Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads discoid, the fls all tubular and perfect, or the plants sometimes dioecious by abortion; invol bracts imbricate, usually some or all of them spine- tipped, and in many spp. with a thickened, glutinous, dorsal ridge; receptacle flat to subconic, densely bristly; cors purple to yellowish or white, with slender tube and long, narrow lobes; filaments usually papillose-hairy; anthers with a firm, narrow, apical appendage, evidently tailed at the base; style with a thickened, hairy ring and an abrupt change of texture below the ± connate, papillate branches; achenes basifixed or nearly so, glabrous, firm, thick-compressed, often curved, nerveless, commonly with a cartilaginous, pale yellow collar; pappus of numerous plumose bristles, deciduous in a ring; spiny herbs with alternate, toothed to more often pinnatifid lvs and medium- sized to large heads. 200, N. Hemisphere. (Spp. all sometimes included in Carduus)