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.

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).

From the in­ves­ti­ga­tions that have been described in this and in the previous Papers an insight is obtained into the complexity of response arising from various factors. It has been shown that the sign of response is modified by the intensity of stimulus, by its point of application, and by the tonic condition of the responding tissue. The fundamental reactions have been found to be essentially the same in pulvinated, in growing and non-growing organs. The results described enable us to enunciate general Laws of Effects of Direct and Indirect stimulus on tissues in normal and in sub-tonic condition.

LAWS OF EFFECTS OF DIRECT AND INDIRECT STIMULUS.

1. THE EFFECT OF DIRECT STIMULUS IS NEGATIVE VARIATION OF TURGOR, NEGATIVE MECHANICAL AND ELECTRICAL RESPONSE, NEGATIVE VARIATION, OR RETARDATION OF RATE OF GROWTH.

a. SUB-MINIMAL STIMULUS GIVES POSITIVE RESPONSE.