2. Lemna obscura (Austin) Daubs, Illinois Biological Monographs. 34: 20. 1965.
Lemna minor Linnaeus var. obscura Austin in A. Gray, Manual ed. 5, 479. 1867
Roots to 15 cm, tip mostly rounded; sheath not winged. Stipes white, small, often decaying. Fronds floating, 1 or 2--5 or more, coherent in groups, obovate, flat or gibbous, 1--3.5 mm, 1--1.5 times as long as wide, margins entire; veins 3, greatest distance between lateral veins near middle; very distinct papillae near apex, some smaller indistinct ones on midline of upper surface; lower surface very often red colored (more intensely than on upper), coloring beginning at attachment point of root, lower upper surface sometimes with red spots; air spaces shorter than 0.3 mm; distinct turions absent. Flowers: ovaries 1-ovulate, utricular scale with narrow opening at apex. Fruits 0.5--0.7 mm, not winged. Seeds with 10--16 distinct ribs, staying within fruit wall after ripening. 2n = 40, 42, 50.
Flowering (occasional) spring--fall. Mesotrophic to --eutrophic, quiet waters, in temperate to subtropical regions with mild winters; 0--800 m; Ala., Ark., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Md., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., S.C., S.Dak., Tenn., Tex., Va., Wis.; c Mexico; South America (Colombia, Ecuador); Hawaii.