Small aquatic or subaquatic herbs, annual or rarely perennial. Roots numerous, unbranched. Stem very short, branching to form a leafy tuft, often with minute multicellular hairs in leaf axils. Leaves all basal, linear to filiform, lacking distinct sheaths or ligules, 1-veined, glabrous; stomata lacking subsidiary cells or absent. Inflorescence a heterogamous or homogamous capitulum, terminal, sessile or scapose, with an involucre of 1–4 pairs of opposite, membranous, glabrous, 1-veined bracts often forming an apparent whorl. Flowers numerous, minute, unisexual, lacking perianth or bracteoles. Male flower a solitary stamen; anther elliptic to linear, basifixed, bilocular, dehiscing by lateral slits. Female flower a solitary unicarpellate pistil; ovary stalked, unilocular; ovule 1, pendulous, anatropous; stigma terminal, sessile, with 2–10 hairs, each a single row of distended cells. Fruit membranous, dehiscent or indehiscent. Seed solitary, ovoid; embryo minute, peripheral, covered by an operculum; endosperm replaced by starchy perisperm.
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