| 1 | Floating water plants. Fronds round to oblong or obovate, with 1 root. Slit of budding pouch coinciding with the margin of the frond, rarely ventral to the margin. Subg. Lemna. |
| 1' | Submerged water plants, only floating at the surface when flowering. Fronds oblong to linear-lanceolate, often without a root. Slit of budding pouch just dorsal to the margin of the frond. Subg. Staurogeton. |
| 2 | Root sheath with lateral wings. Fruit asymmetric; ovule 1, orthotropous. |
| 2' | Root sheath without wings. Fruit symmetric; ovules amphitropous or anatropous. |
| 3 | Frond mostly symmetric, ventral side flat to slightly convex, never inflated. Cells on ventral side not visible to the naked eye. Fruit without wings. Ovule 1, amphitropous. Slit of budding pouch coinciding with the margin of the frond. |
| 3' | Frond mostly asymmetric, ventral side flat to strongly inflated. Cells on ventral side clearly visible to the naked eye. Fruit with lateral wings. Ovule 1, amphitropous or ovules 2-4, anatropous. Slit budding pouch ventral to the margin of the frond. |
| 4 | Margin denticulate towards the apex. Stipe usually strongly elongate, green, persistent. |
| 4' | Margin entire, Stipe short, hyaline, fugacious. |
