The effect of mechanical stimulus on growth is thus similar to that induced by electrical stimulus. Moderate stimulus of rough contact induces an incipient contraction, seen in retardation of growth, the recovery being complete in the course of an hour; but intense stimulation, induced by wound, gives rise to greater and more, persistent retardation of growth.
Mechanical stimulus induces incipient contraction or retardation of rate of growth, the effect being similar to that induced by electric stimulus.
Stimulus by contact or friction induces a retardation which is, relatively speaking, moderate. On the cessation of stimulus the normal rate of growth is restored within an hour.
Intense stimulation caused by the wound gives rise to greater and more persistent retardation of growth.
The next subject of inquiry is the _normal_ effect of light on growth. I speak of the normal effect because, under certain definite conditions, to be described in a later Paper, the response undergoes a reversal. The Crescograph is so extremely sensitive that it records the effect of even the slightest variation of light. Thus, as I have already mentioned, the opening of the blinds of a moderately-lighted room induces, within a short time, a marked change in the record of the rate of growth. The conditions of the experiment would thus become more precise if the growth-rate in the absence of light is taken as the normal. The specimens are, therefore, kept for several hours in darkness before the experiment. But this should not be carried to the extent of lowering the healthy tone of the plant.
I shall, in the present Paper, determine the characteristic response to light in variation of growth, the latent period of response, the effects of light of increasing intensity and duration, and the effects of the visible and invisible rays of the spectrum.
The plant was placed in a glass chamber kept in humid condition. The sources of light employed were: (1) an arc-lamp with self-regulating arrangement for securing steadiness of light, and (2) an incandescent electric lamp. Two inclined mirrors were placed close behind the specimen so that it should be acted on by light from all sides.
[Illustration: FIG. 75.--Normal effect of light. N, normal; S, retarded rate of growth in response to light; N, recovery on cessation of light.]