Wakeman—Pigments of Flowering Plants. 771 
either the para quinone or the ortho quinone grouping, the 
double bonds being satisfied by oxygen or any other divalent 
element or group or by combinations of any of them. 
Armstrong’s theory has received a great deal of attention, 
much evidence both for and against it having been produced. 
Though our present knowledge of the structure of colored com¬ 
pounds would indicate that by no means all colored substances 
are of quinoidal character, yet a surprisingly large number, if 
not all, of the substances of a quinoidal configuration are col¬ 
ored. The quinoidal configuration is in fact one of the best 
known and most reliable chromogens. 
In the study of plant pigments the quinone theory has proved 
of much more value than Witt’s theory of chromophorous 
groups. The constitution and the properties of a very large 
number of vegetable dyestuffs, and of other colored substances 
derived from plants, have been accounted for by assuming a 
quinoidal structure. Moreover the closely related quinhydrone 6 
hypothesis of pigmentation in some plants promises to explain 
biochemically the existence of many colors and shades of color, 
as well as many changes of color, which have hitherto been un¬ 
accounted for. 
In considering quinone and quinhydrone hypotheses of pig¬ 
mentation mention should be made of Richter’s 7 oxonium 
theory of quinones. According to Richter the characteristics 
of oxonium salts, namely simple addition of the components 
in their formation, ready decomposition in solution, and upon 
sublimation, and marked increase in intensity of color, are also 
those of the quinhydrones. For these and other reasons 
hydroquinones, phenoquinones, etc., are regarded by Richter as 
oxonium compounds formed by the addition of pnenols, etc., to 
the tetravalent oxygen of quinones. 
Assuming that the doubly bound oxygen of quinones acts 
as a tetravalent oxygen does not impair the validity of the 
quinhydrone hypothesis as suggested by Kremers and Brandel. 
It brings the quinone pigments into line with the anthocyanin 
pigments, as interpreted by Willstaetter, and explains how 
many quinones, as well as many flavone and xanthone deriva¬ 
tives, also haematin and brazilin, dissolve in acids with an in- 
6 Ph. Rev., 19, p. 200. 
7 Ber., 43, p. 3603. 
