Lemna perpusilla Torr.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Thallus ± convex on both sides, with a prominent papilla at node and apex above, and usually with smaller papillae along the midline, obliquely obovate, asymmetrical and ± falcate, 1–2.5 mm, commonly 1–2 times as long as wide, solitary or up to 5 together, not anthocyanic, with 1–3 obscure nerves or nerveless; root-sheath winged at base (unique in the genus); spathe reduced, open; fr asymmetrical, with a persistent lateral style; seed solitary, ovoid, strongly ribbed; 2n=40. Nearly cosmop., and found in most of our range. (L. trinervis)