Erect, annual, profusely branched, stout glabrous herbs; stems and branches more or less terete or obtusely tetragonous; leaves opposite, sometimes whorled, linear-lanceolate, acute, entire, slightly cordate at base; stipules absent; flowers numerous, densely clustered in axils; bracteoles usually 2; calyx campanulate, 3-5-toothed, often with minute interposed folds, teeth short, triangular; petals 5, small, inserted between the calyx teeth; stamens 5 (2-8), inserted on the calyx tube; ovary enclosed in the calyx tube, 1-5 celled, septa very thin, ovules many, axile placenta; style filiform, short; stigma capitate; capsule membranous, globose-ellipsoid or hemispheric, enclosed in persistent calyx, 2-3 valved, irregularly breaking up or circumsciss; seeds many, small, smooth, rounded or obovoid.