Perennial or annual herbs or low shrubs. Leaves opposite or whorled; stipules absent. Flowers bisexual, irregular, usually arranged in cymose heads with involucral bracts at the base of the head and each flower subtended by a receptacle bract. Petals connate, imbricate. Stamens 2–4, alternating with the petals, epipetalous. Ovary inferior, with a solitary pendulous ovule. Fruit an inferior nut. 11 gen., cosmop.