exhibited not merely by the motile pulvinus of _Mimosa_, but by other pulvini as well, such as those of _Erythrina indica_, and of the ordinary bean plant. _Action of red and yellow lights._--The pulvinus gave little or practically no response to these lights. _Action of blue light: Experiment 96._--Light was applied for 10 seconds and the amplitude of response was similar to that induced by white light (Fig. 90). [Illustration: FIG. 90.--Effect of blue light on pulvinus of _Mimosa_.] _Action of Ultra-violet rays: Experiment 97._--The source of light was a quartz mercury-vapour lamp. The effect was so 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). [Illustration: 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. 92). Attention is here drawn once more to the antagonistic 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 effective in exciting the sensitive _Mimosa_ and in retarding the rate of growth. [Illustration: FIG. 92.--Effect of infra-red rays on the pulvinus of _Mimosa_.] DIVERSE MODES OF RESPONSE TO STIMULUS. In _Mimosa_ excitation is followed by the striking manifestation of the fall of the leaf. But in rigid trees contraction under excitation cannot find expression in movements. I have shown elsewhere that even in the absence of realised movement, the state of excitation can be detected by the induced electromotive change. I have shown that not only every plant but every organ of every plant is sensitive and reacts to stimulus by electric response of galvanometric negativity.[X] [X] BOSE--Friday Evening Discourse--Royal Institution of Great Britain, May 1901.