Small aquatics, floating at the surface or sometimes completely submerged in which case they come to the surface only in the flowering period. Inflorescence 1, lateral, developing from one of the budding pouches, consisting of 1 ♀ and 2 ♂ flowers enclosed by a membranous spathe. Ovary with 1 orthotropous or amphitropous ovule or with 2-4 anatropous ovules. Fruit symmetric or asymmetric, more or less compressed, with or without wings. Seeds longitudinally ribbed, rarely smooth.
Arctic regions absent, worldwide present
9 spp., worldwide with the exception of the Arctic regions.
The genus can be subdivided into 2 subgenera:
- Subg. Lemna. Floating water plants with round, elliptic, oblong or obovate fronds. Root 1, with a distinct root sheath. Slit of budding pouch coinciding with the margin of the frond, rarely ventral to the margin. Ovary with 1-4 ovules.
- Subg. Staurogeton . Submerged water plants, only rising to the surface in the flowering period. Fronds oblong to linear-lanceolate, with or without a root. Root sheath indistinct. Slit of budding pouch just dorsal to the margin of the frond.