Lemna gibba L.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Thallus orbicular to obovate, 3–6 mm, nearly symmetrical, solitary or often in groups of 2–4, very often anthocyanic beneath, mottled yellow-green above, or anthocyanic on both sides, ± evidently 3–5-nerved, usually gibbous, the upper surface flat or nearly so and with ± prominent papillae, the lower usually strongly bulged, the tissue with larger and more prominent air-spaces than in no. 3 [Lemna minor L.]; spathe sac-like, open only at the top; fr narrowly winged; seeds (1)2–3(–6), distinctly many- ribbed; 2n=40, 50, 70, 80. Nearly cosmop., but rare and irregular with us.