Description from Flora of China
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.