[Illustration: FIG. 84.--Renewal of autonomous activity in _Desmodium gyrans_ at standstill by action of light. Up-curve represents up-movement. The horizontal lines below represent durations of exposure to light.] As regards the action of light, the pulsation continued for a time, even on the cessation of light. This persistence of autonomous activity increases with the intensity and duration of incident stimulus, that is to say with the amount of incident energy. In the present case a duration of five minutes’ exposure gave rise to a single pulsation, after which the movement of the leaflet came to a stop. The next application lasted for ten minutes and this gave rise to four pulsations, two during application, and two after cessation of light. The next application was for forty-five minutes, and the pulsation persisted for nearly an hour after the cessation of light. The experiments on sub-tonic specimens show clearly that the energy supplied by the environment becomes as it were latent in the plant, increasing its potentiality for work. The renewal of autonomous activity in a sub-tonic tissue by the action of external stimulus, will be found in every way parallel to the renewal of growth in a sub-tonic organ. REVIVAL OF GROWTH UNDER STIMULUS. [Illustration: FIG. 85.--Record of responses of a mature style in which growth had come to a stop. Up-curve shows contraction under stimulus. Renewal of growth at sixth response, after which growth-elongation is shown by the trend of the base-line downwards.] _Renewal of growth under stimulus: Experiment 91._--I find that application of electric stimulus renews growth in specimens where, on account of extreme sub-tonicity growth has come to a state of standstill. The resumption of growth in grass haulms under the stimulus of gravity is a phenomenon probably connected with the above. The causes which bring about cessation of growth in a mature organ are unknown; that there is a potentiality of growth even in a fully grown grass haulm is evidenced by the fact of its renewed growth under fresh stimulation. That this is not an exceptional phenomenon appears from the record which I obtained with a fully grown style of _Datura alba_. I subjected it to periodic stimulation, and obtained from it a series of contractile responses. After recovery from stimulus it regained its normal length which remained constant for some time as seen in the horizontal base-line. But as a result of successive stimulations, the mature style resumed its growth with increasing acceleration. This is seen in the recovery overshooting its former horizontal limit (Fig. 85).