#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 becoming effective after repetition, in the relation between stimulus and response, and in the effects produced by temperature; also in the phenomenon of after-oscillation. These similarities went even further, the very abnormalities of retinal response finding their reflection in the inorganic. [Illustration: FIG. 117.--ABOLITION OF RESPONSE IN NERVE, PLANT, AND METAL BY THE ACTION OF THE SAME 'POISON' The first half in each set shows the normal response, the second half the abolition of response after the application of the reagent.]