462 PLANT RESPONSE 



that in which it is below par, the plant being in a sub-tonic 

 condition of arrested growth, we find that external stimulus 

 gives rise to little expression in direct response. External 

 .stimulus is found under such circumstances mainly to increase 

 the store of internal energy, in consequence of which we 

 obtain the indirect response of renewed growth. In such a 

 condition, the plant has a great capacity for the absorption of 

 external energy. After growth has commenced, the energy 

 of incident stimulus finds bifurcated expression, by inducing 

 direct or immediate response, and by the indirect or negative 

 after-effect, of enhanced rate of growth (p. 434). The sum 

 total of these two — external stimulus being the same — 

 remains approximately constant, till the optimum tonic con- 

 dition is reached. The direct and indirect effects are thus, 

 up to this point, complementary to each other. After 

 passing this point, however, when the plant is possessed of 

 excess of energy, its power of absorbing stimulus appears to 

 undergo diminution. The direct effect of stimulus is then 



o 



found reduced, and there is no negative after-effect, due to 

 the absorbed component of the external stimulus. 



From these considerations we are enabled to understand 

 the curious growth-response that was observed in varying the 

 temperature of the plant from 34 C. to 35 C. (fig. 182, 



p. 445)- 



The change of temperature was in that case accomplished, 



as will be remembered, by changing the intensity of the 

 electrical heating current. The change from 34 C. to 35 C. 

 thus produced was not, however, brought about at once, but 

 took place in the course of a period of three minutes. We 

 had consequently the stimulating effect of variation of tem- 

 perature bringing about contraction, followed by the accele- 

 rated rate of growth which constituted the negative after- 

 effect of that stimulus, plus the accelerated rate of growth 

 due to rising temperature. That the first of these two factors 

 played a considerable part in this acceleration, is seen from the 

 fact that the permanent increase in the rate of growth charac- 

 teristic of the higher temperature of 35 C. is smaller than the 



