Wakeman—Pigments of Flowering Plants. 785 
Of the oxidation products of dihydrocymene, three, and pos¬ 
sibly four, are believed to exist in plants. Of these thymo- 
quinone and dihydroxy thymoquinone have actually been iso¬ 
lated, while there are strong reasons for believing that one or 
both of the monohydroxy thymoquinones occur in Monarda 
species either in the free state or as labile compounds. 
Thymoquinone together with the corresponding hydroquinone 
has been isolated from several species of Monarda , also from 
the oil from the wood of Thuja articulata. 2 Hydrothymo- 
quinone also exists in the oil from the fruit of Foeniculum 
vulgare , 3 also as dimethyl ether in the oil of Eupatorium trip- 
linerve 4 (E. Ayapana) and in the oil from Eupatorium capil- 
lifolium . 5 Inasmuch as in the diethers the original phenol hy¬ 
drogens are replaced by alkyl radicals they are not prone to 
oxidation in like manner as the phenols, hence, presumably do 
not take part in pigment formation. 
Monohydroxy thymoquinone 6 ) is believed to occur in Monarda 
fistulosa , Monarda citriodora f and perhaps in other species of 
Monarda. 
Dihydroxy thymoquinone 7 ) has been isolated from the vola¬ 
tile oils of Monarda fistulosa and Monarda citriodora. Its pres¬ 
ence has been indicated in Monarda didyma. 
The chemical relationship of the thymoquinones to some of 
the other constituents of the Monardas is very close and is 
worthy of notice here. From the volatile oils of the several 
species of Monarda so far examined, have been isolated both 
of the monohydroxy phenols, thymol and carvacrol, and prob¬ 
ably eymene 8 ) the hydrocarbon underlying not only these 
phenols but hydrothymoquinone as well. 
The relation of the pigment substances to each other and to 
the volatile constituents of the Monardas, also the role which 
some of the non-colored volatile and non-volatile substances 
2 C. r., 139, 927. 
3 Schimmel, Gesch. Ber. 1906, Apr. p. 28. 
4 Gildemeister—The Volatile Oils, p. 479. 
3 Personal communication from Prof. E. R. Miller, Laboratory of Plant 
Chemistry, University of Wisconsin. 
8 Bulletin of the University of Wisconsin No. 448, p. 31-34. 
7 Bulletin of the University of Wisconsin No. 448, p. 31-34. 
8 Ph. Rund., 13, p. 207; Ph. Rev., 14, p. 223. (The writer has not suc¬ 
ceeded in identifying^ eymene in her study of the hydrocarbons in the oil 
of M. punctata.) 
50—S. A. L. 
