564 COMPARATIVE ELECTRO-PHYSIOLOGY 



a somewhat discordant character. I shall, however, be able 

 to show that their complexity is due to the combination of 

 the different effects of the polarisation current on conductivity 

 and on excitability. These separate effects may, according 

 to circumstances, either conspire or act antagonistically. 

 Hence the great variety of results, which appears at first 

 sight incapable of a consistent explanation. 



In order, then, to discover the laws by which an electric 

 current induces a variation of conductivity and excitability, 

 we" must first determine the pure effect of the current on 

 conductivity, apart from any excitatory variation ; and, 

 secondly, its effect on excitability, uncomplicated by any 

 variation of conductivity. 



To take conductivity first : the ideally perfect arrange- 

 ment would be to have the polarising electrodes, in relation 

 to the region whose conductivity-variation is to be tested, at 

 a distance so great that they could exert no predominant 

 an- or kat-electrotonic influence upon it. In a led-off 

 circuit, moreover, a differential action, unless proper pre- 

 cautions are taken, is exerted on two electrodes placed side 

 by side. It is, therefore, desirable to remove one of these 

 outside the sphere of action. Such, then, being the con- 

 ditions to be observed, in order to eliminate the effect of 

 the poles themselves, and thus determine the influence of the 

 direction of current on conductivity alone, I took a petiole 

 of fern 20 cm. in length, and connected its ends through a 

 reversing-key with a Daniell cell (E.M.F. = I volt). The 

 responding galvanometer-circuit had one electrode about the 

 middle of the petiole, near the insertion of a certain lateral 

 leaflet, the other electrode being connected with the lamina 

 of the same leaflet, whose midrib, however, was cut across to 

 prevent transmission of the excitatory effect (figs. 343, 344). 

 It Is thus seen that the led-off electrodes are at a relatively 

 great distance from the polarising electrodes, and further, 

 owing to one electrode being placed out of the way, on the 

 lateral leaflet, and the other symmetrically between anode 

 and kathode, it is clear that anodal or kathodal action is 



