not being dead-beat
shows after-oscillation); the abnormal 'up' is converted into normal
'down' after continuous stimulation. (M) is the record for metal,
the abnormal 'down' being converted into normal 'up' after like
stimulation.]

But these modified responses usually become normal when the specimen is
subjected to stimulation either strong or long continued (fig. 116).

#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 irresponsive 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 same drug, administered in large or
small doses, might have opposite effects, and in preceding chapters we
have seen that the same statement holds good of plants and metals also.

#Stimulus of light.#--Even the responses of such a highly specialised
organ as the retina are strictly paralleled by inorganic responses. We
have seen how the stimulus of light evokes in the artificial retina
responses which coincide in all their detail with those produced in the
real retina. This was seen in ineffective stimuli becomi