vi.] VEGETABLE "SKIN' 109 



65. A vegetable surface. In contrast with the preceding, 

 let me now show you another group of four trials on a vegetable 

 surface of which the formula has been drawn up beforehand, to 

 be verified presently. I have chosen a geranium leaf. 



But let us pause a moment to reflect upon the conditions of 

 experiment if we were limited to the use of two electrodes, and 

 had no previous knowledge of the direction in which the re- 

 sponse takes place. We should be obliged to apply our elec- 

 trodes to two external points of the leaf, the deflection would be 

 the sum or the difference between two opposed responses, and 

 we could not tell which was the greater of the two. Suppose, 

 e.g., that we applied the two electrodes to opposite points on 

 the upper and lower surfaces, and observed a response directed 

 from upper to lower surface ; this might result either from 



two outgoing currents A 

 that at B exceeding 1 



or from two ingoing 

 currents, that at A 



1 



that at A : B + exceeding that at B : B 



and until we know whether the effects are ingoing or outgoing, 

 we could not decide between the two alternatives. We are 

 obliged to use three electrodes to enable us to test separately 

 the point A and the point B, by connecting first one and then 

 the other point through the galvanometer with an indifferent 

 point C. 



We proceed then with the experiment, B C A 

 having clearly realised the simplicity and 

 necessity of its apparent complication. 

 The results, as you see, come out precisely 

 as figured in the diagram before you : 



All four reactions are ingoing ; Nos. 

 i and 4 are homodrome post-anodic ; 

 IN' os. 2 and 3 are antidrome post-kathodic. 



66. Surface against surface. And now, having learned 

 that the reactions of the external surface of a leaf are ingoing, 

 we may make use of the resultant effect when only two elec- 

 trodes are used, to learn whether one of two points or of two 

 surfaces acts more powerfully than the other. Is, e.g., the 

 ingoing blaze of the upper or of the lower surface predominant 



