158 COMPARATIVE ELECTRO-PHYSIOLOGY 



yet, owing to the irresponsive condition of 15, there is a 

 resultant response, and the direction of this action-current 

 is found to be from A to B. We have thus experimentally 

 verified the assumption that in the same tissue an uninjured 

 portion will be thrown into a greater excitatory state than 

 an injured, by the action of the same stimulus. 



When the point B is injured, there is generally speaking 

 a more or less persistent current set up which flows from 

 B to A. But we saw that the direction of the action-current 

 was opposite that is to say, from A to B. This will explain 

 the reason why the action-current causes a diminution or 

 negative variation of the current of injury, so called. One 

 method of doing this is to cause injury to one of the two 

 points. If this be such as to kill the tissue, then its 

 excitability is permanently abolished. Or by causing the 

 excessive stimulation of injury, we may simply depress the 

 excitability of the tissue for a longer or shorter time. 



I shall now give a few instances of response in plants by 

 negative variation. Taking the petiole of turnip, we injure an 

 area on its surface, say B. A current is now observed to 

 flow in the petiole from the injured B to the uninjured A. 

 The induced difference of potential depends on the condition 

 of the plant, and the season. In the experiment here 

 described, its value was "13 volt. A sharp mechanical tap 

 was now given to the petiole, between A and B, and a sudden 

 diminution, or negative variation, of current occurred, the 

 resting potential difference being decreased by -026 volt. A 

 second and stronger tap induced a second response, causing 

 a greater diminution of potential difference by '047 volt. 



In another experiment, the specimen employed was a 

 petiole of cauliflower (Brassica oleraced]. The first up-line 

 to the right indicates the current of injury. The three re- 

 sponses which succeed are induced by a given intensity of 

 stimulus, the next series of six, being in response to stimulus 

 nearly twice as strong, exhibit signs of fatigue (fig. in). 



The current of injury generally undergoes a diminution 

 with time. This is often, as has been explained, on account 



