+--------------------------------------------------------------+ |Normal rate of growth 0·44 µ per sec. | |Retarded rate immediately after stimulation 0·20 µ " " | |Recovery and enhancement after 15 minutes 0·50 µ " " | +--------------------------------------------------------------+ The immediate and after-effects of mechanical stimulus on the tendril are therefore the same as that of electric stimulus. The incipient contraction under direct mechanical stimulus, moreover, is not the special characteristic of tendrils, but of growing plants in general. For I have shown (page 203) that the growth of flower stalk of _Zephyranthes_ is also retarded after mechanical friction, from the normal rate 0·48 µ to 0·11 µ after stimulation. We shall find later that different plant organs, after moderate stimulation, exhibit acceleration of growth as an after-effect. The phenomenon of responsive reaction of tendril is therefore not unique, but similar to that of other organs under all forms of stimulation. The only speciality in tendril is that owing to anatomical peculiarities, the perceptive power of the organ for mechanical stimulus is highly developed. We are now in a position to offer an explanation of the induced concavity of the stimulated side of the tendril, and its recovery after brief contact. The experiments that have been described show that: (1) the proximal side contracts because it is directly stimulated, and the distal side, being indirectly stimulated, expands; the curvature is thus due to the joint effects of contraction of one side, and expansion of the opposite side, and (2) the recovery of the tendril after brief contact is hastened by the after-effect of stimulus, which is expansion and acceleration of growth. The results given above will also be found to explain Fitting's important observations[5] that (_a_) the stimulated side of the tendril undergoes transient contraction with subsequent acceleration of growth, and that (_b_) the distal or convex side undergoes an immediate enhancement of growth. [5] Pfeffer--_Ibid_--Vol. III, p. 57. I give below a record given by a tendril of _Cucurbita_ in response to unilateral contact of short duration (Fig. 105). Successive dots in the record are at intervals of three seconds. The latent period was ten seconds, and the maximum curvature was attained in the course of two and a half minutes. The curvature persisted for a further period of two minutes after which recovery was completed in the course of 12 minutes. Feeble stimulation is attended by a recovery within a short period, but under strong stimulus the induced curvature becomes more persistent. [Illustration: FIG. 105.--Positive curvature of tendril of Cucurbita under unilateral stimulus of contact at x.] INHIBITORY ACTION OF STIMULUS.