The relative effectiveness of various rays on growth will be seen more strikingly demonstrated in records of photo-tropic curvature to be given in a succeeding Paper.
SUMMARY.
The normal effect of light is incipient contraction or retardation of the rate of growth.
The latent period may in some cases be as short as 2 seconds. In large number of cases it is about 15 seconds. The latent period is shortened under stronger intensity of light.
Increasing intensity of light induces increasing retardation and arrest of growth. Under continued action of light of strong intensity the growing organ may undergo an actual shortening.
In these reactions the action of stimulus of light resembles the effects of electric and mechanical stimuli.
The ultra-violet rays induce the most intense reaction in retardation of growth. The less refrangible yellow and red rays are practically ineffective. But the infra-red rays induce a marked retardation of growth.
The effects of light and warmth are antagonistic. The former induces a retardation and the latter an acceleration of growth.