Annual or perennial herbs, glabrous or glandular-hairy; rootstock usually woody and tuberous. Leaves opposite, petiolate. Inflorescence a terminal cyme, flowers rarely solitary; flowers subtended by 5 sepal-like involucral bracts. Flowers bisexual; perianth trumpet-shaped, variously coloured, showy, 5-lobed, deciduous; stamens 5–6, equal to or exceeding perianth tube; style simple, stigma capitate. Fruit (4–)5-ribbed or tuberculate, without glands.
About 60 species mainly from America; 1 species naturalised in Australia.