446 COMPARATIVE ELECTRO-PHYSIOLOGY 



interior tissue is really killed by scalding, and unless this is 

 thoroughly done it is easy to see that the inner cells may 

 continue to conduct excitation. 



There is, moreover, another possibility, that of pseudo- 

 conduction, by which the effect of stimulus might appear to 

 be transmitted across dead areas. In Haberlandt's experi- 

 ment, even if the intervening tissues had been killed, there 

 would still be two masses of tissue, separated from each other 

 by an intervening area of dead tissue. A strong stimulus 

 applied to either of these might then cause an excitatory 

 expulsion of water, capable, when transmitted across the 

 dead area, of imparting a mechanical blow to the second 

 living tissue, sufficient to set up excitation de novo in that 

 portion of the petiole. 



I shall, however, be able to show that conduction is 

 brought about in the plant by the same transmission of 

 excitatory protoplasmic changes which occurs in the animal ; 

 and that those agencies such as cold, anaesthetics, fatigue, 

 and the polar effects of currents which induce its variation 

 in the one case, have the same identical effect in the other 

 also. The electrical responses, again, afford us a crucial 

 method of distinguishing between hydrostatic and excitatory 

 effects. For, had the transmission of excitation taken place 

 in plants by means solely of the propagation of hydrostatic 

 disturbance, its electrical sign would then have been one of 

 galvanometric positivity alone. But we have found, on the 

 contrary, that the sign of the true excitatory reaction is 

 always of galvanometric negativity. The distinct characters 

 of these true excitatory and hydrostatic waves have already- 

 been demonstrated in various experiments, in some of which 

 the hydrostatic has been seen as a positive twitch preceding 

 the true excitatory negative, while in others the transmission 

 of the negative was abolished by the application* of a selective 

 block, such as chloroform, thus bringing about the exhibition 

 of the positive alone (p. 66). So true indeed is it that 

 excitatory changes are propagated in the plant as in the 

 animal, that I have actually been able to isolate certain 



