Tropic curvature also increases with the duration of exposure. The intensity of induced tropic effect is determined by the quantity of incident light. XXXII.--THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS _By_ SIR J. C. BOSE. When a plant organ is subjected to the continued action of unilateral stimulus of light, it exhibits increasing tropic curvature, which in certain cases reaches a limit; in other instances a reversal takes place, seen in neutralisation, or in the conversion of the positive into negative curvature. I shall in this chapter enter into a detailed study of the phototropic curve, and determine its characteristics. As the vague terminology at present in use has been the source of much confusion, it is necessary here to define clearly the various terms which will be employed in this investigation. It is first of all necessary to distinguish between cause and effect, between external stimulus and the excitation induced by it. As regards stimulus itself I have shown elsewhere[16] that its effective intensity becomes summated by repetition. This was demonstrated by the two following typical experiments carried out with the pulvinus of _Mimosa_. [16] "Irritability of Plants"--p. 54. (1) The intensity of a single electric shock of intensity of 0·5 unit was found to be ineffective in inducing excitation; but it became effective on being repeated four times in rapid succession. (2) The same specimen was next subjected to a feebler stimulus of intensity of 0·1 unit, and it required a repetition of 20 times for the stimulus to become effective. The total stimulus in the first case was 0·5 × 4 = 2, and this was found to be the same as 0·1 × 20 = 2 in the second case. Thus the intensity of stimulus is increased by repetition; in the limiting case where the interval between successive stimulus is zero, the stimulus becomes continuous. Bearing in mind the additive effects of stimulus we see that its effective intensity increases with the _duration_ of application. This important conclusion found independent support from the results of Experiment 133 given in the last chapter.