298 PLANT RESPONSE 



excitatory disturbance. But the difficulty is to determine 

 the point at which the latent energy is stored up, and from 

 which the excitatory disturbances subsequently proceed. As 

 we are accustomed to think that stimulus must be due to 

 some sudden variation, one is tempted to suppose, as the 

 simplest explanation, that in the case of autonomous move- 

 ments the stimulation is caused by variations in the rate of 

 absorption of food material, which is probably being carried 

 on, in an intermittent manner only, by the roots and leaves. 

 In this case, if the stimulus proceed from the root, we may 

 expect the excitation to travel to the motile leaflets through 

 the stem, outwards, or in a centrifugal direction. If the 

 assimilating leaves, however, be the source of excitation, we 

 shall look to see the wave of excitation proceeding from 

 without inwards, or centripetally. 



In order, therefore, to localise the source of stimulation 

 in Desmodium gyrans, for example, I first tried to find 

 out in which direction the excitatory impulse proceeded. 

 Unfortunately in this case, the leaf being provided with only 

 a single pair of motile leaflets, it is impossible to obtain the 

 indication of direction which is given by Biophytum, by the 

 successive closure of leaflets in a definite order. I was 

 therefore obliged to have recourse to other expedients. 



I have already shown how the transmission of stimulus 

 can be arrested by local application of ether, or by the anodic 

 block. If the pulsating stimulus, therefore, should be central, 

 that is to say, proceeding from the stem, we should then 

 expect the application of ether, or the anodic block, at the 

 junction of the leaf with the stem, to arrest the movement of 

 the leaflets. If, on the other hand, the source of the stimula- 

 tion should be peripheral, a block produced in the manner 

 described, at a point between the terminal expanded leaflet 

 and the small motile leaflets, would prevent the response of 

 the latter. In carrying out these experiments, however, I 

 found that no such arrest took place in either case. 



The fact that the source of stimulation is not central, is 

 also made evident by the following consideration. Had it 



