Description from Flora of China
Annuals, perennials, or shrubs. Stems erect, decumbent, or prostrate, branched. Leaves usually cauline, opposite throughout or opposite (proximal) and alternate or mostly alternate, sessile or petiolate; blade deltate, elliptic, filiform, lanceolate, linear, obovate, ovate, or rhombic, usually pinnately, sometimes palmately, lobed, both surfaces hairy or glabrate, usually gland-dotted or stipitate glandular, ultimate margin entire or toothed. Capitula discoid, unisexual, female proximal to or intermixed with male, male capitula usually arranged in racemes or spikes; rarely single plants all or mostly male or female. Female capitula: phyllaries 1-8-seriate, outer (1-)5-8 distinct or connate, herbaceous, remainder connate, usually with free tips forming tubercles, spines, or wings; florets 1(-5); corollas 0. Male capitula: involucres cup-shaped to saucer-shaped, 1.5-6 mm in diam.; phyllaries 1-seriate, connate; receptacle flat or convex; paleae spatulate to linear, membranous, sometimes villous, hirtellous, and/or gland-dotted or stipitate glandular, sometimes absent; florets 5-60; corollas whitish or purplish, funnelform, 5-lobed, erect or incurved; staminal filaments connate, anthers free or weakly coherent. Achenes black, ovoid or fusiform, enclosed within globose to obovoid, pyramidal, pyriform, obconical, or fusiform, hard, smooth, tuberculate, spiny, or winged burs; pappus absent. x = 18.
See Payne, J. Arnold Arbor. 45(4): 401-438. 1964.
About 43 species: tropical to subtropical and temperate New World, mostly North America; three species (all introduced) in China.