EXCITATORY CHARACTER OF SUCTION AL RESPONSE 381 



pensator at a uniform rate in larger quantity than necessary, 

 a movement of the index is induced in a direction opposite 

 to that of suction. Here the slope of the curve, due to over- 

 balance, is down, and should the stimulus cause any enhance- 

 ment of suction, the steep down-curve must be replaced by a 

 less steep, or horizontal, or up-curve. The Method of Balance 

 is, as I have said, the most delicate, but this Method of Over- 

 balance enables us to detect the after-effect of stimulus in a 

 striking manner. 



As regards the form of this response by movement of 

 water, reference has already been made to previous results, 

 in which we saw that the excitatory effect in the plant, as 

 studied by electrical response, was of two different types, 

 according to certain phasic conditions. Highly excitable 

 roots, we found, gave response by galvanometric negativity, 

 indicating secretion or expulsion of water. Less excitable 

 roots, on the other hand, gave response by positive variation, 

 most probably indicating responsive absorption. These two 

 opposite effects actually occur, as I find, under different 

 phasic conditions, in the suctional responses which I am 

 about to describe. 



I shall deal first with the results which I invariably 

 obtained in carrying out experiments on Croton and certain 

 other plants during the month of February. The Indian 

 winter was just over, and the spring had not yet fully set in. 

 The nights were still cold, though the days were growing 

 warmer. The plants, therefore, owing to these peculiar 

 climatic conditions, must be regarded as having been some- 

 what sub-tonic. My first experiment related to the initiation 

 of suctional activity in a specimen which, in this respect, 

 had been previously at standstill. Stimulus might here be 

 expected to renew that multiple activity on which suctional 

 response, according to our theory, depends. We may here 

 refer once more to the initiation of multiple activity by 

 stimulus in a leaflet of Desmodium previously sat standstill. 



I have shown that when this plant is deprived of its 

 store of latent energy by unfavourable conditions, then the 



