778 Wisconsin Academy of Sciences, Arts, and Letters . 
gained by a study of simply the pigments themselves; but that 
each pigment should be considered not only in relation to the 
other colored substances in the same and related plants, but 
also to the non-colored substances as well. A close and pecu¬ 
liar relationship has often been found to exist between the col¬ 
ored and the non-colored constituents not only of the same 
plant, but sometimes of the related species of a whole plant 
family. 
As the work progressed a complete revision of the literature 
on plant pigments became necessary. Very little literature 
of a general nature upon plant pigments was found to exist, 
almost nothing in fact beyond Brandel’s excellent monograph. 
Several treatises upon vegetable dye stuffs, it is true, are avail¬ 
able. Among these may be mentioned two by Thomas, Les 
Matieres Colorantes Naturelles, and Les Planets Tinctoriales, 
also Rupe’s more recent Chemie der natuerlichen Farbstoffe 
(1909) and volume 6 of the Biochemisches Hand-Lexicon, Farb¬ 
stoffe der Pflanzen und der Tierwelt. (1911), as well as chap¬ 
ters on natural dye stuffs in various treatises on dye stuffs in 
general. By far the larger literature on plant pigments, how¬ 
ever, is scattered through the chemical and botanical journals 
of the past fifty years, some extending much further back. 
Before attempting to proceed further with work upon plants 
and plant products it seemed desirable to review this literature 
in order, 
1. To avoid useless repetition of work already done. 
2. To interpret new work in the light of the old, and the old 
in the light of recent experimentation. 
3. To make comparisons, draw conclusions, and formulate 
theories as a guide to future work. 
In the course of this review of the literature upon plant pig¬ 
ments it was found that by arranging the pigments according 
to the degree of saturation, as calculated from the underlying 
hydrocarbons, certain relationships were brought out which 
could not well be observed in any other way. Among the import¬ 
ant relationships emphasized by this classification are: 
1. The influence of unsaturation upon the production of color 
in a molecule. 
2. The influence of so called chromophorous groups upon the 
production of color in a molecule. 
3. The existence of homologous series of pigments. 
