480 BOTANICAL GAZETTE [JUNE 
and hairs that beset the burs, so that the rows of plants appear 
reddish brown long before the burs begin to dry. Type II develops 
much less anthocyanin, while pigment is entirely wanting in type I. 
When the burs have dried this difference is manifest in different 
shades of brown. The burs are lightest in color in type I, darkest 
in type II. 
Fic. 6.—X. pennsylvanicum Wallr., at the same age as X. globosum Shull in fig. 7 
Since the three types tend to remain distinct in the field, and 
since all of them show distinctive features while presenting no 
definite evidence of Mendelian segregation, they should be con- 
sidered distinct species. 
The genus Xanthium is in need of careful study, and possibly 
revision, on the basis of breeding tests. The principal difficulty 
in the genus from the taxonomic point of view is to interpret just | 
what the old authors meant by X. canadense Mill. and X. amert- 
