The series of experiments were carried out in the following order. The specimen was first rotated through +90° so that A was above. The responsive electric variation rendered it galvanometrically positive. The root was rotated back to neutral position when the current disappeared. The root was next rotated through -90° and the responsive current became reversed, the upper B becoming electro-positive (Fig. 173). The alternative rotations through +90° and -90° were carried out six times in succession with consistent results. The interval allowed between one stimulation and the next was determined by the period of complete recovery. Growing fatigue was found to increase this period; at first it was seven minutes, at the second repetition it was ten minutes, and at the third time it was prolonged to fifteen minutes. [Illustration: FIG. 173.--Diagrammatic representation of geo-electric response of growing region of root. (_a_) Rotation through -90° makes B, galvanometrically positive. (_b_) Vertical and neutral position. (_c_) Rotation through +90° places A above and renders it galvanometrically positive. (_d_) Additive effect on current of response, root-tip a negative, and growing region A positive.] I give below the series of electric responses induced by alternate rotations through +90° and -90°. The upper position was occupied by A in the odd series, and by B in the even series. In every case the upper side became galvanometrically positive. TABLE XXXVIII.--GEO-ELECTRIC RESPONSE OF ROOT IN THE REGION OF GROWTH. +---------------------------------------------------------------+ | Odd |Galvanometer deflection| Even |Galvanometer deflection| |series.| A, positive. |series.| B, positive. | +-------+-----------------------+-------+-----------------------+ | 1 | 20 divisions. | 2 | 18 divisions. | | 3 | 16 " | 4 | 18 " | | 5 | 10 " | 6 | 12 " | +---------------------------------------------------------------+ ADDITIVE ACTION-CURRENT AT THE TIP AND THE GROWING REGION. It has been shown that under geotropic stimulus the upper side of the tip, _a_, becomes galvanometrically negative, while the point A, higher up in the growing region, becomes galvanometrically positive. If now we make the two galvanometric connections with _a_ and A, the induced electric difference is increased, and the galvanometric response becomes enhanced.