1915] SHULL—XANTHIUM 479 
During the summer of 1914, volunteer seedlings of types I and 
II were allowed to grow in the field under natural conditions, 
in competition with plants of the same types and other native 
weeds. The same differences in the burs and in the blooming 
times occurs under these circumstances, but the differences in the 
foliage loses some of its sharpness due to type I showing less crink- 
ling of the mesophyll when crowded. There are wide differences 
—Seedlings of Xanthium: to the left, upper and lower of X. pennsylvani- 
cum oli to the right, upper and lower of X. globoswm Shull. 
in the habit of the plants when crowded and when cultivated. 
The plants branch profusely when space permits, but are frequently 
unbranched or but slightly branched when closely crowded. Such 
differences will greatly alter the anatomical details of the plant 
structure, and tend to render such distinctions uncertain." 
One more difference between the three types remains to be 
noted. Type II develops considerable anthocyanin in the prickles 
* The ag aaa ii anatomy of these three types of Xanthium, with X. —— 
going under the n of X. americanum, has been described by Nora E. DALBE 
(Kan. Univ. Sci. Bull, 9:57-65. 1914). 
