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See list of 6 species in this genus

Specimens of Lemna are rarely collected with flowers or fruit, L. perpusilla being the only species that reproduces sexually with any frequency. The veins are sometimes difficult to observe on fresh or dried specimens. Therefore, splitting the collection and using both traditional methods (drying and mounting on acid-free paper) along with chemical preservation assist greatly with identification and vouchers the collection. Preserving thalli in 70% ethanol (C₂H₅OH) makes them transparent, which allows veins and aerenchyma tissue to be more easily viewed (place the thallus on a slide to permit light to pass through it). Dried specimens can be boiled in 70% ethanol and then the pigments cleared with 10% sodium hypochlorite (NaOCl).

  • 1a. Thalli (3–) 6–15 mm long, tapering to a long stalk, with distally denticulate margins; lateral thalli remaining attached to the parent thallus by a narrow, green stalk, forming tangled colonies suspended just below the surface of the water
  • 1b. Thalli 1–6 (–8) mm long, sessile or nearly so, with entire margins; lateral thalli 
separating from the parent thallus or remaining attached by a thin, white stipe and 
forming floating rosettes
    • 2a. Mature thalli with a single vein (the vein sometimes obscure and then the thallus appearing unveined)
      • 3a. Vein usually not extending past aerenchyma tissue, extending less than of distance from root node to tip of thallus; thalli 1–2.5 (–4) mm long, 1–2 times as long 
as broad, broad-elliptic to broad-oblong or obovate, usually solitary or paired, symmetrical at the base
      • 3b. Vein usually extending past aerenchyma tissue, extending at least ¾ of the distance from the root node to the tip of the thallus; thalli 2–4 mm long, 1.3–3 times as long as wide, elliptic to lanceolate or obovate, often in clusters of (2–) 4–8 (–10), asymmetrical at the base
    • 2b. Mature thalli with 3 (–5) veins
      • 4a. Root sheath winged at base; roots up to 3.5 cm long, usually sharply pointed at apex; thalli without anthocyanic pigment, usually with a distinct apical papilla
      • 4b. Root sheath not winged at base; roots to 15 cm long, frequently longer than 3.5 cm, rounded at apex; thalli often with at least some spotting or diffuse coloring with anthocyanic pigment, with or without a distinct apical papilla
        • 5a. Plants sometimes with small green-brown to brown rootless turions, these 
0.8–1.6 mm in diameter and sinking to bottom; thalli often with anthocyanic coloration on lower surface, the coloration more intense than that of upper surface, usually with a distinct row of minute papillae along the midvein
        • 5b. Plants without turions; thalli typically green on lower surface, when slightly anthocyanic, the coloration less intense than that of upper surface, with or without 
a distinct row of minute papillae along the midvein

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 Show photos of:   Each photo represents one species in this genus.