7IO COMPARATIVE ELECTRO-PHYSIOLOGY 



period which elapses ; before the initiation of this responsive 

 variation to external stimulus, is longer when the plant is in 

 a sub-tonic condition than in the same plant when its tonic 

 condition has been slightly raised by previous stimulation. 

 Crucial experiments, finally, have been described, showing 

 that water-movement is a mode of excitatory response. 



A difficult problem in connection with electrical response 

 is that of the discharge from the electrical organs of certain 

 fishes. In a large number of cases, of which Torpedo may 

 be taken as the type, the discharge takes place in a direction 

 from the anterior or nervous to the posterior and non- 

 nervous surface. Pacini's generalisation that the responsive 

 discharge is always from the anterior to the posterior surface 

 is negatived by the instance of Malepterurus, in which it is 

 from the modified glandular posterior to the anterior surface. 

 Another peculiarity of the response of electric organs in 

 general is that the responsive current is always in the same 

 direction that, namely, of the organ-discharge whether 

 the exciting shock be homo- or hetero-dromous. No theory 

 has yet been found which will fully explain all these 

 peculiarities. 



I have shown, however, that this phenomenon is not alone 

 of its kind ; nor is it dependent on any specific characteristic 

 of the animal nerve-and-muscle, or gland, of which different 

 electric organs are modifications. The response of the electric 

 organ simply constitutes an extreme case of differential 

 excitability, and follows the general law of response in 

 anisotropic organs namely, that on diffuse stimulation the 

 responsive current flows from the more to the less excitable. 

 The peculiarity of the organ simply depends upon the fact 

 that owing to the serial arrangement of its elementary aniso- 

 tropic plates the terminal electro-motive effect becomes very 

 large by summation. We find vegetal analogues to the two 

 types of electrical plates of Torpedo and Malepterurus, in 

 the leaves of Pterospermum, and the pitcher, of Nepenthe. In 

 the first of these, Pterospermum y as in Torpedo, the anterior 

 nervous surface is relatively more excitable than the mass 



