Spirodela Schleid.

Fronds with (5)7-16(21) veins, with several layers of air spaces, with pigment cells, and with raphids and druses, surrounded at the base by a bifid scale (prophyll) covering the place of attachment of the roots; roots (5)7-16(21) per frond of which only 1-5 (never all) perforate the scale. Utricular scale-like leaf surrounding the flowers with narrow opening at the top; Pistil with 1 amphitropous ovule or 2-5 anatropous ovules; seeds 0.7-2.0 mm long, with 12-22 longitudinal ribs. Two spp., one widely distributed, one in South America.