344 



AMBROSIACEAE. 



VOL. III. 



Bur 10" long or less, the prickles 2" -3" long. 

 Bur densely prickly, its pubescence brown. 

 Bur loosely prickly, its pubescence yellowish. 

 Body of the bur oblong, more than twice as long as thick. 

 Prickles longer than the diameter of the body of the bur. 

 Prickles shorter than the diameter of the body of the bur. 

 Bur narrowly oblong. 

 Bur broadly oblong. 

 Bur and its prickles glabrous, or merely puberulent ; beaks nearly straight. 



3. X. echinatum. 



4. X. glanduliferum. 



5. X. inflexum. 



6. X. pennsylvanicum. 



7. X. commune. 



8. X. americanum. 



i. Xanthium spinosum L. Spiny or 

 Thorny Clotbur, Clotweed or Bur- 

 weed. Fig. 4132. 

 Xanthium spinosum L. Sp. PI. 987. 1753. 



Stem pubescent or puberulent, much branched, 

 ascending or erect, i-3 high. Leaves lanceo- 

 late or ovate-lanceolate, acute or acuminate, 

 lobed, or the upper entire, narrowed at the 

 base, short-petioled, white-canescent beneath 

 and on the whitish veins of the upper surface, 

 2'-5' long; axils each with a short-stalked 

 3-pronged yellow spine nearly i' long; ripe 

 fertile involucre (bur) oblong-cylindric, 4"-6" 

 long, about 2" in diameter, pubescent, armed 

 with short subulate rather inconspicuous beaks, 

 and numerous glabrous spines about i" long. 



In waste grounds, Maine to Ontario, Florida, 

 Illinois, West Virginia, Missouri, Texas, New 

 Mexico and California. Widely distributed as a 

 weed in tropical America. Naturalized from 

 Europe or Asia. Cocklebur. Dagger-cocklebur. 

 Aug.-Nov. 



2. Xanthium speciosum Kearney. Great 

 Clotbur. Fig. 4133. 



Xanthium speciosum Kearney, Bull. Torr. Club 

 24: 574. 1897. 



Very stout, 3-4i high. Stem sharply 

 angled above; lower petioles 4'-6' long; leaf- 

 blades broadly triangular-ovate, the larger 6'-8' 

 wide, 3-S-Iobed, dentate, scabrous on both sur- 

 faces ; burs commonly clustered, oblong to 

 ovoid-oblong, i' long or more, the stout beaks 

 5 "-6" long, somewhat incurved, strongly hooked 

 at the apex, equalling or a little longer than the 

 dense subulate uncinate prickles, which are 

 hispid to above the middle, and 4"~s" long. 



Moist and waste grounds, North Dakota to 

 Wisconsin, Tennessee, Montana, Nebraska and 

 Texas, and locally in waste places eastward. 

 Aug.-Sept. 



