Description from Flora of China
Herbs annual or perennial, ascending or creeping; stem much branched. Leaves opposite, margin entire. Flowers perfect, in sessile or peduncled heads, solitary in axils of bracts. Bracts and bracteoles persistent, membranous. Tepals 5, often unequal, membranous. Stamens 2-5; filaments connate into a tube or a cup at base; anthers 1-loculed; pseudostaminodes entire, dentate, or laciniate. Ovary globose or ovoid; ovule 1, pendulous; style short or long; stigma capitate. Utricles globose or ovoid, indehiscent, falling off with perianth. Seeds lenticular, vertical.
Alternanthera purpurea Y. T. Chang & M. J. Lai (Landscape Architecture [Taipei]  (4): 125. 1990, not Pynaert, 1879) was described from Taiwan. We have seen no specimens of this taxon and are therefore unable to treat it in this account. Further revisionary study is necessary.
About 200 species: mainly in North and South America, widely naturalized elsewhere; five species (four introduced) in China.