Spirodela

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.

FRONDS: floating on the water's surface (only turions sink to the ground), 2 to a few cohering, each obovate to circular, flat or sometimes gibbous, surrounded at the base by a small bifid scale covering the point of attachment of the roots, with two triangular lateral reproductive pouches at the base from which daughter fronds and flowers originate; margins entire; air spaces present; nerves (3 )5 2l; roots (1 )2 21; anthocyanins (especially on the lower surface) and pigment cells present (visible in dead fronds as brown dots). FLOWERS: 1( 2) per frond, surrounded by a small utricular scale, the stamens 2, 4 locular. SEEDS: 1 3, longitudinally ribbed. NOTES: 3 spp.; world wide distribution except arctic and antarctic zones. (Greek: Speira = winding + delos = distinct). REFERENCES: Landolt, Elias. 1992. Lemnaceae. Ariz.-Nev. Acad. Sci. 26(1)2.

Thallus flat or slightly convex on both sides, usually somewhat asymmetrical, marked with 3-15 obscure to evident nerves diverging from a nodal point near the base; roots 2-12+, descending from the nodal point, each with a single vascular strand; reproductive pouches 2, lateral, opposite the nodal point; infl (seldom produced) in one of the lateral pouches, with 2 staminate fls and one pistillate, collectively subtended by a tiny spathe; anther bilocular; ovules 1-4. 4, cosmop.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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