Page:Researches on Irritability of Plants.djvu/94

Rh reversal, with decline of excitability, a state of things which we associate with fatigue.

It must be remembered that in Nature, according to the conditions of its environment, a plant may be found in any of the three states. One specimen may be found in the pre-optimum or subtonic condition; another may be near the optimum condition, and this we shall designate as the normal; a third may be found in the post-optimum condition predisposed to fatigue. The first and third of these conditions may be distinguished from each other by means of testing blows or stimuli. If the plant be in the former condition, these will evoke responses of increasing amplitude; in the latter, they will show a decline.

These three conditions modify not merely the amplitude of response but also exhibit themselves appropriately in other aspects of protoplasmic excitation. These will be seen in the chapters on the Latent Period, and on the Transmission of Excitation.

When—selecting a plant which is neither subtonic nor yet at its optimum—we take a series of responses under uniform stimulation of moderate intensity, allowing sufficient intervals for complete recovery, we obtain uniform responses. This may be accepted as the characteristic effect of a plant in the normal condition.

In fig. 33 is seen a series of such responses taken at intervals of 15 minutes. The ascending portion of each response is here seen to be dotted. This is because of the rapidity of the movement of fall. The successive dots caused by the recorder vibrating ten times per second are widely spaced. In the recovery or down part of the curve, however, as that process is slow, the dots become fused and make a thick continuous line. In the record of the responsive fall, variations of rate of movement may be noticed. At first the speed increases, then very gradually