Wolffia arrhiza (L.) Horkel ex Wimm.

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.

Fronds minute, free floating, 0.8 x 1.5 mm, orbicular to ovate, sub-globose, somewhat flattish. Flowering specimens not seen in Pakistan.

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).

Distribution: Mostly subcosmopolitan.

introduced; Calif.; South America (Brazil); w Eurasia; Africa.

Flowering (very rare) late spring--summer.

Mesotrophic to eutrophic, quiet waters in temperate to subtropical regions with relatively mild winters; 0--800m.

Lemna arrhiza Linnaeus, Mant. Pl. 2: 294. 1771