1. Spirodela Schleiden, Linnaea. 13: 391. 1839.
Duck-meal [Greek speira, winding, and delos, distinct]
Roots (1--)2--21 per frond, abaxial surface at node. Fronds floating (only turions sink to bottom), 2--10, coherent in groups, each obovate-circular, flat or gibbous, 1.5--10 mm, margins entire; air spaces in tissue; reproductive pouches 2, lateral, at base from which daughter fronds and flowers originate, triangular; veins (3--)5--16(--21), from point in proximal part of frond (node); small bifid 2-cleft membranous scale (interpreted as prophyllum) enveloping base of frond, covering point of attachment of roots; anthocyanins present (especially on abaxial surface); pigment cells present (visible in dead fronds as brown dots); turions present or absent, brownish olive, circular-reniform, smaller than growing frond. Flowers 1(--2) per frond, surrounded by small utricular, membranous scale; stamens 2, 4-locular. Seeds 1--3, longitudinally ribbed. x = 10, 18, 23.
Species 3 (2 in the flora): nearly worldwide.
