Fronds globular to ovoid, 0.5–1.5 mm, 1–1.3 times as long as wide, 1.2–1.5 times as deep as wide, slightly pointed at apex, papilla absent; upper surface intensely green (not transparent), with 10–100 stomates; pigment cells absent in vegetative tissue. 2n = 30, 40, 42, 44–46, 50, 60, 62 (Europe, Africa).
Phenology: Flowering (very rare) late spring–summer.
Habitat: Mesotrophic to eutrophic, quiet waters in temperate to subtropical regions with relatively mild winters
Elevation: 0–800 m
Introduced; Calif., South America (Brazil), w Eurasia, Africa.
Wolffia arrhiza was reported from Alberta because of the size of epidermal cells (G. C. D. Griffiths 1988). The plants are within the variation shown by W. columbiana, however.