PLANT RESPONSE 47 



subsidence of the disturbance at A, the wave reached B. 

 The effect of this was to , produce a current in the 

 opposite direction. This apparently hastened the 

 recovery of A (from 60 seconds to 12 seconds). The 

 excitation of A now disappeared, and the second phase 

 of response, that due to excitation of B, was fully 

 displayed. 



Positive after-effect. If we regard the response due 

 to excitation of A as negative, the later effect on B would 

 appear as a subsequent positive variation. 



In the response of nerve, for example, where con- 

 tacts are made at two surfaces, injured and uninjured, 

 there is sometimes observed, first a negative variation, 

 and then a positive after-effect. This may sometimes 

 at least be due to the proximal uninjured contact first 

 giving the usual negative variation, and the more 

 distant contact of injury giving rise, later, to the 

 opposite, that is to say, apparently positive, response. 

 There is always a chance of an after-effect due to 

 this cause, unless (1) the injured end be completely 

 killed and rendered quite irresponsive, or (2) there 

 be an effective block between A and B, so that the dis- 

 turbance due to stimulus can only act on one, and not 

 on the other. 



I have found cases where, even when there was 

 a perfect block, a positive after-effect occurred. It 

 would thus appear that if molecular distortion from 

 stimulus give rise to a negative variation, then during 

 the process of molecular recovery there may be over- 

 shooting of the equilibrium position, which may be 

 exhibited as a positive variation. 



