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 white to pink or blue, pistillate; invol bracts subequal to strongly imbricate, scarious-margined, with green or greenish midrib or tip; receptacle hemispheric or conic, naked; disk-fls yellow, fertile; style-branches flattened, with short, lanceolate, externally hairy appendage; achenes obovate, strongly flattened, ± wing- margined; pappus of several minute bristles and 2(4) longer awns, these commonly reduced or wanting in the ray-achenes, sometimes also in the disk; glabrous, often stoloniferous, short- lived perennials with relatively narrow, chiefly entire lvs and several to very many small to middle-sized hemispheric heads. 5, U.S.A.