Minute gregarious floating water-plants, without distinct stems or true leaves, consisting of green scale-like fronds free from one another or 2–3 cohering by their margins, either rootless or more generally giving off 1 or several capillary rootlets from the under-surface. Flowers very seldom produced, most minute, placed in clefts on the edges of the frond, or sunk on its surface, naked or enclosed in a spathe, usually a single female with 1 or 2 males by its side. Perianth wanting in both sexes. Male flower: Stamens 1 or 2; filaments short; anthers 1–2-celled. Female: Ovary sessile, 1-celled, narrowed into a short and stout style; stigma simple; ovules 1–7. Fruit a somewhat fleshy utricle, with 1 or several seeds; albumen fleshy or wanting; embryo straight, axile.

An order of 2 genera and 20 species, found in still waters in all countries, both temperate and tropical. It contains the smallest of all known flowering-plants, all of them being of exceedingly simple structure, and very seldom found in flower.