With regard to the question of immediate and after-effect of stimulus, I find great difficulty in drawing a line of demarcation. Owing to physiological inertia there is a delay between the application of stimulus and the initiation of responsive reaction (latent period); owing to the same inertia, the physiological reaction is continued even on the cessation of stimulus. All responsive reactions are thus after-effects in reality. The latent period is shortened under strong stimulus, but the contractile reaction becomes more persistent. When the stimulus is moderate or feeble, the recovery from incipient contraction takes place within a short time. Stimulus, under certain circumstances, is found to improve the ‘tone’ of the tissue, and as we shall presently see bring about, as the after-effect, an enhancement of the rate of growth. The effect of electric stimulus is thus an incipient or actual contraction. SUMMARY. In normal conditions electric stimulus induces an incipient contraction exhibited by the retardation of the rate of growth. Growth is often affected by an electric stimulus which is below human perception. Under increasing intensity of stimulus, the contractile reaction opposing growth elongation becomes more and more pronounced. At a critical intensity of stimulus growth becomes arrested. Under stronger intensity of stimulus growing organ undergoes an actual shortening in length. There is continuity between the incipient contraction seen in retardation, arrest of growth, and contraction of the organ under stronger stimulus. The latent period of responsive variation of growth is shortened under stronger stimulus, but the period of recovery becomes protracted. XV.--EFFECT OF MECHANICAL STIMULUS ON GROWTH _By_ SIR J. C. BOSE. Amongst the various stimuli which induce excitation in _Mimosa_ may be mentioned the irritation caused by rough contact, by prick, or wound. Friction causes moderate stimulation, from which the excited pulvinus recovers within a short time. But a prick or a cut induces a far more intense and persistent excitation; the recovery becomes protracted, and the wounded pulvinus remains contracted for a long period. I shall now describe the effect of mechanical irritation on growth. For moderate stimulus, I employ rough contact or friction; more intense stimulation is caused by a prick or a cut. EFFECT OF MECHANICAL IRRITATION. [Illustration: FIG. 74.--(_a_) N, normal rate of growth; F, retarded rate immediately after friction; A, partial recovery after 15 minutes. (_b_) N, normal; W, immediately after wound; C, an hour after. (Successive dots at intervals of 5″.)]