Intense stimulation caused by the wound gives rise to greater and more persistent retardation of growth. XVI.--ACTION OF LIGHT ON GROWTH _By_ SIR J. C. BOSE, _Assisted by_ GURUPRASANNA DAS. The next subject of inquiry is the _normal_ effect of light on growth. I speak of the normal effect because, under certain definite conditions, to be described in a later Paper, the response undergoes a reversal. The Crescograph is so extremely sensitive that it records the effect of even the slightest variation of light. Thus, as I have already mentioned, the opening of the blinds of a moderately-lighted room induces, within a short time, a marked change in the record of the rate of growth. The conditions of the experiment would thus become more precise if the growth-rate in the absence of light is taken as the normal. The specimens are, therefore, kept for several hours in darkness before the experiment. But this should not be carried to the extent of lowering the healthy tone of the plant. I shall, in the present Paper, determine the characteristic response to light in variation of growth, the latent period of response, the effects of light of increasing intensity and duration, and the effects of the visible and invisible rays of the spectrum. METHOD OF EXPERIMENT. The plant was placed in a glass chamber kept in humid condition. The sources of light employed were: (1) an arc-lamp with self-regulating arrangement for securing steadiness of light, and (2) an incandescent electric lamp. Two inclined mirrors were placed close behind the specimen so that it should be acted on by light from all sides. NORMAL EFFECT OF LIGHT. [Illustration: FIG. 75.--Normal effect of light. N, normal; S, retarded rate of growth in response to light; N, recovery on cessation of light.] _Experiment 78._--I shall first give records obtained with _Kysoor_ on the action of light. The first series exhibits the normal rate of growth in darkness; in the next the retarding effect of light is seen in the shortening of spacings, as compared with the normal, between successive dots. The light was next cut off and record taken once more after half an hour. Growth is now seen to have recovered its normal rate (Fig. 75). With regard to the after-effect of light I may say in anticipation that there are two different results, which depend on the physiological condition of the tissue. In a tissue whose tonic condition is below par, the after-effect is an acceleration; but with tissues in an optimum condition, the immediate after-effect is a retardation of the rate of growth. This is specially the case when the incident light is of strong intensity and of long duration. DETERMINATION OF THE LATENT PERIOD. [Illustration: FIG. 76.--Latent period and time-relations of response to light, applied at thick line. Successive dots at intervals of 5 sec.]