I90 PLANT RESPONSE 



or below, inducing depression. Anaesthetics, moreover, tem- 

 porarily, and poisons permanently, abolish response. It will 

 be shown further, in Chapter XVIII, that the transmission 

 of excitation may be very much diminished, or even arrested, 

 by the application of cold or ether. 



Theory of protoplasmic change. — It is thus seen that the 

 hydro-mechanical theory is incapable of explaining the facts 

 of the case. I shall now, therefore, proceed to demonstrate 

 that the excitatory change in plants is brought about in the 

 same manner as in animals, and that the transmission of 

 excitation depends upon the propagation of protoplasmic 

 changes, in the one case as in the other. This may be 

 determined by a crucial experiment as to whether vegetable 

 tissue exhibits those peculiar polar effects of the electric 

 current on excitability, which are seen in the protoplasm of 

 animal tissues. In the animal tissue, for example, it is the 

 kathode that, under normal conditions, produces excitation, 

 the effect of the anode being the reverse. In the case of animal 

 tissues, again, the anode will even act as a block to the trans- 

 mission of stimulus. 



Crucial tests applied by means of polar excitation. — 

 Such effects are incapable of explanation by the hydro- 

 mechanical theory, and if we succeed in discovering similar 

 phenomena in the case of vegetable tissues, we shall establish 

 the existence of a fundamental property of protoplasm 

 common to the animal and vegetable alike. With this end 

 in view I have carried out numerous experiments on plants, 

 both sensitive and ordinary. As specimens of the former 

 class, I used Biophytum, Mimosa, and Averrhoa. The in- 

 vestigation resolves itself into the determination of the differ- 

 ences of excitatory effects, at the anode and kathode, both at 

 make when the circuit is completed, and at break when 

 it is interrupted. The presence of the excitatory effect is 

 indicated in the case of ' sensitive ' plants by the mechanical 

 responses of the motile organ. In order to separate the 

 effects of the anode and kathode, we may use the Mono-polar 

 method, i.e. have one electrode near a motile organ, and the 



