354 COMPARATIVE ELECTRO-PHYSIOLOGY 



We have seen that a young root of Colocasia, when fresh, 

 gives the normal response of galvanometric negativity. 

 Taking next an older root of the same plant, and employ- 

 ing the same intensity of stimulus as before, I found the 

 responses to take place, generally speaking, by galvanometric 

 positivity (fig. 215). This would appear to suggest a ten- 

 dency towards specialisation of function, galvanometric nega- 

 tivity being associated, as we have seen, with secretion, and 

 positivity, in all probability, with the opposite namely, 

 absorption. A similar specialisation of certain cells for 

 secretion and others for absorption is manifested more 

 unmistakably in the digestive organs 

 of the higher animals. 



Thus in the young roots the pre- 

 dominant reaction would seem to be 

 secretion, reversed under continuous 

 stimulation to absorption. In the older 

 roots, on the other hand, the pre- 

 dominant reaction must be supposed 

 to be absorptive. Here, then, judging 

 from the electrical indications, we 

 would seem to have proof of that 

 Physiological activity in virtue of which 

 Colocasia water is taken up by the root, thus 



giving rise to the so-called ' root- 

 pressure.' We can also see how, by the summated activities 

 of numerous roots, this * root-pressure ' is kept approximately 

 constant for a certain length of time. Taking longer periods 

 into account, further, we can see that this physiological 

 activity is likely to undergo periodic change, a fact which is 

 evidenced by the known periodic variation of root-pressure. 

 The question, however, of the actual influence of excitation 

 on the process of the ascent of sap will be dealt with in the 

 next chapter. 



One form of stimulus to whose action the roots must often 

 be subjected is that of the chemical substances present in 

 the soil, and I undertook to test the electrical variations 



