PIGMENTS 



103 



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 con- 

 tains neither choline nor phosphorus, the earlier observations being 

 due to mixtures of various other materials with the true chloro- 

 phyll in the extracts which were examined. Beginning with 1912, 

 Willstatter 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 " chloro- 

 phyll." In 1912, Willstatter 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 &," 

 associated with two yellow pigments, carotin and xanthophyll, and, 

 in some cases, with the reddish-brown fucoxanthin. The per- 

 centages of total pigment materials, and the relative proportions 

 of the five different pigments, in several types of plants, are as 

 follows : 



The two chlorophylls have the following formulas; chloropnyll 

 a, C55H72O 5 N4Mg, and chlorophyll 6, CooHroOe^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. ChloroplMl a is blue-black, is easily soluble in most 

 organic solvents, and when saponified by alcoholic potash gives a 



