Fig. 77.—Action of light of increasing intensities: 1: 2: 3 in retardation of growth.
Experiment 80.—I next studied the action of light, the intensity of which was increased in arithmetical progression. The intensity of white light given by a half-watt incandescent electric lamp of 200 candle power, placed at a distance of a metre, is taken as the unit. Much feebler light would have been sufficient, but it would have required much longer exposure. The intensity was increased by bringing the lamp nearer the plant; marks were made on a horizontal scale so that the intensity of incident light increased at the successive marks of the scale as 1: 2: 3: and so on. The duration of exposure was same in all cases, namely, 5 minutes. After each experiment suitable periods of rest were allowed for the plant to recover its normal rate of growth. Records in Fig. 77 show increasing retardation induced by stronger intensities of light. Table XVIII gives the result of a different experiment.
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. |
EFFECT OF CONTINUOUS LIGHT.
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. 78a) 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.
Fig. 78.—Effects of continuous (a) electric and (b) photic stimulation of moderate intensity, taken on a moving plate.