490 



Prof. J. C. Bose. Influence of Electric 



The electrical resistance offered by the 95-mm. length of stem and petiole 

 will be from two to three million ohms. The intensity of the constant 

 current flowing through the plant can be read by unplugging the key which 

 short-circuits the microammeter G. The choking coil C prevents the 

 alternating induction current from flowing into the polarising circuit and 

 causing direct stimulation of the pulvinus. 



Before describing the experimental results, it is as well to enter briefly into 

 the question of the external indication by which the conducting power may 

 be gauged. Change of conductivity may be expected to give rise to a 

 variation in the rate of propagation or to a variation in the magnitude of the 

 excitatory impulse that is transmitted. Thus we have several methods at 

 our disposal for determining the induced variation of conductivity. In the 

 first place, the variation of conductivity may be measured by the induced 

 change in the velocity of transmission of excitation. In the second place, 

 the transmitted effect of a sub- maximal stimulus will give rise to enhanced 

 or diminished amplitude of mechanical response, depending on the increase 

 or decrease of conductivity brought about by the directive action of the 

 current. And, finally, the enhancement or depression of conductivity may 

 be demonstrated by the ineffectively transmitted stimulus becoming effective, 

 or the effectively transmitted stimulus becoming ineffective. 



Exclusion of the Factor of Excitability. — The object of the enquiry being 

 the pure effect of variation of conductivity, we have to assure ourselves that 

 under the particular conditions of the experiment the complicating factor of 

 polar variation of excitability is eliminated. It is to be remembered that 

 excitatory transmission in Mimosa takes place by means of a certain conduct- 

 ing strand of tissue which runs through the stem and the petiole. If a point 

 on the petiole of a given leaf be subjected to strong stimulation, an excitatory 

 impulse will not merely be transmitted across its own pulvinus, but will 

 travel farther along the stem, inducing the fall of other leaves. Conversely, a 

 strong stimulus applied on the stem gives rise to an impulse which passes 

 through the pulvinus to the petiole and thence to the sub-petiole, as evidenced 

 by the successive closure of its leaflets. The main pulvinus may thus be 

 regarded as a contractile indicator of excitation, interposed in the path of 

 the conducting strand which connects the stem with the petiole. In the 

 experiment to be described, the polarising current enters by the tip of the 

 petiole and leaves by the stem, or vice versa, the length of the intrapolar 

 region being 95 mm. The point of application of stimulus on the petiole is 

 40 mm. from the electrode at the tip of the leaf. The responding pulvinus 

 is also at the same distance from the electrode on the stem. The point of 

 stimulation and region of response are thus at the relatively great distance 



