Under ordinary circumstances, the upper half of the pulvinus is, on account of its favourable position, more effectively stimulated by geotropic stimulus; in consequence of this the leaf assume a more or less horizontal position of "dia-geotropic" equilibrium. But when the plant is inverted the more excitable lower half of the organ now occupies the favourable position for geotropic excitation. The leaf now erects itself till it becomes almost parallel to the stem. The response of the same pulvinus which was formerly "dia-geotropic" now becomes "negatively geotropic"; but an identical organ cannot be supposed to possess two different specific sensibilities. The normal horizontal position assumed by the leaf is, therefore, due to differential geotropic excitabilities of the two sides of a dorsiventral organ. I have explained (p. 401) that when the pulvinus of _Mimosa_ is subjected to lateral stimulation of any kind, it undergoes a torsion, in virtue of which the less excitable half of the organ is made to face the stimulus. Experiments will be described in a subsequent chapter which show that geotropic stimulus also induces similar torsional response. The results obtained from this method of enquiry give independent proof: (1) that the lower half of the pulvinus is geotropically the more excitable, and (2) that the direction of incident geotropic stimulus is the vertical line of gravity which impinges on the upper surface of the organ. SUMMARY. The stimulus of gravity is shown to induce an excitatory reaction which is similar to that induced by other forms of stimulation. The direct effect of geotropic stimulus is an incipient contraction and retardation of rate of growth. The upper side of a horizontally laid shoot is more effectively stimulated than the lower side, the excited upper side becoming concave. Electrical investigation also shows that it is the upper side that undergoes direct stimulation. Tropic reactions are said to be positive, when the directly stimulated side undergoes contraction with the result that the organ moves to meet the stimulus. According to this test, the geotropic response of the stem is _positive_. The geotropic response is delayed by the bending down of the horizontally laid shoot. Reduction of weight is found to shorten the latent period; in the case of the petiole of _Tropæolum_ this is shorter than 20 seconds. The latent period of geotropic response is found to be of the same order as the "migration period" of the hypothetical statoliths. The complete geotropic curve shows characteristics which are similar to tropic curves in general. In a dorsiventral organ the geotropic excitabilities of the upper and lower sides are different. In the pulvinus of _Mimosa_ the geotropic excitability of the lower half is greater than that of the upper half. The differential excitabilities of a dorsiventral organ modifies its position of geotropic equilibrium. XL.--GEO-ELECTRIC RESPONSE OF SHOOT _By_