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,
Willstaetter 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, Willstaetter 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, | Algae,
| 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 (se