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 yellow, pistillate and fertile, their ovaries imbricate in 2–3 series; invol bracts subequal or imbricate in 2–several series, firm, herbaceous or partly membranous-chartaceous; receptacle flat, chaffy, its bracts (or sometimes in part the inner bracts of the invol) subtending rays as well as disk-fls; disk-fls sterile, with undivided style; ray-achenes glabrous, strongly flattened parallel to the invol bracts, wing- margined, pappus none, or of 2 awns confluent with the wings of the achene; coarse perennial herbs with medium-sized or large subhemispheric heads. 15, U.S.