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.
[29] I shall in what follows take the direction of vertical lines of gravity as that of movement of falling bodies, from above towards the centre of the earth.
[30] "Plant Response"—p. 505.
[31] Haberlandt—"Physiological Plant Anatomy"—p. 603.
[32] Jost—Ibid, p. 437.
[33] Exception to this will be found in page 336, where explanation is offered for the difference.