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Annual or perennial herbs. Leaves opposite. Capitula solitary, axillary or in branch forks, sessile or shortly stalked, heterogamous, radiate, enlarging in fruit; involucre of two series, the outer herbaceous, the inner closely enveloping the outer florets; receptacle paleate. Outer florets female, radiate with small ray; style branches strongly divergent, linear and obtuse. Inner florets bisexual but functionally male, campanulate or infundibuliform; style branches connate, club-shaped; anthers with truncate bases and ovate apical appendix. Achenes of the female florets covered by the indurate and echinate phyllaries, the achene itself smooth; achenes in disc florets abortive; pappus absent.
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.
