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. [Illustration: 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.