TABLE XVIII.--EFFECT OF LIGHT OF INCREASING INTENSITY ON THE RATE OF GROWTH.
+-----------------------------------------+ | Intensity of light. | Rate of growth. | +---------------------+-------------------+ |0 (Normal) | 0.47 µ per sec. | |1 Unit | 0.28 µ " | |2 " | 0.17 µ " | |3 " | 0.10 µ " | |4 " | Arrest of growth. | +---------------------+-------------------+
_Experiment 81._--The continued effect of light of moderate intensity in bringing about increasing retardation of growth will be seen in Fig. 78(_b_) side by side with the record of effect of continuous electric stimulation (Fig. 78_a_) on growth. In both the cases the effect of continuous stimulation is seen to be the same, namely, a growing retardation, which in the given instances culminated in arrest of growth. This is true of stimulus of moderate intensity. Under a more intense stimulation the incipient contraction does not end in a mere arrest of growth, but the responding organ undergoes an actual shortening.
[Illustration: FIG. 78.--Effects of continuous (_a_) electric and (_b_) photic stimulation of moderate intensity, taken on a moving plate.]
EFFECTS OF DIFFERENT RAYS OF THE SPECTRUM.
Different observers have found[V] that it is the more refrangible rays which exercise the greatest influence upon growth and tropic curvature. The relative effects of different lights will, however, become more precise from the curves of response to the action of different rays. For this purpose, I first employed monochromatic lights from different parts of the spectrum, produced by prism of high dispersion. In practice, the usual colour filters were found very convenient, as they allowed the application of more intense light. A thick stratum of bichromate of potash solution transmitted red rays, a thinner stratum allowed the transmission of yellow in addition; ammoniated copper sulphate solution allowed the blue and violet rays to pass through. It should be borne in mind that certain complicating factors are introduced by the incidence of light on the organ; there may be a slight rise of the temperature. We have seen however that moderate rise of temperature induces an acceleration of the rate of growth (p. 175). I shall later describe other experiments which will demonstrate the antagonistic effects of light and warmth on growth. Warmth again may induce a certain amount of dessication, but this is reduced to a minimum by maintaining the plant-chamber in a humid condition. The heating effect of the red is, relatively speaking, much greater than that of the blue rays. But in spite of this it is found that while red rays are practically ineffective, the blue rays are most effective in inducing responsive retardation of growth.