This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol3", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
X. macuiatum Raf. Am. Month. Mag. 344. 1818.
X. oviforme Wallr. Beitr. Bot. 1: 240. 1842.
Stem rough, purplish or purple-blotched, 1°-2° high. Leaves firm, scabrous, with scattered short papillose hairs, obtusely toothed and lobed, somewhat resinous-glandular beneath; burs commonly clustered in the axils, ovoid to oval, 7"-11" long, 4"-6" thick, glandular; prickles very dense, densely hispid from the base to the middle or beyond, subulate, hooked, the longer about 2k" long, and about equalling the stout hispid beaks.
Xanthium glanduliferum Greene, Pittonia 4: 61. 1899.
Similar to X. echinatum. Leaves very thick and scabrous with short stout papillae; burs oval, 5"-8" long, 3"-41/2" thick, yellow, the prickles scattered, bristly-hispid nearly to the hooked apex, scarcely as long as the conic-subulate short-bristly beaks.
In dry soil, North Dakota to Assiniboia, British Columbia and Nebraska. Adventive in Missouri. June-Sept.
Xanthium Macounii Britton, known only from Lake Winnipeg, Manitoba, differs by a longer bur, 10" long and 4" thick.
Xanthium inflexum Mackenzie & Bush, Rep. Mo. Bot. Gard. 16: 106. 1905.
Glabrate, or papillose-roughened above, 3°-41/2° high. Leaves long-petioled, broadly ovate, more or less cordate, mostly 3-lobed, crenate-dentate; burs 1' long or less, the body oblong, more than twice as long as thick, 3"-31/2" in diameter, glandular-pubescent; prickles hooked, stiff, longer than the diameter of the bur, glandular-pubescent below, glabrous above; beaks stout, about 5" long, bent at the middle, strongly inflexed, hooked.
Sandy river-bottoms, Courtney, Missouri. Aug.-Sept.
Xanthium pennsylvanicum Wallr. Beitr. Bot. I: 236. 1842.
Stem comparatively slender, smooth below, roughish above, 1°-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" long, 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 gorunds, Quebec to Minnesota, New York, Missouri and Colorado. Aug.-Oct. Referred in our first edition to Xanthium canadense Mill., a name which has been variously applied to different plants by authors.
Xanthium commune Britton, Manual 912. 1901.
Stem rather slender, 1°-2°, roughish. Leaves broadly ovate, more or less lobed, scabrous, especially above; burs commonly solitary in the axils, oblong, 7"-12" 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, Missouri, Utah and Arizona. Aug.-Oct.
X. americanum Walt. Fl. Car. 231. 1788.
X. macrocarpum glabratum DC. Prodr. 5: 523.
1836. X. glabratum Britton, Manual 912. 1901.
Rough, 1°-61/2° high. Leaves slender-petioled, broadly ovate to orbicular, 3-ribbed and cordate 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"-9' 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 edition, to the Old World X. strtimarium L., from which it proves to be distinct. Sheep- or clot-bur. Button-bur. Small or lesser burdock. Aug.-Oct.
Xanthium orientale L. (X. canadense Mill.), an Old World tropical species, is naturalized in the West Indies.
Key to Genera.