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 or sometimes discoid, the rays pistillate and fertile, yellow to orange or occasionally reddish, invol bracts ± herbaceous, essentially equal, uniseriate or subbiseriate, often with some bracteoles at base; receptacle flat or convex, naked; disk- fls perfect and fertile, yellow to orange or reddish; style-branches flattened, truncate, penicillate; achenes subterete, 5–10-nerved; pappus of numerous, usually white, entire or rarely barbellulate capillary bristles; herbs (ours) or sometimes woody plants, with alternate (or all basal), entire to variously dissected lvs and mostly small to medium-sized heads. 1000+, cosmop. Spp. 4–14 ill- defined, their chromosomes also subject to miscount because of B-chromosomes.