368 PHYSIOLOGY 



ucts very like those of chlorophyllin, suggesting that the two pigments 

 have structural similarities. That both have peculiar relations with 

 carbon dioxid is interesting, but cannot yet be explained. 



When chlorophyllin disappears in the autumn, the yellow pigments become 

 prominent, and some of its decomposition products have a share in reddening the 

 tissues. The red pigments are then dissolved in the cell sap ; the yellows are still 

 in the chloroplasts. The autumnal coloring, however, is not yet fully understood. 



Carotin. The chemical composition of carotin is certainly very 

 different from that of chlorophyllin. Its formula, probably CseHsg or 

 C^M, shows that it lacks both O and N. It is widely distributed in 

 plants, and to it chiefly the orange and yellow tints of flowers, fruits, 

 seeds, roots, etc., are due. 



(3) THE ENERGY 



Light. While the intricate chemical relations of chlorophyll are 

 yet unknown, one of its physical features is known to be of the greatest 

 importance. That is its capacity to absorb radiant energy. When the 

 radiant energy coming from the sun is passed through prisms of rock 

 salt, glass, or other appropriate media, or is reflected from a minutely 

 striate surface, the various wave lengths are unequally refracted or 

 reflected, so that the physiological and other effects of energy of dif- 

 ferent wave lengths can be studied. Certain of these wave lengths 

 (if they were sound waves one might say about i octave out of n) affect 

 our eyes, and this physiological effect is what we know as light. By 

 a figure of speech the cause is likewise so named, and the waves them- 

 selves are calle.1 " light." But they differ only in length and frequency 

 from the much greater number, both longer and shorter, slower and 

 faster, which we cannot perceive with our eyes. Other physiological 

 effects, such as inflammation of the skin and the development of pig- 

 ment ("sunburn "), are produced by light waves. On the plant, like- 

 wise, waves of different lengths produce different effects according as 

 certain parts are attuned to them (see p. 449). 



Absorption spectrum. The chlorophyll is so constituted that it can 

 absorb waves of certain lengths, all falling within the range of our vi- 

 sion. On the plant this energy cannot produce the effect that it does on 

 our eyes, and hence for the plant it is " light " only by a convenient 

 figure of speech. There are seven separated groups of waves whose 

 absorption is more or less complete. When we look at a spectrum of 



