_Neutralisation and reversal under increased intensity of light: Experiment 127._--It is advisable to employ thin specimens (in which the transverse distance is small) for the exhibition of reversal effect. I took a hypocotyl of _Sinapis nigra_ and subjected it to unilateral action of light from a 16 candle-power incandescent electric lamp placed at a distance of 10 cm. A maximum positive curvature was induced in the course of 50 minutes. The intensity of light was afterwards increased by bringing the lamp nearer to a distance of 6 cm. This resulted in a process of neutralisation of the preceding response; after an exposure of 70 minutes the specimen assumed a dia-phototropic position in which it remained in equilibrium. Sunlight was next applied, and in the further course of 30 minutes there was a pronounced reversal into negative phototropic curvature. [Illustration: FIG. 121.--Positive and negative phototropic responses of _Oryza_ under continued unilateral stimulus of intense light from arc lamp.] _Neutralisation and reversal under continuous stimulation: Experiment 128._--In the last experiment the different changes in the response were brought about by successive increase in the intensity of light. In the present experiment, very strong light was applied from the beginning, and continuous record was taken of the change in the response. In order to reduce the period of experiment I employed a mercury vapour lamp which emits the most effective violet and ultra-violet rays. The specimen used was a seedling of the rice plant (_Oryza sativa_). The first effect of light was a positive curvature which attained its maximum; after this there was a neutralisation in less than six minutes after the application of light. The further continuation of light induced a pronounced negative curvature (Fig. 121). I shall in the next chapter give other instances which will show that all organs (pulvinated and growing) possessed of power of transverse conduction, exhibit a transformation of response from positive to negative under continued action of strong light. Thus an identical organ, under different conditions of intensity and duration of stimulus, exhibits _positive_ phototropic, _dia_-phototropic, and _negative_ phototropic curvatures, proving conclusively that the three effects are not due to three distinct irritabilities. The responsive movements are, on the other hand, traced to a fundamental excitatory reaction, remaining either localised or increasingly transmitted to the distal side. NEGATIVE PHOTOTROPISM OF ROOTS.