1 6 COMPARATIVE ELECTRO-PHYSIOLOGY 



simpler than the theory of electro-motive molecules, is 

 available. It will also be proved that the electrical response 

 due to true excitation is quite distinct from that which is 

 brought about by the hydrostatic disturbance, its sign being 

 in fact opposite. This true excitatory electrical response, 

 again, will be shown to be modified by all those conditions 

 which affect the physiological state of the tissue. And, 

 lastly, it will be proved that there is no breach of continuity 

 as between the electrical responses in plant and animal, 

 for not only is the sign of response in both cases the 

 same, but it is also true that every type of response and 

 modification of response, which occurs in the animal tissue, is 

 to be found under parallel circumstances in that of the 

 plant also. 



In order to determine what is the electrical response 

 characteristic of excitation, we first select for experiment a 

 sensitive plant, say Mimosa, because here, in the responsive 

 fall of the leaf, we have a visible indication of the excitatory 

 reaction. It is desirable at this point to say a few words re- 

 garding the use of the terms ' excitatory ' and ' true excitation.' 

 We are all familiar with the fact that, when muscle is excited 

 by stimulus, it responds in a conspicuous manner by con- 

 traction. This is universally accepted as the phenomenon of 

 excitation, its electrical concomitant being galvanometric 

 negativity. Having once applied the term ' excitatory ' to this 

 particular aspect of molecular response in living tissues, it is 

 of course important that we should henceforth distinguish 

 carefully between it and its possible opposite, namely, ex- 

 pansion, with concomitant galvanometric positivity. Under 

 stimulation there is a contraction of, and expulsion of water 

 from, the excited pulvinus, which brings about the depression 

 of the leaf. It is generally supposed that only the lower half 

 of the pulvinus is excitable. This, however, is an error, for 

 both upper and lower halves are excitable, and contract 

 under stimulation. If localised stimulus be applied on the 

 upper side, that side contracts, and, by the concavity thus 

 induced, the leaf is erected. But, though both halves are 



