_Effect of light of moderate intensity: Experiment 120._--The source of light was a small arc lamp placed at a distance of 50 cm., the intensity of incident light was increased or decreased by bringing the source of light nearer or further away from the plant. Two inclined mirrors were placed behind the plant so that the specimen was acted on by light from all sides. A seedling of wheat was mounted on the Balanced Crescograph, and record was first taken under exact balance; this gives a horizontal record. The up-curve represents retardation, and down-curve acceleration of rate of growth. The source of light was at first placed at a distance of 50 cm. from the plant, and exposure was given for 4 minutes at the point marked with an arrow (Fig. 115a). We shall find in the next chapter that the _intensity of phototropic effect is proportional to the quantity of incident light_. This quantity at the beginning proved to be sub-minimal, and hence there was an acceleration at the beginning. Continued action induced the normal effect of retardation, as seen in the subsequent resulting up-curve. On the cessation of light, the balance was upset in an opposite direction, the resulting down-curve showing an acceleration of the rate of growth above the normal. This acceleration persisted for a time, after which the normal rate of growth was restored, as seen in the curve becoming once more horizontal. _The after-effect of light of moderate intensity is thus a temporary acceleration of rate of growth above the normal._ _Effect of strong light: Experiment 121._--The same specimen was used as in the last experiment. By bringing the source of light to a distance of 25 cm. the intensity of light was increased fourfold; the duration of exposure was kept the same as before. The record (Fig. 115b) shows that a retardation of rate of growth occurred from the very beginning without the preliminary acceleration. This is for two reasons: (1) the increased intensity was now above the critical minimum, and (2) the tone of the organ had become improved by previous stimulation. On the cessation of light, the after-effect showed no enhancement of rate of growth, the recovery from retardation to the normal rate being gradual. In the next experiment (the result of which is not given in the record) the intensity of light was increased still further; the retardation now became very marked, and it persisted for a long time even on the cessation of light. We thus find that: (1) The immediate effect of light of moderate intensity is a preliminary acceleration, followed by normal retardation. The acceleration is the effect of sub-minimal stimulation. The immediate after-effect is an acceleration above the normal. (2) The immediate effect of strong light is a retardation from the beginning; the immediate after-effect shows no acceleration, the growth rate being gradually restored to the normal.