Page:Popular Science Monthly Volume 49.djvu/87

Rh converted into heat and other forms of energy. This energy is directly available to the protoplasm containing the chlorophyll, and by means of it the synthesis of complex substance may be accomplished. Moreover, the amount of synthesis accomplished by plants exposed to separate portions of the spectrum will be directly proportional to the amount of that portion which can be absorbed and converted into useful forms of energy. This is graphically illustrated in Fig. 2, The amount of synthesis is shown to be greatest in the red light between B and C, where the greatest absorption takes place. (See Fig. 1, I.)

Chlorophyll is a very complex and highly unstable substance, and during the absorption of light it is slowly broken down, but ordinarily it is rebuilt by the protoplasm as fast as it is decomposed. If, however, the chlorophyll and the leaf containing it are exposed to a light of such intensity that the chlorophyll is decomposed faster than it can be rebuilt, then damage must ensue, which if sufficiently extensive will result in the death of the entire leaf. The intensity of the light which induces a maximum of activity in any plant, and which it may receive without damage, is determined by its specific constitution. The intensity of light falling on a plant in an open plain during twenty-four hours ranges from almost total darkness to the blaze of the noonday sun, and varies almost momentarily. As an adjustment to this condition many plants are able to regulate the intensity of the light impinging on the chlorophyll-bearing masses of protoplasm by altering the position of the surfaces of the leaves. In others in which this movement is not possible—such, for example, as the leaf-like duckweeds which float on the surface of the water—the intensity of the light received is regulated by alternations in the position and distance of the chlorophyll from the surface of the organ. (See Fig. 3.)

In many plants growing in the bright glare of the sun a thickened cuticle or a heavy coat of hairs serves to protect the