Description from Flora of China
Herbs annual, perennial, or subshrubs, erect or prostrate, richly branched. Leaves opposite, petiolate; leaf blade margin serrulate. Flowers numerous, arranged in axillary cymes or clustered in leaf-axils, rarely solitary, very small. Sepals 5, free, leathery, with conspicuous midrib, margin membranous, apex long acuminate. Petals 5, free, membranous. Stamens 5-10. Ovary ovoid-globose or subglobose, 5-loculed, many-ovuled, apex subacute; styles short; stigmas capitate. Capsule sub-bony, 5-septicidal. Seeds oblong, slightly curved, reticulate with elliptic pits.
About 25 species: tropical and warm-temperate regions; three species in China.