Annual or perennial herbs. Leaves opposite, cauline and/or rosetted at base, usually sessile. Inflorescence a compact, cylindrical spike or subglobose head, terminal or axillary, usually sessile between 2 leaves, extending beyond leaves in fruit. Flowers bisexual, each subtended by a membranous bract and 2 bracteoles; perianth of 5 segments, free or fused near base, midvein thickened, margins membranous, outer surface sometimes woolly; stamens 5, fused into a membranous cup for part or all of their length, stamens alternating with 5 staminode-like lobes; ovary superior, unilocular; stigma shortly bi-lobed. Fruit an indehiscent, membranous utricle.
About 130 species, mainly in tropical regions of America and Australia, extending to Timor; about 37 species in Australia, 2 naturalised.