[V] PFEFFER--Physiology of Plants--Vol. II., p. 104 (English Translation)
_Effect of red and yellow light._--These rays had little or no effect in inducing variation of growth.
_Effect of blue light: Experiment 82._--The blue rays exerted a marked retarding effect on growth. Light was applied for 34 seconds and retardation was initiated within 14 seconds of the incidence of light, and the retarded rate was two-fifths of the normal (Fig. 79B).
[Illustration: FIG. 79.--N, normal. B, effect of blue light, and V, of ultra-violet light. The records are on a moving plate.]
_Effect of ultra-violet light: Experiment 83._--Ultra-violet light was obtained from a quartz mercury vapour lamp. The effect of this light in retardation of growth was very marked. Response was induced within 10 seconds, the maximum retardation being one-sixth of the normal rate (Fig. 79V).
_Effect of infra-red rays: Experiment 84._--In passing from the most refrangible ultra-violet to the less refrangible red rays, the responsive retardation of growth undergoes a diminution and practical abolition. Proceeding further in the infra-red region of thermal rays, it is found that these latter rays become suddenly effective in inducing retardation of growth.
A curve drawn with the wave length of light as abscissa, and effectiveness of the ray as ordinate shows a fall towards zero as we proceed from the ultra-violet wave towards the red; the curve, however, shoots up as we proceed further in the region of the infra-red. In connection with this it should be remembered that while the thermal rays induce a retardation of growth, rise of temperature, up to an optimum point, gives rise to the precisely opposite reaction of acceleration of growth.
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.
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.