Page:Life Movements in Plants.djvu/254

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LIFE MOVEMENTS IN PLANTS

become circumscribed, and the intensity and duration of stimulus necessary to convert the positive to negative will be feebler and shorter. It will be very seldom that a plant is likely to be found at the optimum. Hence plants in general may be expected to give a feeble positive response under sub-minimal stimulus.

These considerations led me to look for the positive response under sub-minimal stimulation; the tracings which I have obtained with my highly sensitive Crescograph and other recorders show that my anticipations have been justified.

Positive response under sub-minimal stimulus: Experiment 89.—In normal specimens, light of strong intensity induces a retardation of growth. When the source of light is placed at a distance, the intensity of light undergoes great diminution. Under the action of such feeble stimulus I obtained an acceleration of growth even in specimens which may he regarded as moderately vigorous (Fig. 83). Similar acceleration of growth was also
Fig. 83.—Acceleration of growth under sub-minimal light stimulus. Record on moving plate; stimulus applied at 5th dot, and subsequent erection of curve exhibits acceleration of growth. Last part of curve shows recovery of normal growth on cessation of stimulus.

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