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 occasionally discoid, the rays mostly 12–45, yellow, pistillate and fertile; invol ± resinous, its bracts herbaceous-tipped, imbricate or subequal; receptacle flat or convex, naked; disk-fls yellow, the inner and often also the outer sterile; style-branches flattened, with externally hairy, lance-linear or occasionally very short appendages; achenes compressed to subquadrangular, scarcely nerved; pappus of 2–several firm, deciduous, often minutely serrulate awns; taprooted herbs, sometimes woody at the base, with alternate, ± resinous-punctate lvs and relatively large, hemispheric heads, the disk 1–3 cm wide. 50, New World.