Sp. Pl. 2: 970. 1753; Gen. Pl. ed. 5; 417, 1754
Roots 1 per frond. Fronds floating or submersed, 1 or 2–20 or more, coherent in groups or forming chains, lanceolate-ovate, flat or gibbous, 1–15 mm, margins entire or denticulate, upper surfaces sometimes with small conic papillae along veins (especially at node and near apex); air spaces in tissue; reproductive pouches 2, lateral, at base from which daughter fronds and flowers originate, triangular; veins 1–5(–7), originating from point in proximal part of frond (node) or if more than 3 veins present, outer ones sometimes branching distally from inner ones; scale at base of frond absent; anthocyanins sometimes present; pigment cells absent (red pigmentation present in some species); turions absent (sometimes present in L. turionifera). Flowers 1(–2) per frond, surrounded by small utricular, membranous scale; stamens 2, 4-locular. Seeds 1–5, longitudinally ribbed. x = 10, 21, 22
Worldwide except arctic and antarctic regions.
In Lemna the connection to the mother frond is formed by a thin white stipe at the base that falls off or decays after the frond is grown (frond of L. trisulca is narrowed at base into a green stalk persisting on frond).
Species 13 (9 in the flora).
|1||Fronds submersed (except when flowering or fruiting), margins denticulate distally, 2–3.5 times as long as wide, base suddenly narrowed into green stalk, 2–20 mm.||Lemna trisulca|
|1||Fronds floating, margins entire, 1–3 times as long as wide.||> 2|
|2||Fronds with 1 vein.||> 3|
|2||Fronds with 3–5(–7) veins.||> 4|
|3||Vein mostly prominent, longer than extension of air spaces, or running through at least 3/4 distance between node and apex||Lemna valdiviana|
|3||Vein sometimes indistinct, very rarely longer than extension of air spaces, not longer than 2/3 distance between node and apex||Lemna minuta|
|4||Root sheath winged at base; root tip usually sharp pointed; roots to 3(–3.5) cm; fronds without reddish color or spots of anthocyanin, mostly with 1 very distinct papilla near apex on upper surface.||> 5|
|4||Root sheath not winged; root tip mostly rounded; roots often longer than 3 cm; fronds often with reddish tinge or spots of anthocyanin, with or without distinct papilla near apex on upper surface.||> 6|
|5||Seeds with 35–70 indistinct ribs (to count ribs remove membranous pericarp), staying within fruit wall after ripening; root sheath wing 2–3 times as long as wide||Lemna perpusilla|
|5||Seeds with 8–26 distinct ribs, falling out of fruit wall after ripening; root sheath wing 1–2.5 times as long as wide||Lemna aequinoctialis|
|6||Plants forming small, olive to brown, rootless turions, which sink to bottom||Lemna turionifera|
|6||Plants usually without distinct turions.||> 7|
|7||Largest air spaces longer than 0.3 mm; if red-colored on lower surface, coloring beginning from margin; ovary (1–)2–7-ovulate||Lemna gibba|
|7||Largest air spaces 0.3 mm or shorter; if pigmented on lower surface, red coloring beginning from attachment point of root; ovary 1-ovulate.||> 8|
|8||Fronds not reddish on lower surface (or at least much less so than on upper); greatest distance between lateral veins near or proximal to middle.||Lemna minor|
|8||Fronds often reddish on lower surface (more intensely so than on upper); greatest distance between lateral veins near or distal to middle.||> 9|
|9||Fronds flat, with mostly distinct papillae on midline of upper surface; seeds with 30–60 indistinct ribs||Lemna turionifera|
|9||Fronds often gibbous, with very distinct papillae above node and near apex on upper surface but not between node and apex; seeds with 10–16 distinct ribs||Lemna obscura|