Erect or prostrate annual or perennial herbs, stem tetragonal. Leaves opposite, young leaves fascicled or falsely verticillate, sessile or shortly petioled; stipules connate with petiole, more or less sheathing both sides into many setae -like structures or fimbriate. Flowers usually in axillary or terminal heads or verticillate, generally subtended by leafy bracts, sometimes cymose or corymbose, small, white or Noe. Calyx-tube ovate, persistent 2-4 lobed. Corolla infundibular, hemispherical or salverform, 4-lobed. Stamens 4, exserted. Stigma bifid or undivided. Capsule crowned with persistent calyx-limb, bilocular, mature capsule dehiscing septicidaly from the apex; coccus one seeded with an equal longitudinal cleft, dehiscing longitudinaly. Seeds ovate, oblong, ventrally marked with longitudinal grooves.