Wolffia arrhiza

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).

Flowering (very rare) late spring--summer. Mesotrophic to eutrophic, quiet waters in temperate to subtropical regions with relatively mild winters; 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.