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.		5 Lemna trisulca
+	Fronds floating, margins entire, 1--3 times as long as wide.		(2)
2 (1)	Fronds with 1 vein.		(3)
+	Fronds with 3--5(--7) veins.		(4)
3 (2)	Vein mostly prominent, longer than extension of air spaces, or running through at least 3/4 distance between node and apex		8 Lemna valdiviana
+	Vein sometimes indistinct, very rarely longer than extension of air spaces, not longer than 2/3 distance between node and apex		9 Lemna minuta
4 (2)	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)
+	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 (4)	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		6 Lemna perpusilla
+	Seeds with 8--26 distinct ribs, falling out of fruit wall after ripening; root sheath wing 1--2.5 times as long as wide		7 Lemna aequinoctialis
6 (4)	Plants forming small, olive to brown, rootless turions, which sink to bottom		4 Lemna turionifera
+	Plants usually without distinct turions.		(7)
7 (6)	Largest air spaces longer than 0.3 mm; if red-colored on lower surface, coloring beginning from margin; ovary (1--)2--7-ovulate		1 Lemna gibba
+	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 (7)	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.		3 Lemna minor
+	Fronds often reddish on lower surface (more intensely so than on upper); greatest distance between lateral veins near or distal to middle.		(9)
9 (8)	Fronds flat, with mostly distinct papillae on midline of upper surface; seeds with 30--60 indistinct ribs		4 Lemna turionifera
+	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 KSN:>2. Lemna obscura		(1)