Wakeman—Pigments of Flowering Plants. 781 
hydrocarbons. This relationship is very similar to that exist¬ 
ing between pigments referable to hydrocarbons of different 
degrees of saturation discussed in the following paragraph. 
4. The existence of series of compounds referable to similar 
symmetrical, or nearly symmetrical hydrocarbons of different 
degrees of saturation. 
A relationship quite similar to that expressed by homology 
under the same degree of saturation is noted between the hydro¬ 
carbons of different degrees of saturation to which any of the 
plant pigments are referable. This relationship is best ex¬ 
pressed by the accompanying chart of graphic formulae repre¬ 
senting the hydrocarbons to which a large majority of the plant 
pigments falling under the degrees of saturation C n H 2n _ 10 
to 0 n H 2n _ 18 are referable. 
In addition to the hydrocarbons listed in this table, attention 
should here be called to pigments, or pigment forming substances, 
referable to benzene and dihydrobenzene, naphthalene and dihy- 
dronaphthalepe anthracene and dihydroanthracene series of hy¬ 
drocarbons. It is also interesting to note the symmetrical or 
almost symmetrical character of all of the above hydrocarbons. 
Whether this symmetry of arrangement of the underlying hy¬ 
drocarbon is only coincident with the conditions which produce 
color in the molecule, or whether the symmetrical arrangement 
is itself one of the conditions does not become manifest. 
In order to bring out the relationships just discussed the 
plant pigments of known constitution have here been classified 
according to the underlying hydrocarbon. A second classification, 
according to plant families, showing the relationship which exists 
between the pigments and the noncolored constituents of the 
same and related plants, was intended to be included. This 
classification was, however, found to be too long for the purposes 
of this paper, therefore will be published later as supplementary 
to it. The experimental work of the writer 1 , where heretofore 
published has been referred to in the same manner as that of 
other investigators. Some work not previously published has 
been briefly described, i. e. the study of the pigment of red ger¬ 
anium blossoms. In addition to the above a large amount of 
1 Quantitative determinatino of oxidase in the leaves of Monarda fistulosa. 
Ph. Rev., 26, p. 314. 
Thymoquinone and Hydrothmoquinone. 
Ph. Rev., 26, p. 
Higher oxidation products of thymoquinone. 
Proc. A. Ph. A., 58, p. 979. 
The Monardas, a phytochemical study. 
Bull, of Univ. of Wis., Sci. Ser., Vol. 4, No. 4, pp. 81-128. 
