Fronds nearly globular, 0.5--1.4 mm (x 0.4--1.2 mm), 1--1.3 times as long as wide, 1--1.3 times as deep as wide, rounded at apex, papilla absent; upper surface transparently green, with 1--10(--30) stomates; pigment cells absent in vegetative tissue. 2n = 30 (H, U), 40 (U), 42 (D), 500 (U), 70 (70), 80 (U). Flowering (very rare) summer--early fall. Mesotrophic to eutrophic, quiet waters in temperate to subtropical regions; 0--1100 m; Alta., Man., Ont., Que., Sask.; Ala., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., S.C., S.Dak., Tenn., Tex., Vt., Va., W.Va., Wis.; Mexico; West Indies (Curaçao); Central America; South America. I know of no specimens of Wolffia columbiana from Rhode Island.
OTHER REFERENCES Dore, W. G. 1957. Wolffia in Canada. Can. Field-Naturalist 71: 10--16. Brooks, J. S. 1940. The Cytology and Morphology of the Lemnaceae. Ph.D. thesis. Cornell University. Landolt, E. and Urbanska-Worytkiewicz K. 1980. List of the studied Lemnaceae samples: origin and chromosome numbers. Veröff. Geobot. Inst. Rübel Zürich 70: 205--247.
Thallus light green and translucent, broadly ellipsoid to globose, asymmetrical, mostly 0.8-1.4 נ0.6-1.2 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.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.