transmitted red rays, a thinner stratum allowed the trans- mission of yellow in addition; ammoniated copper sul- phate solution allowed the blue and violet rays to pass through. It should be borne in mind that certain compli- cating factors are introduced by the incidence of light on the organ; there may be a slight rise of the temperature. We have seen howch-r that moderate rise of temperature induces an acceleration of the rate of growth (p. 175). I shall later describe other experiments which will demonstrate the antagonistic effects of light and warmth on growth. Warmth again may induce a certain amount of dessication, but. this is reduced to a minimum by maintaining the plant-chamber in a humid condition. The heating effect of the red is, relativllly speaking, much greater than that of the blue rays. But in spite of. this it is found that while red rays are practically inett‘ective, the blue rays are most effective in inducing responsive retardation of growth.

[greet of red and yellow light.-——'l‘liese rays had little or no etl'ect in inducing variation of growth.

Injfl'ecl, of blue light : Islrpvriment 82.—-—The blue rays exert- ed a marked retarding etl'ect on growth. Light was applied {UP 34 “0011.15 and retardation was initiated within 14 seconds of the incidence of light, and the retarded rate was two-fifths of the normal (Fig. 7915).

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Flu. 7tl.—--N. normal. it. effect of blue light. and V, of nltrmviolet “gm. The records are on a moving plate.