Annual to perennial herbs, rarely (not in Australia) undershrubs, usually hairy. Leaves opposite; stipules absent; bracts herbaceous to slightly scarious. Flowers usually in dichasia, rarely solitary, usually 5-merous, bisexual; sepals free; petals white, emarginate to bifid, sometimes absent; stamens usually 10 (rarely 8), sometimes 5 or fewer; ovary 1-celled, with many ovules, styles usually (3–)5(–6). Capsule oblong, usually more or less curved, opening by twice as many teeth as styles; seeds numerous, discoid or slightly nautiloid, tuberculate.
60 species, almost cosmopolitan but none indigenous to Australia, 8 species naturalised.
Because of the frequently inadequate Australian herbarium material and the observed variation in it, together with problems still apparent in the European taxonomy of this difficult genus, infraspecific taxa are not used here.