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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 he noticed (Fig. 76) that a retardation of growth was induced
Fig. 76.—Latent period and time-relations of response to light, applied at thick line. Successive dots at intervals of 5 sec. 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.

Experiment 80.—I next studied the action of light, the intensity of which was increased in arithmetical