Annual or perennial herbs; stems 4-angled. Leaves and stipules more or less equal, in whorls of 4–9(–12). Inflorescences usually upper-axillary cymes, sometimes reduced to single flowers. Flowers bisexual; calyx usually absent; corolla rotate, 4-lobed, often hairy, white or yellow; corolla-tube very short; stamens 4, with filaments scarcely fused to corolla-tube, anthers lacking appendages; ovary inferior, 2-celled, ovules 1 per cell, style 2-lobed, stigmas capitate. Fruit dry, 2-lobed, breaking freely into 2 mericarps.
About 400 species, cosmopolitan; 29 species in Australia, 20 native.