Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads radiate, the rays large, yellow, neutral; invol bracts subequal to imbricate, usually green; receptacle flat to low-conic, chaffy, its bracts clasping the achenes; disk-fls perfect and fertile; style-branches flattened, hispidulous on both sides (at least distally), the stigmatic lines poorly developed; achenes thick, moderately compressed at right angles to the invol bracts, glabrous or sometimes hairy; pappus of 2 readily deciduous awns with enlarged, thin, paleaceous base, rarely with some additional short scales; coarse annual or more often perennial herbs with simple lvs, at least the lowermost ones opposite; x=17. 50, N. Amer. Hybrids abound.