The cause of this difference in the reactions to geotropic and other stimuli lies in the fact that in the latter case, energy is supplied from outside. But in geotropism the force of gravity is by itself inoperative; it is only through the weight of the cell contents that the stimulus becomes effective. Want of recognition of this fundamental difference has led many observers in their far-fetched and sweeping attempt, to establish an identity of reaction of the root to geotropic and photic stimulations, in spite of facts which plainly contradict it. Thus the root moves away from the incident vertical line of gravity; but under light, the root very often moves towards the stimulus. The negative phototropic response of the root of _Sinapis_ is an exceptional phenomenon for which full explanation has been given in page 376. We shall next consider whether the particular distribution of the falling starch-grains (which offers a rational explanation of geotropic stimulation) in the shoot and in the root, is capable of furnishing an explanation of the different geotropic responses in the two organs. In this connection, the results of investigation of Haberlandt and Nemec are highly suggestive. Haberlandt finds statoliths present in the responding region of the stem; the geotropic stimulation of the stem is therefore direct. Nemec's investigation on the distribution of statoliths in the root show, on the other hand, that it is the central portion of the root cap that contains the falling starch grains, and this would account for the indirect geotropic stimulation of the root. The theory of statoliths is, however, not essential for the explanation of the opposite geotropic effects in the shoot and in the root. The observed fact, that the perceptive region in the root is separated from the responding region, is sufficient to explain the difference of geotropic action in the two organs. Through whatever means the stimulus of gravity may act, it is inevitable, from the fact that the stimulation of the shoot is direct and of the root indirect, that an identical stimulus should in two cases induce responsive reactions of opposite signs. It will thus be seen that the postulation of two different irritabilities in the shoot and in the root is wholly unnecessary and unwarranted by facts. For the irritability of the root has been shown to be in no way different from that of other organs; an uniformity is thus found to exist in the reaction of all vegetable tissues. SUMMARY. On subjection of the tip of the root to the stimulus of gravity, the upper side exhibits excitatory reaction of galvanometric negativity. This shows that the root-tip undergoes direct stimulation. The electric response in the growing region above the stimulated point of the root-tip is positive, indicative of increase of turgor and expansion. This is due to the effect of indirect stimulus.