Page:Life Movements in Plants.djvu/235

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The next subject of inquiry is the normal ell‘ect of light on growth. I speak of the normal eli'ect because, under certain definite conditions, to be described in a later ’aper, the response undergoes a reversal. ’l‘he Crescograph is so extremely sensitive that it records the eti'ect 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 condi- tions of the experiment would thus become more precise it' 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 l\Vt‘rillg 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 ell'ects of light of incr‘asing intensity and du‘ation, 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