Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Genus Description - Heads mostly radiate, the rays pistillate or neutral, mostly yellow, cuneate, 3-lobed, not very numerous; invol bracts in 2–3 series, subequal or the inner shorter, ± herbaceous, generally soon deflexed, the outer sometimes joined at the base; receptacle convex to ovoid or conic, naked; disk-fls very numerous, perfect, the cor-lobes glandular-hairy; style branches flattened, with ventromarginal stigmatic lines and dilated, subtruncate, penicillate tip; achenes 4–5-angled, with as many intermediate ribs, pubescent on the angles and ribs, or glabrous; pappus of 5–10 scarious or hyaline, often awn-tipped scales; herbs with alternate, glandular-punctate, usually decurrent lvs and solitary to numerous heads. 40, New World.