Wakeman—Pigments of Flowering Plants. 861 
tion of the hydroxyl group in the pyrylium cycle, all of the 
anthocyanin pigments so far known possessing a hydroxyl group 
in the flavonol position. For example, luteolin and cyanidin 
are both represented by A he formula C 15 H 10 O 6 . 
The formula for free cyanidin is not known. 
The hydrochloride of luteolin Willstaetter represents by for¬ 
mula I, and that of cyanidin by formula II. below. 
While this difference of position of a hydroxy group may be 
sufficient to explain the difference in properties of the two 
groups of pigments in acid combination, something more seems 
to be required to explain this difference in the acid free form, 
since many of the flavone pigments as well as the anthocyanin 
pigments are flavonols. Moreover it does not appear to be 
sufficient to explain the difference between such isomers as, 
