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Rh Action of Ultra-violet rays : Experiment 97. — The source of light was a quartz mercury-vapour lamp. The effect was 80 intense that, to keep the record within the plate, I had to reduce the period of exposure to half, i.e., to five seconds. The responsive movement was initiated within six seconds of the application of light. The intensity and the rapidity of reaction is independently evidenced by the more erect curve of response (Fig. 91).

Fig. 91.— Effect of ultra-violet rays on the pulvinus of Mimosa

Action of Infra-red rays : Experiment 98. — The obscure thermal rays also caused a strong excitatory reaction (Fig. 921. Attention is here drawn once more to the antasfonistic reactions of temperature and radiation effects of heat.

It has been shown that the rays which cause the most intense excitations in Mimosa also induce the greatest retardation in the rate of growth. Thus ultra-violet is not only the most effective in causing excitation in Mimosa but also in retardation of growth. Next in order comes the blue rays : the yellow and red are practically ineffective in both the cases. Infra-red rays are, however, very