POLAR EFFECTS UNDER STRONG CURRENTS 263 



Type II., was obtained. With a current of 20 micro- 

 amperes the result was Km Am Ab, or Type III. And 

 finally, with 30 micro-amperes the characteristic reaction of 

 Type IV. — namely, Km Kb Am Ab — was observed. 



These results were confirmed with ten different speci- 

 mens. 



I have thus described about a hundred experiments 

 with different species of sensitive plants which conclusively 

 demonstrate the existence of Types III. and IV. of polar 

 reaction. 



There remains an alternative hypothesis in regarding the 

 effects seen in Types III. and IV., as in some way due to 

 the production of secondary poles. The exact physical 

 conditions under which the formation of the secondary pole 

 is possible so as to cause complications in the excitatory 

 phenomena are clearly shown in the experiments of Engel- 

 mann and Biedermann on the ureter of rabbit. In an 

 insulated specimen, under a moderate current, it was found 

 that excitation took place only at the kathode at make 

 and at anode at break. But when the specimen was laid 

 on a good conducting support, such as salt clay, then the 

 polar reactions were found to be exactly reversed — that 

 is to say, the anode excited at make and the kathode at 

 break. This opposition of effects under differing circum- 

 stances is explained by the fact that in the second of these 

 cases we have a conducting-sheet which gives rise to diffusion 

 of the current and a rich development of secondary poles. 

 Thus, opposite to the kathode, there are produced numerous 

 secondary anodes, and conversely, opposite to the anode, 

 there are numerous secondary kathodes. It is these 

 secondary kathodes which are effective in causing excitation 

 in the neighbourhood of the anode at make. In the neigh- 

 bourhood of the primary kathode, on the other hand, excita- 

 tion is prevented by the depressing influence of the secon- 

 dary anodic points. The presence of secondary poles thus 

 induces an apparent reversal of the normal effects, which 

 is simultaneous at the two electrodes. The simultaneity 



