There is a general impression that it takes from several minutes to more than an hour for the light to react on the growing organ. This underestimate must have been due to the want of sufficient delicate means of observation. For my recorders indicate in some cases a response within less than 2 seconds of the incidence of light. This was found, for example, in the record of response given by a seedling of _Cucurbita_, to a flash of ultra-violet light. In the majority of cases the response is observed within 15 seconds of the incidence of light. _Experiment 79._--For the determination of the latent period, a record of the effect of arc light of 30 seconds’ duration was taken on a moving plate. It will be noticed (Fig. 76) that a retardation of growth was induced within 35 seconds of the incidence of light. The incipient contraction induced by light is thus similar to that induced by any other form of stimulus. Growth became restored to the normal value, 5 minutes after the cessation of stimulus. EFFECT OF INTENSITY OF LIGHT. [Illustration: FIG. 77.--Action of light of increasing intensities: 1: 2: 3 in retardation of growth.] _Experiment 80._--I next studied the action of light, the intensity of which was increased in arithmetical progression. The intensity of white light given by a half-watt incandescent electric lamp of 200 candle power, placed at a distance of a metre, is taken as the unit. Much feebler light would have been sufficient, but it would have required much longer exposure. The intensity was increased by bringing the lamp nearer the plant; marks were made on a horizontal scale so that the intensity of incident light increased at the successive marks of the scale as 1: 2: 3: and so on. The duration of exposure was same in all cases, namely, 5 minutes. After each experiment suitable periods of rest were allowed for the plant to recover its normal rate of growth. Records in Fig. 77 show increasing retardation induced by stronger intensities of light. Table XVIII gives the result of a different experiment. TABLE XVIII.--EFFECT OF LIGHT OF INCREASING INTENSITY ON THE RATE OF GROWTH. +-----------------------------------------+ | Intensity of light. | Rate of growth. | +---------------------+-------------------+ |0 (Normal) | 0.47 µ per sec. | |1 Unit | 0.28 µ " | |2 " | 0.17 µ " | |3 " | 0.10 µ " | |4 " | Arrest of growth. | +---------------------+-------------------+ EFFECT OF CONTINUOUS LIGHT.