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 
promintiic, 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 C^R^ or 
C^x, 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 called " 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 
