Wolffia borealis (Hegelm.) Landolt
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Thallus bright green and shiny above, pale beneath, ellipsoid to oblong-ovoid, symmetrical, mostly 0.1–1.2 × 0.2–0.5 mm, 1.3–2+ times as long as wide, ± pointed at both ends, the upper surface floating just above the water, punctate (post mortem) with brown pigment-cells, elevated distally into a terminal papilla; stomates 25–50; 2n=20, 30, 40. Quiet water; widespread in the U.S. and W.I., n. to Conn., s. N.H., s. Ont., and Minn., often with no. 3 [Wolffia columbiana H. Karst.], but more consistently forming a single layer with the upper surface exposed. (W. punctata, misapplied)