[Illustration: FIG. 172.--Diagrammatic representation of geo-electric response of root-tip. The middle figure shows root in vertical position. Rotation through +90° places _a_ above, which becomes galvanometrically negative. Rotation through -90°, places _b_ above and makes it negative.] After due observance of these precautions the electric response obtained is found to be very definite; when the root is made horizontal, by rotation of the root through +90°, the point _a_ is above, and the responsive current is found to flow from _b_ to _a_, _the upper side of the tip_ becoming galvanometrically negative; when the root is brought back to the vertical, the responsive current disappears; rotation through -90° makes the point _b_ occupy the upper position, and the responsive current is from _a_ to _b_; the upper side thus exhibits in every case, an excitatory electric change of galvanometric negativity (Fig. 172). The root-tip thus exhibits the characteristic response to direct stimulation. Experiments carried out with 12 different specimens gave concordant results. The following table gives the absolute values of electro-motive force induced at the tip under geotropic stimulus. TABLE XXXVII.--GEO-ELECTRIC RESPONSE OF THE ROOT TIP (_Vicia Faba_). +--------------------------+ |Specimen.|Induced E. M. F.| +---------+----------------+ |1 | 0·0005 volt. | |2 | 0·0011 " | |3 | 0·0010 " | |4 | 0·0015 " | +--------------------------+ ELECTRIC RESPONSE IN THE GROWING REGION. _Experiment 183._--I next undertook an investigation on the electric variation induced in the growing region under the stimulus of gravity. The experimental difficulties are here greatly reduced, since the available area of contact for galvanometric connection is not so restricted as in the case of the root-tip. The specimen is securely mounted so that the root is vertical. It is next rotated in the vertical plane through +90°, so that the point A in the growing region occupied the upper position. The electric response in the growing region took place in a short time and was very distinct. The induced electric change at A was now galvanometric _positivity_ indicative of increase of _turgor and expansion_.