Description from Flora of China
Herbs annual or perennial, erect, ascending, or prostrate, sometimes with tuberous roots. Stems viscid glandular pubescent or glabrescent. Leaves opposite. Inflorescences axillary, in 1-3-flowered cymes, sometimes panicles, or rarely 1-flowered, enclosed within a campanulate involucre; involucre lobes 5, reticulate-venose, enlarged and membranous after flowering. Flowers bisexual, inconspicuous, opening in morning; pedicel 20-25 mm. Perianth constricted beyond the ovary, 0.6-0.8(-1) cm, limb rose-red or light red-purple, plicate, campanulate or short funnelform, often oblique, ca. 0.8 cm in diam., deciduous. Stamens (2-)3(-5); filaments capillary, circinately incurved, jointed at the ovary base. Style filiform; stigma capitate. Fruit oblong, fusiform, obovoid, or terete, small, smooth or tuberculate, without sticky glands. Seed: embryo curved, cotyledons enclosing the endosperm, radicle elongated.
About 25 species: mainly in warm regions of America, one species in Asia.