Plants annual or perennial, growing emerged in fresh waters. Roots without septations. Stems rhizomatous. Leaves emerged or submersed; emerged leaves petiolate, petioles mostly triangular, rarely terete, blades linear to broadly ovate, with pellucid markings absent or present as dot or lines; submersed leaves mostly sessile phyllodes, blades mostly linear to rarely ovate, pellucid markings present as dots or lines, or absent. Inflorescences erect, emerged, racemose or paniculate scapes, rarely umbelliform, whorls 1-18; pedicels ascending to recurved. Flowers bisexual, subsessile to pedicellate; sepals herbaceous to coriaceous, without sculpturing, reflexed to spreading or rarely slightly appressed; petals white, larger than sepals; stamens 9 to many, filaments glabrous, anthers versatile or basifixed; gynoecium of many carpels, carpels 1-ovuled, styles terminal or lateral. Achenes terete, often longitudinally ribbed and glandular, rarely with a dorsal keel, separating when mature, beak lateral or terminal, erect, horizontal, or recurved.
Americas: Argentina: present Chile: present Guianas: present Neotropics: present northern U.S.A: present
Native to the Americas, consisting of 27 species distributed from northern U.S.A. to Argentina and Chile; 26 species in the Neotropics; 8 species in the Guianas.