Page:Life Movements in Plants.djvu/241

Rh Eﬂ‘ect of ultra-violet light: Experimmt 8e'3.—-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).

Eﬁ'w-t of infra-red rag/s: Experiment 8-1.--In passing from the most refrangible ultra-violet. to the less refrangiblo 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 eti'ective 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 denionstrated in records of photo- trOpic curvature to be given in a succeeding ’aper.

The normal effect of light is incipient cont'action or retardation of the ‘ate of growth.

The latent period may in some eases 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.