ABSORPTION OF FOOD BY PLANT 353 



were found to be in the positive phase, giving rise to response 

 by galvanometric positivity. 



In the case of the root, it is interesting to find that 

 there is a similar alternation of responsive phases. For 

 this demonstration I again took the root of Colocasia, and 

 recorded its responses to equi-alternating electric shocks. 

 Among the mass of roots there are naturally some which 

 are dead and decaying. One of these was selected for one 

 electrical contact, while the other was made with a young 

 and vigorous root. The responsive reaction, under these 

 conditions, was found to take place 

 by galvanometric negativity (fig. 214). 

 Under long-continued stimulation, 

 however, I have often found this 

 normal response by galvanometric 

 negativity to be reversed to its 

 opposite, positivity. From this we 

 may pass to the consideration of 

 response in older roots, where the 



phasic reaction is typically positive. 



XT . . FIG. 214. Photographic 



Now we have seen in previous Record of Normal Nega- 



chapters, as will be remembered, that ve Response of Young 



Root of Colocasia 



there are two different conditions 



under which the positive may be substituted for the normal 

 negative response. The first is that of reversal under long- 

 continued stimulation, which we ( have just seen. And the 

 second occurs when the stimulus falls below the critical 

 level which is necessary to the evoking of true excitation. 

 In this latter case, as we saw further, the incident stimulus 

 increases the internal energy, and causes expansion, positive 

 turgidity-variation, and galvanometric positivity. It would 

 thus appear that one identical stimulus may induce one effect, 

 that of galvanometric negativity, in a highly excitable tissue, 

 and the opposite, or galvanometric positivity, in a tissue that 

 is less excitable. In connection with this question the ex- 

 perimental results which I am about to describe are very 

 significant. 



A A 



