Wakeman—Pigments of Flowering Plants. 
871 
There exist in plants a large number of compounds, referable 
to these three hydrocarbons, most of which are used as dyestuffs. 
So far as is known, all except possibly the aloins, are hydroxy 
derivatives, and their alkyl or sugar ethers, of quinone oxida¬ 
tion products of these hydrocarbons, viz. anthraquinone and 
methyl anthraquinones. The aloins are possibly hydroxy de¬ 
rivatives of dihydro methyl anthracene. 
Anthraquinone, which forms pale yellow crystals, is a quinone 
having its two carbonyl groups in p - position with reference to 
each other, a configuration which in itself is supposed to give 
color to a molecule. The intensity of the color, and especially 
the dyeing property, of the substance appears to depend upon 
the number and the position of free hydroxy groups introduced 
into the molecule. 
As in the Xanthone and Flavone groups the compound ap¬ 
pears to be more highly colored and to possess better dyeing 
properties when there is a hydroxy group in position -1- rela¬ 
tively ortho to the carbonyl group, so the anthraquinone pig¬ 
ments used particularly as dyes contain at least one hydroxy 
group in ortho position to one of the quinone oxygens. In 1887 
