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.

Fig. 76.—Latent period and time-re­la­tions of re­sponse to light, applied at thick line. Suc­ces­sive dots at in­ter­vals of 5 sec.

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.