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, 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.