Annuals [perennials], 10-30(-70+) cm. Stems erect to decumbent, branched from bases or throughout. Leaves cauline; mostly opposite (distal sometimes alternate); blades (1-)2-3-pinnatisect, primary lobes linear to linear-cuneate, ultimate margins entire or toothed, faces puberulent (little, if at all, setaceous at bases, on teeth, or at tips of lobes, oil-glands submarginal). Heads radiate, borne singly or in 2s or 3s [pseudocephalia]. Calyculi of  1-9 ± linear bractlets (lengths 1/2-1 phyllaries, bearing oil-glands). Involucres turbinate to campanulate [hemispheric], 5-10 mm diam. Phyllaries persistent, 6-12 in ± 2 series (distinct to bases or nearly so, oval-oblanceolate, each bearing 1-7 round to elliptic oil-glands). Receptacles convex, ± pitted (socket margins fimbrillate to setose), epaleate. Ray florets usually 5-8, pistillate, fertile; corollas yellow-orange. Disc florets 12-50[-100+], bisexual, fertile; corollas yellow to orange, tubes shorter than cylindric throats, lobes 5, deltate to lance-deltate. Cypselae obpyramidal to obconic, subsericeous or glabrescent; pappi persistent, of 15-20 scales in ± 2 series (each scale comprising 5-10 basally connate, unequal bristles). x = 13.
Heads radiate, the rays pistillate and fertile, yellow or orange; invol bracts biseriate, the inner distinct or weakly connate at the base; receptacle naked, flat to convex; disk-fls numerous, tubular and perfect; style-branches flattened, subtruncate, shortly penicillate around the tip, and with a minute terminal cone; achenes slender, ±striate; pappus of 12-22 scales, each scale cleft into 5-10 bristles; aromatic herbs with pinnatifid to pinnately dissected, mainly opposite lvs, these and the invol beset with scattered embedded oil-glands. 7, New World.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.