EXCITATORY CHARACTER OF SUCTIONAL RESPONSE 389 



negative. We saw, further, that there was reason to 

 associate this positive response with the process of absorp- 

 tion, and the negative, conversely, with that of expulsion 

 or secretion. With various kinds of tissues, moreover, we 

 have found, and shall see further, that as a general rule 

 positive response is obtained, either when the tissue is 

 sub-tonic, and very slightly, excitable, or when the stimulus- 

 intensity is feeble, and when the tissue is fatigued by over- 

 stimulation. Negative response, on the other hand, is 

 characteristic of highly excitable tissues. Under natural 

 conditions, then, when the roots are subjected to the 

 moderate stimulation of such factors as contact with soil, 

 water, and food, we might expect their response to be 

 positive or absorptive. In cut branches, also, in which the 

 tissue is not extremely excitable, a simple terminal applica- 

 tion of stimulus induces, as we saw, the absorpto-positive 

 effect, either by initiating suction, or by enhancing that 

 which was already taking place. 



But in highly excitable young roots, in contact with a 

 stimulating supply of .inorganic food, the characteristic 

 response, as we have seen, is by secretion. The electrical 

 response also of young roots we found to be negative. 

 Between these two extremes of positive and negative, then, 

 there must be an intermediate case in which the responsive 

 action to external stimulus will be zero. 



Applying this to the parallel case of suctional response 

 in cut branches, we should expect to meet with two 

 different cases besides that already given. In one, where 

 the tissue is very highly excitable, the response under simple 

 terminal stimulation will be negative or expulsive. In 

 tissues, however, which are not so highly excitable, but more 

 excitable than those sub-tonic specimens whose characteristic 

 responses I have already described, it might be possible to 

 find cases in which the suctional response to stimulus will 

 be zero. In the sub- tonic tissues referred to, we have already 

 seen that in consequence of increase of internal energy by 

 external stimulus, the suctional response tends to reach a 



