AMBROSIACEAE. 



VOL. III. 



6. Xanthium pennsylvanicum Wallr. 

 Pennsylvania Clotbur. Fig. 4137. 



Xanthium pennsylvanicum Wallr. Beitr. Bot. i : 

 236. 1842. 



Stem comparatively slender, smooth below, 

 roughish above, i-3 high. Leaves thin, long- 

 petioled, sharply toothed and some of them 

 3-5-lobed, smoothish, or the upper surface 

 scabrate, glandular ; burs clustered in the axils, 

 7"~9" l n g, about one-third as thick, puberu- 

 lent and resinous-glandular, and commonly 

 with a few longer hairs ; prickles numerous, 

 subulate, hooked, more or less hispid or gla- 

 brate, the longer ones nearly as long as the 

 diameter of the bur; beaks slender, but stouter 

 than the prickles, incurved and hooked. 



Moist gerunds, Quebec to Minnesota, New 

 York, Missouri and Colorado. Aug.-Oct. Re- 

 ferred in our first edition to Xanthium canadense 

 Mill., a name which has been variously applied to 

 different plants by authors. 



7. Xanthium commune Britton. Cockle- 

 bur or Clotbur. Fig. 4138. 



Xanthium commune Britton, Manual 912. 1901. 



Stem rather slender, i-2, roughish. Leaves 

 broadly ovate, more or less lobed, scabrous, espe- 

 cially above ; burs commonly solitary in the axils, 

 oblong, /"-I2" long, half as thick, or less, the 

 subulate-conic beaks slightly incurved, hooked at 

 the apex, about as long as the subulate uncinate 

 prickles, which are hispid to about the middle with 

 brown hairs and shorter than the diameter of 

 the bur. 



Moist grounds, Quebec to Alberta, Maryland, Mis- 

 souri, Utah and Arizona. Aug.-Oct. 



8. Xanthium americanum Walt. Amer- 

 ican Cocklebur or Btirweed. 

 Fig. 4I39- 



X. americanum Walt. Fl. Car. 231. 1788. 



X. macrocarpum glabratum DC. Prodr. 5: 523. 



1836. 

 X. glabratum Britton, Manual 912. 1901. 



Rough, i-6i high. Leaves slender-petioled, 

 broadly ovate to orbicular, 3-ribbed and cor- 

 date or cordate-reniform at the base, the lower 

 often 10' wide, the margins dentate, or more 

 or less 3-5-lobed, both surfaces roughish and 

 green ; bur oblong, glabrous or merely puberu- 

 lent, 6"-g" long, about 3" in diameter, its 2 

 sharp conical-subulate 2-toothed beaks straight 

 or nearly so, equalling or slightly longer than 

 the glabrous spines. 



Moist grounds, Ontario to Florida, Michigan, 

 Tennessee and Kansas. Referred, in- our first edi- 

 tion, to the Old World X. strumarium L., from 

 which it proves to be distinct. Sheep- or clot-bur. 

 Button-bur. Small or lesser burdock. Aug.-Oct. 



Xanthium orientals L. (X. canadense Mill.), an 

 Old World tropical species, is naturalized in the 

 West Indies. 



