Annuals (sometimes persisting), 10-60(-120) cm. Stems erect to ± prostrate (repeatedly 'forked'). Leaves cauline; opposite; petiolate or ± sessile; blades mostly elliptic to deltate, rhombic, or ovate, sometimes lyrate, ultimate margins entire or toothed, faces usually pilosulous to sericeous or scabrellous, sometimes glabrate or glabrescent, usually gland-dotted. Heads radiate, 1(-3) in 'forks' of branches (terminal, appearing axillary by sympodial growth). Involucres ± hemispheric, 3-5 mm diam. (becoming ± rotate in fruit). Phyllaries persistent (outer) or falling, 10-13 in 2 series (outer 4-6 herbaceous, inner 5-8 each investing a ray ovary, enlarging in fruit to form a perigynium, shed with enclosed cypsela). Receptacles convex, paleate (paleae cuneate to spatulate, ± conduplicate or flattish, membranous). Ray florets 5-8, pistillate, fertile; corollas yellowish (tubes shorter than to equaling laminae, laminae ovate to elliptic or linear). Disc florets 3-8(-12+), functionally staminate; corollas yellowish, tubes shorter than funnelform or campanulate throats, lobes 5, deltate. Cypselae each enclosed within and shed with an often hardened, ± prickly perigynium (the ultimate 'fruits' plumply ellipsoid to fusiform, or ± compressed); pappi 0 or rudimentary. x = 11.
Heads radiate, the rays mostly 5-10, minute, yellow, pistillate and fertile; invol biseriate and dimorphic, the outer bracts 4-6, herbaceous elliptic to ovate, the inner larger, ±prickly or spiny, as many as and individually enclosing the ray- achenes, ripening with them into burs; receptacle small, convex, chaffy throughout, its bracts soft, loosely folded or convex and embracing the 5-15 disk-fls, these sterile, with undivided style; ray-achenes thick, radially somewhat compressed, glabrous; pappus none; diffuse, often dichotomous annuals with opposite, toothed or entire lvs and small heads solitary in the axils or in the forks of the stem. 10, trop. Amer.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.