350 COMPARATIVE ELECTRO-PHYSIOLOGY 



that these organs secrete acids, by means of which insoluble 

 substances are made soluble. It is equally clear, further, that 

 the inorganic solids so dissolved are afterwards absorbed by 

 the plant. Thus it will be seen that these alternating pro- 

 cesses of secretion and absorption of food-material, as they 

 take place in the vegetable organism, are not very different 

 in their essential features from the ordinary phenomenon of 

 digestion as known to us. The chief distinction between the 

 two would now seem to lie in the fact that in the animal the 

 supply of food is in the main organic, and in the plant inor- 

 ganic. Even here, however, we meet with connecting links 

 in the form of insectivorous plants, in whose case the organic 

 supply is obtained by means of the digesting leaf, and the 

 inorganic through the roots. We may regard digestion, 

 therefore, in its widest sense, as a process of absorption of 

 insoluble food rendered soluble, whether such food be organic 

 or inorganic. Apparently, then, in the case of the plant the 

 root functions as a digestive organ. But whether or not this 

 analogy is merely superficial can only be determined by an 

 experimental inquiry into the parallelism which may or may 

 not exist between the various excitatory reactions of the 

 root on the one hand and a typical digestive organ on the 

 other. 



In order to obtain the large quantity of inorganic material 

 which is necessary to the nutrition of a tree, for instance, it 

 is clear that fresh quantities of charged fluid must be con- 

 stantly taken up. In order further that this process may be 

 maintained continuously it must be possible to get rid of the 

 useless water, which is accordingly passed off, chiefly from the 

 transpiring leaves, in the form of vapour. The absorption of 

 food and the ascent of sap, or transpiration-current, would 

 appear therefore to be related phenomena. I shall, in the 

 course of the present and following chapters, then, take up in 

 detail the consideration of these two aspects of the problem, 

 which will thus constitute two main lines of inquiry: 



(i) Whether or not the excitatory reaction of the root has 

 any similarity to that of digestive organs in general ; and 



