THE ELECTRO-MOTIVE RESPONSE OF PLANTS 15 



of leaves ' in their prime' to be somewhat different A strong 

 negative phase was now observed on stimulation, but pre- 

 ceded by a short-lived positive reaction. These results will 

 be discussed in detail in a subsequent chapter, but it may be 

 said here that from the records given by Burdon Sanderson 

 it is difficult to know which of the various responsive phases 

 are to be regarded as those of true excitation, and which as 

 the results of some other cause. 



It will thus be seen that the results arrived at and the 

 theories advanced by different observers in this field are some- 

 what at variance with each other. This must have been due 

 to the difficulties met with in disentangling the fundamental 

 reaction from those various subsidiary effects which are apt 

 to be found in combination with it. Chief among these 

 difficulties was (i) the fact that positive and negative 

 variations are generally measured in terms of an existing 

 current of rest. But, as a matter of fact, these two 

 apparently opposite responsive variations, positive and 

 negative, are not always indicative of opposite reactions. 

 For an identical excitatory reaction, added algebraically to 

 the resting-currenr, might appear, according to circumstances, 

 as either of the two. There was also (2) the difficulty of 

 discriminating two opposed electrical effects, one of which 

 was due, as I shall show, to true excitation, and the other 

 to increase of internal energy brought about by mechanical 

 movement of water. Under different conditions, it may be 

 either the one or the other of these which becomes prominent. 

 I shall hope to show that each of them is definite and dis- 

 tinguishable from the other. 



With regard to the response of plants in general, then, 

 it may be well to state here that the first fact to be demon- 

 strated in the course of the present work is that such 

 responsive phenomena as may be observed in the case of 

 sensitive plants like Dioncea, are not unique, but occur 

 under similar circumstances even in ordinary plants, and are 

 characteristic of all plant organs. I shall be able to show, 

 moreover, that an explanation of these phenomena, much 



