Currents on Transmission of Excitation. 



485 



the polarising current is intense or of prolonged duration, in which case it is 

 always retarded. The presence of two polar regions, a cathodic accelerating 

 and an anodic retarding, causes the one change to counterbalance the other."* 

 The above would appear to indicate that a current has either no effect or a 

 retarding action on conduction of excitation. These conflicting results are no 

 doubt due to the disturbing influence of the two poles. But this is not the 

 only source of uncertainty in this investigation. Far more serious is the 

 difficulty which arises, as we shall see, from the escape of the induction 

 current employed as the test stimulus. In the course of this paper I shall 

 show how these experimental difficulties have been overcome, and how definite 

 is the characteristic variation of conductivity caused by the directive action 

 of an electric current. The object of my present paper is primarily the 

 demonstration of the selective conductivity induced in the conducting tissue 

 of the plant by the passage of an electric current. After giving the results 

 of this enquiry, I next deal with the question whether the various effects 

 observed in the plant have their parallel in the case of the animal also. 



Method of Conductivity Balance. — I have previously carried out an electrical 

 method of investigation dealing with the influence of electric current on 

 conductivity. The method of Conductivity Balance which I devised for this 

 purposef was found very suitable. Isolated conducting tissues of certain 

 plants were found to exhibit transmitted effect of excitatory electric change 

 of galvanometric negativity, which at the favourable season of the year was 

 of sufficient intensity to be recorded by a sensitive galvanometer. A long 

 strand of the conducting tissue was taken and two electric connections were 

 made with a galvanometer, a few centimetres from the free ends. Thermal 

 stimulus was applied at the middle, when two excitatory waves with their 

 concomitant electric changes were transmitted outwards. By suitably moving 

 the point of application of stimulus nearer or further away from one of the two 

 electric contacts, an exact balance was obtained. This was the case when the 

 resultant galvanometer deflection was reduced to zero. If now an electrical 

 current be sent along the length of the conducting tissue, the two excitatory 

 waves sent outwards from the central stimulated point will encounter the 

 electric current in different ways ; one of the excitatory waves will travel 

 with and the other against the direction of the current. If the power of 

 transmitting excitation is modified by the direction of an electric current, 

 then the magnitudes of transmitted excitations will be different in the two 

 cases, with the result of the upsetting of the conductivity balance. From the 



* Gotch, " Polar Excitation of Nerve " in 1 Text-Book of Physiology,' edited by 

 Schafer, 1900, p. 502. 

 t Bose, "Comparative Electro-physiology" (1907), Longmans, Green and Co. 



2 Q 2 



