Page:Life Movements in Plants.djvu/229

Rh With regard to the question of immediate and after- eﬂ'ect'of stimulus, I ﬁnd great ditﬁcnlty in d'awing a line of demarcation. Owing to physiologiml 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-ell'ects in r Iality. 'l‘he latent period is shortened under strong stimulus, but the contractile r aaction becomes more parsistent. 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 ‘tono’ ol' the tissue, and as we shall presently see bring about, as the al'ter—etl'ect, an enhancement of the rate of growth.

The etl'ect of electric stimulus is thus an ircipient or actual contraction.

In normal conditions electric stimulus induces an inci- pient contraction exhibited by the retardation of the. 'ate of growth. Growth is often all'ected by an electric stimu— lus 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 elf the organ under stronger stimulus.

The latent period of responsivc variation of growth is shortened under stronger stimulus, but the period of recovery becomes protracted.