Description from Flora of China
Herbs annual to perennial. Stems procumbent to ascending, often much branched. Leaves opposite, entire, those of a pair unequal, terete to flat, petiole broadening at base forming a sheath around stem. Inflorescences of solitary or clustered flowers subtended by scarious bracts and bracteoles. Flowers sessile or pedicellate. Perigone tube campanulate, short to long, smooth to hairy; lobes 5, inside white, green, or pink to purple, outside fleshy. Stamens 5 to many, adnate with perigone. Perigynous, ovary with a truncate or impressed apex, 1-loculed; stigma 1. Capsule terete or turbinate, operculum with 1 or 2 seeds falling off as a whole. Seeds globose-reniform to lentil-shaped.
About 28 species: Africa, tropical Asia, Australia, two species in tropical South America, two species pantropical; one species in China.