Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads usually radiate, the rays pistillate and fertile, white or anthocyanic; invol bracts narrow, herbaceous and equal to scarcely herbaceous and evidently imbricate, but without Aster-like green tips; receptacle flat or nearly so, naked; disk-fls numerous, perfect and fertile, yellow; some spp. with rayless pistillate fls between the disk-fls and ray-fls; style-branches flattened, the appendage short (not over 0.3 mm in ours), acute to more often obtuse, or even obsolete; achenes in our spp. 2–4-nerved; pappus of capillary bristles, with or without a short outer series of minute bristles or scales; herbs with alternate lvs and solitary to numerous, hemispheric to turbinate heads. Typically blooming in spring and early summer (unlike Aster), but some weedy spp. continuing until fall. 200, mainly N. Temp.