The complexity of phototropic reaction arises from the summated effects of numerous factors; for explanation of the resultant response it is therefore necessary to take full account of the individual effect of each of them. Among these operative factors in phototropic reaction may be mentioned:-- (1) The difference of effects induced by light at the proximal and distal sides of the organ. (2) The modification of the latent period with the intensity of stimulus. (3) The after-effect of stimulus. (4) The modifying influence of tonic condition on response. (5) The effect of direction of light. (6) The effect of intensity of light. (7) The effect of duration of stimulation. (8) The transmitted effect of light. (9) The effect of unequal excitability in different zones of the organ. (10) The effect of transverse conduction in modification of the sign of response. (11) The effect of temperature on phototropic action. (12) The modification of response due to differential excitability of the organ. (13) Nastic and tropic reactions. (14) The torsional effect of light. The sketch given above will give us some idea of the complexity of the problem. In this and in the following papers I shall describe the investigations I have carried out on the subjects detailed above. ACTION OF LIGHT. I have shown that there is no essential difference between the responses of pulvinated and growing organs, that diminution of turgor induced by stimulus brings about contraction in the one, and retardation of the rate of growth in the other. Indirect stimulation, on the other hand, induces an expansion and acceleration of the rate of growth. The experimental investigation on the tropic effect of light may therefore be carried out both with pulvinated and growing organs. As regards the effect of direct stimulus of light on growing organs we found (p. 208) that it induces an incipient contraction, seen in diminution of the rate of growth; this incipient contraction culminates in an actual contraction under increasing intensity of light. The contraction under direct stimulation is also observed in pulvinated organs. When light acts from above the upper half of the pulvinus undergoes contraction, resulting in erection of the motile leaf or leaflets. As regards the effect of indirect unilateral stimulus of light on the distal side of the organ, we found that its effect is an enhancement of turgor (p. 281). Hence the positive tropic curvature under light is brought about, as in the case of other forms of stimuli, by the contraction of the proximal, and expansion of the distal sides of the organ.