Annual or perennial, small herbs. Roots fibrous. Stem cylindrical, erect or ascending, branched, sometimes stoloniferous at base. Leaves opposite, sessile, united at the base, flat or terete, entire. Inflorescence axillary or terminal, cymose. Flowers minute, bisexual, 3-5-merous. Calyx polysepalous. Corolla polypetalous, white, greenish, pinkish, yellow or red, sometimes shorter than sepals. Stamens in one whorl, as many as and smaller than petals. Carpels free, style short, stigma minute. Follicles 1-many seeded.