choking coil which will be described below, A simpler, though less perfect, device may be employed to reduce and practically eliminate the leakage. This consists of a loop, L, of silver wire placed outsicfe EE', The leakage of induc- tion current is thus diverted along this path of negligible resistance in preference to the longer circuit through the entire petiole, which has a resistance of several million ohms.
Polar action of current on excitability. — It is well known that an electric current induces a local depression of ex- citability at the point of entrance to the tissue, or at the anode, and an enhancement of excitability at the point of exit, or at the cathode. But the excitability is unaffected at a point equally distant from anode and cathode. This is known as the indifferent point. The exciting electrodes EE' are placed at the indifferent point. But when the current enters on the right side, the terminal leaflets to the right have their excitability depressed by the proximity of anode, but the leaflets near the electrodes EE', being at a distance from the anode are not affected by it. Moreover it will be shown that the enhanced conductivity conferred by the directive action of the current overpowers any depression of excitability in the terminal leaflets due to the proximity of the anode. I shall, for convenience, de- signate the transmission as ' up-hill ', when excitation is propagated against the direction of the constant electric current, and ' down-hill ' when transmitted with the direc- tion of the current.
Transmission of excitation ' Up-hill ' : Experiment 38. — I shall give here an account of an experiment which may be taken as typical. I took a vigorous specimen of Averrhoa bilimbi, and applied a stimulus whose intensity was so adjusted that the propagated impulse brought about a fall of only two pairs of leaflets. This gave a measure of normal conduction without the passage of the current. The constant electric current was now sent from right to
