_Electric response to direct stimulation: Experiment 168._--For the determination of electric response at the directly excited proximal side A, we take a shoot with a lateral leaf. The point A, which is to undergo stimulation, is connected with one terminal of the galvanometer, the other terminal being led to an indifferent or neutral point N on the leaf. Application of any form stimulus at A, gives rise to an electric current which flows through the galvanometer from the neutral to the excited point A (Fig. 164b). _The directly stimulated point A thus becomes galvanometrically negative._ The "action" current lasts during the application of stimulus and disappears on its cessation. _Electric response to indirect stimulation: Experiment 169._--We have also seen that application of stimulus at A causes indirect stimulation of the distal point B resulting in an increase of turgor and expansion. The corresponding electric change of the indirectly stimulated point B is found in the responsive current, which flows now through the galvanometer from the indirectly stimulated B to the neutral point N (Fig. 164c). _The indirectly stimulated point thus becomes galvanometrically positive._ Having thus obtained the separate effects at A and B, we next modify the experiment for obtaining the joint effects. For this purpose the neutral point N is discarded and A and B connected directly with the indicating galvanometer. On stimulation of A that point becomes negative and B positive, and the current of response flows through the galvanometer from B to A. The deflection is increased by the joint electrical reactions at A and B (Fig. 164d). The results may thus be summarised:-- TABLE XXXIII.--ELECTRIC RESPONSE TO DIRECT UNILATERAL STIMULUS. +---------------------------------------------------------+ | Electrical change at the | Electrical change at the | | proximal side A. | distal side B. | +----------------------------+----------------------------+ | Galvanometric negativity | Galvanometric positivity | | indicative of contraction | indicative of expansion | | and diminution of turgor. | and increase of turgor. | |---------------------------------------------------------+ | The corresponding tropic curvature is positive movement | | towards stimulus. | +---------------------------------------------------------+ Galvanometric negativity is thus seen to indicate the effect of direct stimulus, and galvanometric positivity that of indirect stimulus. We thus see the possibility of electric detection of the effects of geotropic stimulation. This method would, moreover, enable us to discriminate the side of the organ which undergoes greater excitation. EXPERIMENTAL ARRANGEMENTS FOR OBTAINING GEO-ELECTRIC RESPONSE.