Annual (often ephemeral), glabrous herbs. Leaves sessile. Inflorescence a dichasium or monochasium, or a single flower. Calyx 4-lobed for less than half its length; corolla-lobes 4, spreading, tube narrowed above ovary; stamens 4, shortly exceeding corolla-tube, adnate to tube near base, anthers not or weakly twisted after dehiscence; ovary ovoid, 1-locular, stigma obscurely 2-lobed. Capsule septicidal, splitting in 2 from apex, largely remaining within withered perianth; seeds numerous, small, reticulate.
2 species; 1 from western Europe, north Africa and south-western asia, the other from western USA and western South America, both widely naturalized.