Wakeman — Pigments of Flowering Plants. 875 
1-, 2, 4-anthraquinone; rubiadin, methyl -1-dihydroxy -2, 4- 
anthraquinone; and pseudo purpurin, trihydroxy -1, 2, 4- 
methyl -2- anthraquinone, ali as glucosides. In several other 
instances several of these anthraquinone derivatives are known 
to exist side by side in the plant. In Rheum officinale are 
found emodin, isoemedin, rhein and chrysophanic acid, while 
in Rhamnus purshiana are found emodine, chrysophanic acid 
and chrysarobin, a reduction product of chrysophanic acid. Of 
how the plants build up any or all of these related compounds, 
or pass from one to the other, nothing appears to be known. 
By the aid of structural formulae it can be shown how the plant 
might be able to synthesize anthraquinone from two molecules 
of carbonic acid and two of benzene. 
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By the substitution of phenols or homologues of benzene for 
one or both of the benzene molecules the various anthraquinone 
pigments might be formed. Unfortunately for the probability 
of any such hypothesis little or nothing is known of the volatile 
constitutents of the anthraquinone producing plants. A large 
number of them contain tannic acid, gallic acid, and cinnamic 
acid however. By the condensation of two molecules of gallic 
acid a molecule of the anthraquinone configuration with six 
hydroxy groups would result. 
Unfortunately, again, such an anthraquinone derivative has 
not been isolated from plants. By the substitution of a mole- 
