KEY TO THE SPECIES

1Fronds 5-12 mm long. Roots 7-16, one perforating the ventral scale. Nerves 7-12. Seeds smooth.
Spirodela polyrhiza
1'Fronds 2-5 mm long. Roots 1-9(-12), all perforating the ventral scale. Nerves 3-5. Seeds ribbed.
Spirodela punctata