Under unilateral mechanical stimulus of short duration the directly excited proximal side undergoes contraction, the indirectly stimulated distal side exhibits the opposite effect of expansion. The induced curvature is thus due to the joint effects of the contraction of one side, and the expansion of the opposite side.
As the after-effect of direct stimulus is an acceleration of growth above the normal, the stimulated side undergoes an expansion by which the recovery is hastened.
Unilateral application of direct stimulus induces a positive curvature, but the same stimulus applied at a distance from the responding region induces a negative curvature.
The tendril of Passiflora is excitable both on the upper and under sides: the excitability of the under side is about seven times greater than that of the upper side.
Stimulation of one side of the tendril induces an expansion of the opposite side, even in cases where the contractility of the stimulated side is feeble.
The response to stimulation of the more excitable side of the tendril is thus inhibited by the stimulation of the opposite side. This is because of the neutralisation of the effect of direct by that of indirect stimulation.
[4] Jost—Ibid—p. 490.
[5] Pfeffer—Ibid—Vol. III, p. 57.