Fronds boat-shaped, 0.7--1.5 mm, 1.3--2 times as long as wide, 0.7--1 times as deep as wide, with point at apex bent upward; papilla absent; upper surface intensely green, with 50--100 stomates; pigment cells present in vegetative tissue (visible in dead fronds as brown dots). 2n = 20, 22, 30, 40.
Aquatic herb Flowers: occurring very rarely, borne one per plant body, lacking sepals and petals, with one stamen and one nearly smooth seed. Fruit: bladder-like (utricle) and thin-walled. Roots: absent. Plant body: not differentiated into stem and leaves, solitary or in pairs, green with scattered pigment cells visible under 10x magnification, 0.7 - 1.5 mm long, one and a third to two times as long as wide, three-quarters to one time as deep as wide, lacking projections and air spaces in tissue, always floating (even when crowded). A conical cavity at the tip produces daughter plants, while a cavity beside the midvein of the upper surface produces flowers.
Similar species: Wolffia columbiana is distinguished by having a spherical plant body that is deeper than wide and lacks pigment cells. The plant body of Wolffia brasiliensis differs by having a rounded tip and a tent-shaped projection in the center.
Flowering: summer to early fall
Habitat and ecology: Quiet waters of lakes, ponds, ditches, and slow-moving rivers. This species is likely more common than records show.
Occurence in the Chicago region: native
Notes: The genus Wolffia consists of the world's smallest flowering plants.
Etymology: Wolffia is named after J.F. Wolff, German botanist and physician (1778-1806). Borealis means northern.
Thallus bright green and shiny above, pale beneath, ellipsoid to oblong-ovoid, symmetrical, mostly 0.1-1.2 נ0.2-0.5 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)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.