In the parallel phenomenon of the response of pulvinated organs we found that under condition of sub-tonicity, the response becomes positive and that this abnormal positive is converted into normal negative in consequence of repeated stimulation. In growth, response likewise the abnormal acceleration of growth under light in the sub-tonic specimen of _Kysoor_ was converted into normal retardation after continuous stimulation for half an hour. From the facts given above, we are justified in drawing the following conclusions:
(1) That while light induces a _retardation_ of growth in a tissue whose tonic condition is normal or above par, it brings about an _acceleration_ in a tissue whose condition is below par.
(2) That by the action of the stimulus of light itself a sub-tonic tissue is raised to a condition at par, with the concomitant restoration of normal mode of response by retardation of growth.
Another important question arises in this connection: Is the restoration of normal response due to light as a form of stimulus, or to its photo-synthetic action? An answer to this is to be found from the results of an inquiry, whether a very different form of stimulus which exerts no photo-synthetic action, such as tetanising electric shocks, also induces a similar acceleration of growth in a sub-tonic tissue.
The normal retarding effect of electric stimulus on specimens in active state of growth was demonstrated in record given in Fig. 72, where the normal rate was found greatly reduced after stimulation.
_Abnormal acceleration of growth under electric stimulus: Experiment 88._--For my present purpose I took a sub-tonic specimen of seedling of wheat, its rate of growth being as low as 0.05 µ per second. After electric stimulation the rate was found enhanced to 0.12 µ per second, or about two and-a-half times. I give (Fig. 82) two records obtained with two different specimens. In the first, the record was taken on a stationary plate (Fig. 82). The closeness of successive dots in N show the feeble rate of growth of the sub-tonic specimen, the wider spacing after stimulation, S, exhibit the induced enhancement of growth.
[Illustration: FIG. 82.--Enhancement of rate of growth in sub-tonic specimens of wheat seedling. First series of record on stationary, second series (_b_) on moving plate N, record before stimulation. S, after stimulation.]