Wakeman—Pigments of Flowering Plants. 873 
apparently made no attempt to explain why this is so. Brandel 1 
in his monograph on Plant Pigments offers such an explanation. 
“It is well known that the mordants which are used in dye¬ 
ing with these substances are salts of aluminum, iron and 
chromium, in other words salts of trivalent metals. The pro¬ 
cess of dyeing with mordants depends upon the formation of 
the aluminum, iron or chromium derivative and its deposition 
in the fiber. This being true, one would possibly not expect 
the monohydroxyanthraquinones to have dyeing properties, 
inasmuch as the union of three molecules of the monohydroxy- 
anthraquinone with one atom of aluminum, might hardly be 
expected to take place very readily. 
“On the other hand, by the introduction of more OH groups 
into the same molecule the tendency to form these trivalent 
metallic derivatives would be increased and it would be expected 
to be the greatest in those cases in which the OH groups are 
connected to neighboring carbon atoms. The bonds of the 
aluminum atom would be subject to a less strain as it were, 
than when they united with bonds from different molecules or 
from widely separated bonds in the same molecule. From this 
standpoint, the 2, 3, dihydroxyanthraquinone 
o 
CH II CH 
as well as the 1, 2, dihydroxyanthraquinone 
O 
1 Brandel—Plant Pigments, p. 29. 
