abolition by the action 
of certain reagents—the so-called poisons. We find, 
however, that ‘ poisons’ also abolish the responses in 
plants and metals (fig. 117). Just. as animal tissues 
pass from a state of responsiveness while living to a 
state of irresponsiveness when killed by poisons, so also 
we find metals transformed from a responsive to an ir- 
responsive condition by the action of similar ‘ poisonous’ 
reagents. 
The parallel is the more striking since it has long 
been known with regard to animal tissues that the 
