Lemna
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
Distribution
Worldwide except arctic and antarctic regions
Discussion
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).
Selected References
None.
Lower Taxa
Key
| 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 |