Annual or perennial herbs, or subshrubs; stems often jointed and swollen at nodes. Leaves opposite and decussate, or whorled, simple, entire; exstipulate or less often with small, scarious stipules, often connate at base; often sessile or indistinctly petiolate, petiole often amplexicaul. Inflorescence a terminal, solitary flower, or a dichasial cyme or umbel. Flowers regular, bisexual, rarely unisexual; sepals 4-5, imbricate, free or united in a tubular, 4- or 5-toothed calyx, persistent; petals as many as sepals or sometimes some or all absent, free or nearly so, sometimes bifid and clawed; stamens in 1 or 2 whorls, sometimes in part absent, filaments free, basally united or epipetalous ones adnate to base of petal, anthers 2-locular, versatile, longitudinally dehiscent; ovary superior, sessile or shortly stipitate, 1(-5)-locular, ovules 1-numerous, campylotropous, placentae basal, central or free central, style 1, 1 lobed or 2-5 stigmas, or styles 2-5, free or united at base. Fruit a capsule, valvate or dehiscing by 2-5 apical teeth; seeds usually numerous, small, with mealy endosperm, embryo curved around perisperm.
Cosmopolitan: present Guianas: present
Approximately 2200 species in 86 genera, cosmopolitan, but most widely distributed in temperate regions of the northern hemisphere (with a center in the Mediterranean and Irano-Turanean regions), rarely indigenous to the tropics; in the Guianas 2 species in 2 genera.