126 PLANT RESPONSE 



From certain metallic particles, again, when exposed to a 

 single strong flash of electric radiation, I have obtained 

 pulsatory responses. From the consideration of all these 

 cases, I was led to investigate the question whether a plant- 

 tissue, when acted on by a single strong stimulus, could be 

 found to give similar repeated responses. From this I was 

 led to the discovery of Multiple Response in plants, a pheno- 

 menon which I find to be very prevalent, and which I shall 

 describe fully in Chapter XX. 



(c) Response disproportionately less than stimulus : respond- 

 ing substance raised above par. — It is the last of these sub- 

 cases, however, which we could least easily have foreseen. It 

 is as if here the active expression of the incident stimulus were 

 bifurcated. Returning once more to the mechanical model, 

 we may imagine the responding portion of the system to 

 contain, besides the mechanically responding galvanometer, 

 two plates of lead, immersed in dilute sulphuric acid, by 

 which the energy of the stimulating current is partially stored 

 up. The external stimulating current has now to do two 

 things, first, to cause mechanical response in the galvano- 

 meter, and secondly, to store up an increasing amount of 

 latent energy in the second part of the responding system. 

 It is evident here that, owing to increasing storage, the 

 mechanical response of the galvanometer may become pro- 

 gressively less. We are, perhaps, too apt to ascribe to 

 'fatigue' all cases of diminution of responses. For in this 

 case we shall have an appearance of fatigue which is not due 

 to the run-down of energy, but to its actual increase, in the 

 responding system. 



I have been able to discover a parallel case to this in the 

 response of plants, a case, that is to say, in which the response 

 is disproportionately smaller than stimulus, successive stimuli 

 nevertheless causing an increase of the energy of the system. 

 I took for my experiment a straight tendril of Passiflora in 

 which growth had undergone arrest. It may be pointed out 

 here that a certain tonic condition is necessary to the con- 

 tinuation of growth. This tonic condition is determined, as 



