1915] SHULL—XANTHIUM 477 
began to develop their characteristic mature leaves, a very surpris- 
ing uniformity of the plants belonging to each type was observed. 
This result was wholly unexpected, as it was believed that hybridi- 
zation could hardly have been avoided in nature. Type I has a 
dark green mesophyll with veins almost white, and a much 
Fic. 3.—Type III, Xanthium canadense Mill.; natural size 
crinkled surface; while type II has yellow green foliage, the leaf 
surfaces relatively plane. The foliage of type LI resembles 
that of type I very closely, so that it is difficult to distinguish 
young plants (figs. 6 and 7 show the foliage differences of types 
I and II). 
