METHOD OF CONDUCTIVITY BALANCE.
I have previously carried out an electric method of investigation, dealing with the influence of electric current on conductivity. The method of Conductivity Balance which I devised for this purpose[M] 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 results of experiments carried out by this method on the effect of feeble current on conductivity, the conclusion was arrived at that excitation is better conducted against the direction of the current than with it. In other words, the influence of an electric current is to confer a preferential or selective direction of conductivity for excitation, the tissue becoming a better conductor in an electric up-hill direction compared with a down-hill.
The results were so unexpected that I have for long been desirous of testing the validity of this conclusion by independent method of inquiry. I shall presently give full account of the perfected method, and the various difficulties which had to be overcome to render it practical. Before doing this I shall describe a simple method which I have devised for demonstrating the principal results.
CONTROL OF TRANSMITTED EXCITATION IN AVERRHOA BILIMBI.
The petiole of Averrhoa bilimbi has a large number of paired leaflets, which, on excitation, undergo downward closure. Feeble stimulus is applied at a point in the petiole, and the transmission of excitation is visibly manifested by the serial fall of the leaflets. The distance to which the excitation reaches is a measure of normal power of conduction. Any variation of conductivity, by the passage of an electric current in one direction or the other is detected by the enhancement or diminution of the distance through which excitation is transmitted. I shall describe the special precautions to be taken in carrying out this investigation.
Fig. 44.—Diagram of experimental arrangement for control of transmitted excitation in Averrhoa bilimbi. For explanation see text.
Electric stimulus of induction shock of definite intensity and duration is supplied at the middle of the petiole at EE′ (Fig. 44). The leaflets to the left of E, are not necessary for the purpose of this experiment and therefore removed. The intensity of the induction shock may be varied in the usual manner by removing the secondary coil nearer or farther from the primary. The duration of the shock is always maintained constant. On application of electric stimulus excitation is transmitted along the petiole, the distance of transmission depending on the intensity of stimulus. With feeble stimulus two pairs of leaflets may undergo an excitatory fall; with stronger stimulus the transmission is extended to the end of the petiole, and all the leaflets exhibit movements of closure. We shall now study the modifying influence of a constant current on conduction of excitation. C is an electric cell, R the reversing key by which the electric current could be sent from right to left or in the opposite direction. When the current is sent from right to the left, the excitatory impulse initiated at EE′ travels against the direction of the current in an ‘up-hill’ direction. When the current is reversed it flows in the petiole from left to right and the transmitted impulse travels with the current or in a ‘down-hill’ direction.
Two complications are introduced on the completion of the electric circuit of the constant current: the first, is the distributing effect of leakage of the induction current used for excitation, and second, the polar variation of excitation induced by the constant current.