It has thus been shown that a perfect parallelism exists in the conductivity variation induced in the plant and in the animal by the directive action of the current. No explanation could be regarded as satisfactory which is not applicable to both cases. Now with the plant we are able to arrange the experimental condition in such a way that the factor of variation of excitability is completely eliminated. The various effects described about the plant-tissue are, therefore, due entirely to variation of conductivity. The parallel phenomena observed in the case of transmission of excitation in the animal nerve must, therefore, be due to the induced change of conductivity. The action of an electrical current in inducing variation of conductivity may be enunciated under the following laws, which are equally applicable to the conducting tissue of the plant and the nerve of the animal:-- LAWS OF VARIATION OF NERVOUS CONDUCTION UNDER THE ACTION OF ELECTRIC CURRENTS. 1. The passage of a current induces a variation of conductivity, the effect depending on the direction and intensity of current. 2. Under feeble intensity, heterodromous current enhances, and homodromous current depresses, the conduction of excitation. 3. The after-effect of a feeble current is a transient conductivity variation, the sign of which is opposite that induced during the continuation of current. SUMMARY. The variation of conductivity induced by the directive action of current has been investigated by two different methods:-- (1) The method in which the normal speed and its induced variation are automatically recorded; (2) That in which the variation in the intensity of transmitted excitations is gauged by the varying amplitudes of resulting responses. The great difficulty arising from leakage of the exciting induction current into the polarising circuit was successfully overcome by the interposition of a choking coil. The following summarises the effects of direction and intensity of an electric current, on transmission of excitation through the conducting tissue of the plant. The velocity of transmission is enhanced against the direction of a feeble current, and retarded in the direction of the current. Feeble heterodromous current enhances conductivity, homodromous current, on the other hand, depresses it. Ineffectively transmitted excitation becomes effectively transmitted under heterodromous current. Effectively transmitted excitation, on the other hand, becomes ineffectively transmitted under the action of homodromous current. The after-effect of a current is a transient conductivity change, the sign of which is opposite to that induced during the passage of current. The after-effect of a heterodromous current is, thus, a transient depression, that of homodromous current, a transient enhancement of conductivity.