For determination of normal velocity of transmission of excitation and the induced variation of that velocity, I employed the automatic method of recording the velocity of transmission of excitation in _Mimosa_, where the excitatory fall of the motile leaf gave a signal for the arrival of the excitation initiated at a distant point. In this method the responding leaf is attached to a light lever, the writer being placed at right angles to it. The record is taken on a smoked glass plate, which during its descent makes an instantaneous electric contact, in consequence of which a stimulating shock is applied at a given point of the petiole. A mark in the recording plate indicates the moment of application of stimulus. After a definite interval the excitation is conducted to the responding pulvinus, when the excitatory fall of the leaf pulls the writer suddenly to the left. From the curve traced in this manner the time-interval between the application of stimulus and the initiation of response can be found, and the normal rate of transmission of excitation through a given length of the conducting tissue deduced. The experiment is then repeated with an electric current flowing along the petiole with or against the direction of transmission of excitation. The records thus obtained enable us to determine the influence of the direction of the current on the rate of transmission. I shall presently describe the various difficulties which have to be overcome before the method just indicated can be rendered practical. The scope of investigation will be best described according to the following plan[N]:-- [N] For fuller account _see_ Bose--‘The influence of Homodromous and Heterodromous Electric Current on Transmission of Excitation in Plant and Animal.’ Proc. R. S. B., Vol. 88, 1915. PART I.--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF EXCITATION IN PLANTS. General method of experiment. Effect of feeble current on velocity of transmission of excitation ‘up-hill’ or ‘down-hill.’ Determination of variation of conductivity by the method of minimal stimulus and response. The after-effect of current. PART II.--INFLUENCE OF DIRECTION OF ELECTRIC CURRENT ON CONDUCTION OF EXCITATION IN ANIMAL NERVE. The method of experiment. Variation of velocity of transmission under the action of current. Variation in the intensity of transmitted excitation. PART I.--INFLUENCE OF DIRECTION OF CURRENT ON TRANSMISSION OF EXCITATION IN PLANT. THE METHOD OF EXPERIMENT.