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 | Brown | Green | Plants, | Seaweeds, | Algæ, | Per Cent. | Per Cent. | 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 |

The two chlorophylls have the following formulas: chlorophyll _a_, C_{55}H_{72}O_{5}N_{4}Mg, and chlorophyll _b_, C_{55}H_{70}O_{6}N_{4}Mg. Hence, they differ only in having two hydrogen atoms in the one replaced by one oxygen atom in the other. Both are amorphous powders, from which crystalline chlorophyll (see below) can be obtained by hydrolysis. Chlorophyll _a_ is blue-black, is easily soluble in most organic solvents, and when saponified by alcoholic potash gives a transient pure yellow color. Chlorophyll _b_ is dark green, is somewhat less soluble than the other form, and when saponified by potash gives a transient brilliant red.

=Amorphous and Crystalline Chlorophyll.=--When the chlorophyll of plants is extracted by alcohol and the alcoholic extract evaporated nearly to dryness, beautiful dark green crystals are obtained. Willstätter has shown, however, that in these crystallized forms the ethyl group (from the ethyl alcohol used) has replaced the phytyl group (see below) which is present in the pigments as they exist in the plant tissues; and that, when extracted by other solvents than alcohol, the pigments may be obtained in the amorphous forms in which they exist in the plant.

This change from amorphous to crystalline compounds may be understood from the preliminary statement that the chlorophylls are esters of tri-basic acids, in which one acid hydrogen is replaced by the methyl (CH_{3}) group and a second by the phytyl (C_{20}H_{39}, from phytol, or phytyl alcohol, C_{20}H_{39}OH) group. When treated with ethyl alcohol (C_{2}H_{5}OH) for the purpose of extracting the pigments, the ethyl (C_{2}H_{5}) group replaces the phytyl group, thus yielding a methyl-ethyl ester, and these esters are the crystalline forms of the chlorophylls. This replacement is made possible through the action on the original pigment in the tissues of an enzyme, _chlorophyllase_, which is also present in the tissues, which splits off the phytyl group, forming phytyl alcohol, and leaving a free COOH group in the pigment, with which the alcohol used in the extraction forms the ethyl ester (see