Description from Flora of China
Herbs annual or perennial. Roots thick, obconic. Leaves opposite, petiolate or upper ones sessile. Inflorescence with each flower surrounded by a 5-fid, calyxlike involucre of bracts, arranged in terminal, ± flat-topped cymes, rarely solitary; involucral lobes erect, acuminate, plicate, not enlarged after flowering. Flowers bisexual, opening in afternoon, fragrant or not. Perianth constricted beyond ovary; limb showy, variously colored, expanded, salverform, plicate, distally 5-lobed, deciduous. Stamens 5 or 6, as long as perianth tube or exserted; filaments adnate to tube at base. Ovary ovoid or ellipsoid. Style linear, as long as stamens or longer, exserted; stigma capitate. Fruit globose or obovoid, coriaceous, crustaceous, or chartaceous, ribbed or tuberculate, without sticky glands. Seed: embryo curved, cotyledons folded, enclosing the mealy endosperm.
About 50 species: mainly in tropical America; one species (introduced) in China.