Mostly annual or perennial herbs, sometimes small shrubs; stems often quadrangular, especially when young. Lvs opposite, sometimes alternate or whorled, entire. Fls (4)-6-(8)-merous, actinomorphic, often di- or tristylous, usually solitary in upper lf axils, sometimes in small whorled cymes which may form terminal spikes; bracteoles 2. Calyx tube straight, usually tubular, occasionally broad-campanulate, not spurred or pouched at base; epicalyx present. Petals 4-6, usually conspicuous, occasionally inconspicuous or 0, clawed, usually purplish, inserted in lower part of calyx tube. Stamens 2-12, sometimes in 2 series of different lengths. Ovary sessile or nearly so, 2-celled; stigma capitate; ovules 2-numerous in each cell. Capsule enclosed by calyx, septicidal with 2 valves or dehiscing irregularly.