Roots absent. Fronds floating or submersed (only turions sink to bottom), 1 or 2, coherent, each frond globular, ovoid, or boat-shaped, 3-dimensional, smaller than 1.6 mm, margins entire; air spaces not in tissue; reproductive cavity terminal, at base from which daughter fronds (but no flowers) originate, conic, lower side of cavity with short tract of elongated cells along median line forming connection between node and attachment to mother frond; veins 0; scale at base of frond absent; anthocyanins absent; in some species pigment cells present (visible in dead fronds as brown dots); turions light green, globular, smaller than growing fronds. Flowers 1 per frond, originating in cavity ± on median line of upper frond surface, not surrounded by utricular scale; stamen 1, 2-locular. Seeds 1, nearly smooth. x = 10, 20, 21, 22, 23.

Distribution

Nearly worldwide (except arctic and antarctic regions).

Discussion

Species 11 (5 in the flora).

Selected References

None.

Lower Taxa

Wolffia arrhiza, Wolffia borealis, Wolffia brasiliensis, Wolffia columbiana, Wolffia globosa

Key

1	Fronds boat-shaped, 0.3–1 times as deep as wide, with pigment cells in vegetative tissue.	> 2
1	Fronds globular to ovoid, 1–1.5 times as deep as wide, without pigment cells in the vegetative tissue.	> 3
2	Fronds 1–1.5 times as long as wide, rounded at apex, papilla usually prominent in center of upper surface	Wolffia brasiliensis
2	Fronds 1.3–2 times as long as wide, with point at apex bent upwards, papilla absent	Wolffia borealis
3	Fronds with 10–100 stomates, slightly pointed at apex, the adaxial upper surface intensely green (not transparent)	Wolffia arrhiza
3	Fronds with 1–10(–30) stomates, rounded or slightly pointed at apex, upper surface transparently green.	> 4
4	Fronds 1.3–2 times as long as wide, 0.3–0.5 mm wide	Wolffia globosa
4	Fronds 1–1.3 times as long as wide, 0.4–1.2 mm wide	Wolffia columbiana