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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.

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.

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