Herbs or subshrubs. Leaves opposite, rarely alternate. Flowers perfect, clustered in heads. Bracteoles with distal crest. Tepals 5, equal or not, long hairy or glabrous. Stamens 5; filaments united into a tube or cup at base, lateral appendages present, 3-parted at apex, pseudostaminodes absent; anthers 1-loculed. Ovary 1-loculed, with a pendulous ovule; stigmas 2 or 3, linear, or stigma 2-parted. Utricles globose or oblong, compressed, indehiscent. Seeds shiny, lenticular.

About 100 species: mainly in North and South America and Pacific Islands, naturalized elsewhere; two species (one introduced) in China.