Description from Flora of China
Annuals or perennials. Stems usually erect, branched distally or throughout. Leaves usually cauline, usually opposite, rarely whorled, distal sometimes alternate, petiolate or sessile; blades simple, compound (leaflets petiolulate), or 1-3-pinnatisect or -pinnately lobed, ultimate margin entire, dentate, laciniate, serrate, or toothed, both surfaces usually glabrous, sometimes hirtellous, hispidulous, pilosulose, puberulent, scaberulose, or strigillose. Synflorescence of corymbs, sometimes capitula in 2s or 3s or solitary. Capitula usually radiate or discoid, sometimes disciform; calycular bracts erect to spreading or reflexed, herbaceous; involucres mostly hemispheric or campanulate to cylindric; phyllaries persistent, mostly 2-seriate, usually distinct, sometimes partially connate 0.05-0.1 × their lengths, mostly oblong or ovate to oblong-lanceolate, papery to membranous or scarious, usually striate with brownish veins, margin usually hyaline; receptacles flat or slightly convex, paleate; paleae usually deciduous, usually straw-colored, sometimes yellow to orange, with darker striae, flat to slightly navicular. Ray florets usually 1-seriate, usually neuter, sometimes female and sterile; corollas usually yellow, sometimes white or pinkish. Disk florets bisexual, fertile; corollas usually yellow to orange, sometimes whitish or purplish, tubes shorter than throats, lobes (3-)5, deltate; staminal filaments glabrous; style branch tips deltate or lanceolate to subulate. Achenes usually obcompressed to flat, unequally 3- or 4-angled, and cuneate to oblanceolate or obovate, sometimes (all or inner) equally 4-angled and linear-fusiform, rarely subterete, faces smooth, striate, or tuberculate, glabrous or hairy, each sometimes with 2 grooves, margin usually retrorsely, sometimes patently or antrorsely, barbed or ciliate, apices sometimes attenuate, not beaked; pappus absent, or persistent, of (1 or)2-4(-8) usually retrorsely, sometimes antrorsely, barbellate or ciliate, rarely smooth, awns. x = 12.
See Sherff, Publ. Field Mus. Nat. Hist., Bot. Ser. 16: 1-709. 1937.
About 150-250 species: widespread, especially in subtropical, tropical, and warm-temperate North and South America; ten species (one endemic, one introduced) in China.