PHYSIOLOGICAL ISOLATION OF TYPES IN THE 
GENUS XANTHIUM 
CHARLES A. SHULL 
(WITH SEVEN FIGURES) 
While collecting Xanthium seeds for physiological studies, my 
attention has often been called to the heterogeneity of the Xanthium 
population in the field. Burs collected from several widely separated 
localities belong to the uncertain group listed under the name of 
X. canadense Miller in RoBinson and FERNALD’s seventh edition 
of Gray’s Manual. 
In the annual report of the botanist of the Department of Agri- 
culture for 1886, VASEY mentions X. canadense as the species 
troublesome in the west, while the principal eastern species is called 
X. strumarium. But it is now known that X. strumarium has 
never been introduced into America. As several types were often 
found growing intermingled in Kansas and Kentucky, the possi- 
bility of hybridization suggested itself. 
Desiring seeds of uniform physiological character for certain 
investigations in which great accuracy was necessary, I collected 
burs from the three main types occurring in the fields about Law- 
rence, Kansas. In each case the seeds were chosen from a single 
plant of the type. It was thought that the various forms were 
possibly the result of promiscuous crossing of varieties or elemen- 
tary species, and that a year or two of guarded pollination would 
be necessary to purify the strains so that physiological properties 
as well as morphological characters might be uniform. The use 
of pure bred material for physiological investigations has not yet 
been considered essential, but it may be very desirable, or even 
necessary, for certain kinds of work. Burs from the three types 
chosen are shown in figs. 1, 2, and 3. The original plants stood 
side by side on the northern edge of the Wakarusa floodplain about 
o.5 kilometer south of Mount Oread. After being photographed, 
the burs were opened, and the seeds of the three types were found 
Botanical Gazette, vol. 69] [474 
