DETECTION OF LATENT STIMULUS 461 



Latent component almost vanishing above the opti- 

 mum. — As an extreme instance of this, we may take the 

 response at a temperature beyond the optimum, say at 37 C. 

 Here, by its environmental conditions, the plant is already 

 supplied with an excess of energy. And besides this, there 

 is the fact which we have already noticed, that its general 

 excitability is diminished, so as to be equal to, or less than, 

 that at 30 C. At these two temperatures, then — of 30 C. 

 and 37 C. — we have two conditions of excitability more or 

 less the same, but with a different history. 



Below the optimum there is an unsatisfied capacity for 

 absorption of stimulus, whereas above it this capacity has 

 been fully met. It would therefore appear that the power of 

 the tissue to hold stimulus latent is diminished progressively 

 up to the optimum, till beyond that point it practically 

 disappears. I obtained a remarkable confirmation of this 

 inference in the course of my experiments. In the experi- 

 ment just described, for example, the direct response was 

 a retardation of twenty-four divisions, and there was no 

 indirect effect, showing that little or no stimulus had become 

 latent (fig. 185). 



Variation of receptivity. — In the experiments described, 

 the sum of the direct and the indirect effects, up to the optimum, 

 had been found to be approximately constant, that is to say, 

 a total response of about forty-six divisions. At 37 C, how- 

 ever, we see the total response reduced to twenty-four divisions 

 without any latent component. And since the total response 

 measures for us the amount of stimulus taken up by the 

 tissue, it would appear that at 37 C. not only is the 

 power to hold stimulus latent lost, but also that the general 

 receptivity of the tissue is very much reduced. It is thus 

 seen that the condition of a tissue modifies its receptive 

 power ; hence it is possible for different parts of the same 

 organ — say, for instance, the tip and the growing region — 

 being in different conditions, to possess different receptivities. 



Direct and indirect response of plant in sub-tonic 

 condition. — Turning from this case of excess of energy to 



