Description from Flora of China
Herbs annual or perennial. Stems creeping or suberect, with dichotomous branching. Leaves opposite, shortly petiolate; leaf blade orbicular or ovate-cordate, 3--5-veined; stipules small, bristly, often fugacious. Inflorescence a cyme, terminal. Flowers (4-or)5-merous, pedicellate, small. Sepals free, green, herbaceous. Petals absent or (1--)3--5, 2-lobed, occasionally appendiculate in sinus and/or auriculate at top of claw. Stamens (2--)5, opposite sepals; filaments joined at base. Ovary with few ovules; styles (2 or)3, joined at base. Fruit a capsule, (2-or)3-valved, 1- to many seeded. Seeds ovate or reniform, small, flat; testa tuberculate.
About 48 species: native to Central and South America from Mexico to Patagonia, also widely distributed as weeds in the Old World tropics; two species in China.
(Authors: Lu Dequan; Michael G. Gilbert)