Page:Researches on Irritability of Plants.djvu/105

82 to be borne in mind that the effect of continuous stimulation is, after all, the effect of successive stimuli with the resting interval shortened. On referring back to fig. 38 we notice two phases in the response-series: in the first phase the excitability is increasing; in the second phase, it is decreasing. In the first phase again, we notice that there is a residual contraction, the recovery being incomplete. Owing to this, the base-line is gradually shitting upwards. This, coupled with the enhancing excitability and consequent staircase increase in the individual responses, brings about a maximum additive contraction, as will be understood, by joining the tops of these contractile responses. The additive effect of such contractions would be a responsive fall much greater than could have taken place under any single stimulation.

If we were now to repeat this experiment, shortening the intervals between the successive stimuli, we should obtain a somewhat similar result, with the sole difference that the successive component responses would appear nearer each other and with their recoveries still further reduced. The result of this would be slight notches in an ascending curve. Carrying this process to a limit—that is to say, when the successive stimuli follow each other quickly, as in continuous tetanisation—the notches themselves will disappear and we shall have merely an ascending curve.

Turning to the second phase in the response-series, where the excitability has reached a maximum, we find these phenomena reversed. The leaf having attained its maximum limit of fall, its capacity for further contraction is now reduced. In sharp contrast to the first phase of the series, however, successive contractions now grow smaller and smaller, under growing fatigue, while the relaxations tend to become increasingly large. In the extreme case of continuous tetanisation the resulting record in this phase would be one of relaxation, appearing as a down-curve. Thus under tetanisation we should have a response-curve, showing first the normal contraction, followed in the second place by