From the analogy of opposite responses of shoot and root to stimulus of gravity, it was surmised that the root would respond to light by a negative curvature. This was apparently confirmed by the negative phototropic curvature of the root of _Sinapis_. The supposed analogy is however false; for while the stimulus of gravity acts, in the case of root, only on a restricted area of the tip, the stimulus of light is not necessarily restricted in the area of its action. That there is no true analogy between the action of light and gravitation is seen from the fact that while gravitation induces in the root a movement opposite to that in the stem, in the case of light, this is not always so; for though a few roots turn away from light, others move towards the light. As regards negative phototropic response of the root of _Sinapis_, it will be shown (p. 376) to be brought about by algebraical summation of the effects of direct and indirect photic stimulus. SUMMARY. The normal positive phototropic curvature is modified by transverse conduction of true excitation to the distal side of the organ. The extent of neutralisation or reversal due to internal conduction of excitation from the proximal to the distal side of the organ depends: (_a_) on the intensity of the incident stimulus, (_b_) on the conductivity of the organ in a transverse direction, (_c_) on the thickness of the intervening tissue, and (_d_) on the relative excitability of the distal as compared to the proximal side. The dia-phototropic position is not one of indifference, but of balanced antagonistic reactions of two opposite sides of the organ. The supposition that direct sunlight is phototropically ineffective is unfounded. The response is fully vigorous, but the first positive curvature may in certain cases be neutralised by the transmission of excitation to the distal side. Under light of strong intensity and long duration, the transmitted excitation to the distal side neutralises, and finally reverses the positive into negative curvature. The _positive_-phototropic, the _dia_-phototropic, and the _negative_ phototropic curvatures are not due to three distinct irritabilities but are brought about by a fundamental excitatory reaction remaining localised or increasingly transmitted to the distal side. XXXI.--THE RELATION BETWEEN THE QUANTITY OF LIGHT AND THE INDUCED PHOTOTROPIC CURVATURE _By_ SIR J. C. BOSE, _Assisted by_ SURENDRA CHANDRA DAS, M.A. I shall in this chapter describe experiments in support of the important proposition that _the intensity of phototropic action is dependent on the quantity of incident light_. The proportionality of the tropic effect to the quantity of light will be found to hold good for the median range of stimulation; the deviation from this proportionality at the two ends of the range of stimulation--the sub-minimal and supramaximal--is, as we shall find, capable of explanation, and will be fully dealt with in the next chapter.