ELECTROTONUS 



235 



excitatory action of K, the current due to an E.M.F. of two 

 volts was reduced, by separating A from K, and thus inter- 

 posing a greater resistance. A 

 distance was thus found — i.e. 

 10 cm. — such that, on complet- 

 ing the AK circuit, the excitation 

 was not sufficient to produce 

 response of the leaf. 



The AK circuit was now 

 opened, and adjustments made 

 with the second circuit a'k', 

 with an E.M.F. of two volts, in 

 such a way that, on complet- 

 ing that circuit alone, there 

 was no response of the leaf. 

 Owing to the shorter length of 

 petiole available — i.e. 3 cm. — 

 the current could not be reduced 

 to the requisite amount by 

 simply increasing the interpolar 

 distance. An external resist- 

 ance had therefore to be added, 

 in order to attain the desired 

 condition. Thus either circuit, 

 acting alone, was ineffective. 



Kat-electrotonic increase of excitability. — Now, in order 

 to show the increase of excitability in the pulvinus at K, as 

 induced by the neighbourhood of kathode K', we first com- 

 plete the A'k' circuit. This, as has been said, is ineffective. 

 But now, on making the AK circuit, its previously ineffective 

 stimulus becomes effective, and the leaf responds. From 

 this it will be seen that during the passage of a current, 

 through the a'k' circuit, a point in the neighbourhood of the 

 kathode k', that is to say, the pulvinus, is rendered more 

 excitable. 



This experiment may be varied by first making the AK, 

 which is now the electrotonic, circuit, and then completing a'k' 



Fig. 102. Diagrammatic Repre- 

 sentation of Electrical Connec- 

 tions in Mimosa to Exhibit 

 Variation of Motile Excitability, 

 induced by Anode and Kathode 



In the first of these experiments, 

 the a'k' circuit is electrotonic 

 and the AK circuit excitatory. 

 In the second and third experi- 

 ments the AK circuit is made 

 electrotonic, and a'k' excitatory. 

 In the third experiment, A and K 

 are reversed. 



