Page:Life Movements in Plants.djvu/226

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LIFE MOVEMENTS IN PLANTS

induction shock which is easily graduated by the use of the well known sliding induction coil, in which the approach of the secondary to the primary coil, indicated by the higher reading of the scale, gives rise to increasing intensity of stimulus. The retarding effect of electrical stimulus on growth has already been demonstrated in record taken on a moving plate (Fig. 61).

I shall adopt for unit stimulus, that intensity of electric shock which induces a barely perceptible sensation in a human being. It is very interesting to find, as stated before, that growth is often affected by an electric stimulus, which is below the range of human perception.

Effect of Intensity: Experiment 74.—I shall now describe a typical experiment on the effect of intensity of
Fig. 72.—Effects of electrical stimulus of increasing intensities: of 0.25 unit, 1 unit, and 3 units. Short dashes represent the moments of application of stimulus.

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