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. 
