Description: Erect or floating glabrous herbs with stems ribbed at the base.
Leaves opposite, toothed, petiolate.
Inflorescence terminal, heads in dense corymbs; heads hemispherical; involucral bracts numerous, leaf-like, in 1 or 2 series, obtuse to acuminate, ± fused; receptacle naked, convex, reticulate. Ray florets absent. Disc florets bisexual, tubular, 5-toothed, the lobes shortly fused. Anthers obtuse at base. Style branches elongate, with clavate appendages.
Achenes compressed dorsally, 5-angled, papillose, glabrous; pappus absent.
Distribution and occurrence: World: c. 5 species, Mexico & subtropical South America. Australia: 1 species (naturalized).
Text by M. F. Porteners