Wolffia columbiana H.Karst.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Thallus light green and translucent, broadly ellipsoid to globose, asymmetrical, mostly 0.8–1.4 × 0.6–1.2 mm, 1–1.3 times as long as wide, not punctate, floating low in the water, only a small circular portion of the upper surface in contact with the air; upper surface slightly roughened, sometimes to form ca 3 obscure papillae; stomates 3–5; 2n=30, 40, 50, 70. Quiet water; Mass. and N.H. to s. Ont. and Minn., s. to S. Amer.; often forming more than one layer, some wholly submersed.