566 COMPARATIVE ELECTRO-PHYSIOLOGY 



In fig. 345 b, this is seen to have actually occurred. There 

 will be other cases where, the depression of conductivity 

 induced being not too great, it will be possible to watch the 

 gradually lessening amplitude of response until it ends in 

 actual reversal. We have next to determine the effect on 

 conductivity, of the passage of excitation in an uphill 

 direction that is to say, from the kathodic to the anodic 

 region. For this purpose the polarising current was reversed 



FIG. 345. Photographic Records of Responses taken in last Experiment, 

 when" Excitation was transmitted with and against the Polarising 

 Current 



a, Normal response of petiole of fern to transmitted excitation ; d, 

 Reversal of response when excitation was travelling electrically downhill 

 or with the current ; c, Normal response once more ; d, Enhanced 

 response due to increase of conductivity when excitation travels electri- 

 cally uphill, or against the polarising current. Upward arrow f 

 indicates that polarising current is in same direction as normal 

 response. Downward arrow ^ shows polarising current in opposite 

 direction. The same in the two following figures. 



by means of a reversing key, the left-hand end of the petiole 

 being now made the kathode. Before doing this, however, I 

 stopped the current, and took a second set of records of 

 normal responses. It will be seen that by reason of the 

 cessation of the previously acting left-to-right current, 

 leaving an after-effect, these were slightly enhanced above 

 the first normal responses (fig. 345 c). On now reversing 

 the current, conductivity was found to be enhanced, as seen 

 in the greater amplitude of response (fig. 345 d). 



