CHAPTER VIII
PIGMENTS
Practically all plant structures contain pigments. These may be considered as of two types: (a) the vegetative pigments, which have a definite energy-absorbing rôle in the metabolic processes of the tissues which contain them, and (b) the ornamental pigments. It is probable that the same chemical compound may serve in either one of these capacities under different conditions, but, in general, it is possible to assign either a definite vegetative, or physiological, use, or else a simple ornamental, or biological, significance to each of the common pigments. The first type is found widely distributed through the protoplasm, or cell-sap, of the plant structures; while the ornamental pigments are located chiefly in the epidermal cells, especially of flowers.
With respect to their colors, the plant pigments may be grouped as follows:
Green—the chlorophylls.
Yellow—the carotinoids, flavones, and xanthones.
Red—phycoerythrin, lycopersicin, anthocyanin.
Blue—anthocyan derivatives.
Brown—phycophæin, fucoxanthin.
Of these, the chlorophylls, the carotinoids, phycoerythrin (in red sea-weeds) and phycophæin (in brown sea-weeds) are generally vegetative pigments; while the others form the basis for most of the ornamental pigments, although they may have a definite energy-absorbing effect, in some cases.
THE CHLOROPHYLLS
The importance of the green coloring matter in plants has been understood for more than a century, its connection with photosynthesis having been known as far back as 1819. But definite knowledge as to its chemical constitution is of very recent origin. As recently as 1908, it was asserted that chlorophyll is a lecithin-like body, yielding choline and glycero-phosphoric acid on hydrolysis. It is now known, however, that chlorophyll contains neither choline nor phosphorus, the earlier observations being due to mixtures of various other materials with the true chlorophyll in the extracts which were examined. Beginning with 1912, Willstätter and his collaborators, in a series of classic papers which were finally collected in book form, clearly demonstrated the chemical constitution of the green pigments of plants, which had been previously designated under the single name "chlorophyll." In 1912, Willstätter and Isler first showed that the green coloring matter which is extracted from plants by alcohol, ether, etc., is made up of two definite chemical compounds, to which they assigned the names "chlorophyll a" and "chlorophyll b," associated with two yellow pigments, carotin and xanthophyll, and, in some cases, with the reddish-brown fucoxanthin. The percentages of total pigment materials, and the relative proportions of the five different pigments, in several types of plants, are as follows:
| Land Plants, Per Cent. | Brown Seaweeds, Per Cent. | Green Algæ, Per Cent. | |
|---|---|---|---|
| Total pigment in the dry matter | 0.99 | 0.29 | 0.21 |
| Proportion of: | |||
| Chlorophyll a | 63 | 55 | 44 |
| Chlorophyll b | 22 | 4 | 31 |
| Carotin | 6 | 11 | 7 |
| Xanthophyll | 9 | 10 | 18 |
| Fucoxanthin | 20 |