Wayne Armstrong's On-line Description of Wolffia arrhiza (Lemnaceae)
Home Keys To Genera List Of Species Search Lemna Spirodela Wolffia Wolffiella Wayne's Word

Wolffia arrhiza

(Linnaeus) Horkel ex Wimmer

Common Watermeal

Fl. Schles. 3: 140; 1857.


Roots: None.

Shape of Plant Body: Ovoid to nearly spherical with a distinctly flattened, dark green dorsal surface (transparent below), floating with only central portion of dorsal surface above water; without brown pigment cells.

Size: 0.8-1.3 mm long.

Veins: None.

Budding Pouch Position: One funnel-shaped pouch at basal end.

Flower (Fruit) Position: Within floral cavity on dorsal surface, not enclosed by a spathe.

Arrangement of Clonal Clusters: Solitary or 2 connected.

Habitat: Floating at surface of quiet streams and ponds, often mixed with other Lemnaceae and aquatic plants.

Range: Rare in California and probably introduced, generally in mild coastal and interior valleys of central and southern California; not known north of California or elsewhere in the United States; wide distribution throughout temperate, subtropical and tropical regions of Europe, Africa, western Asia and eastern Brazil.

Other Information:

May be distinguished from the similar and closely related W. columbiana Karsten by its dark green, distinctly flattened dorsal surface (which appears darker when viewed from above with substage illumination only) and more numerous stomata (15-100); W. columbiana has a rounded dorsal surface (with only a small flattened portion), is transparent green throughout and has fewer stomata (generally fewer than 15).

See group comparison photo with Wolffia arrhiza (6), W. brasiliensis (4) and W. borealis (5) next to each other. The transparent green species without dark green dorsal surface, W. globosa (2) and W. columbiana (3), appear darker because of the above-stage illumination. The reverse is true when viewed with substage illumination only. I.e. W. arrhiza appears darker and W. columbiana appears lighter and more translucent (see backlit image) of these latter two species.

Selected Images:

Return To The Lemnaceae Home Page
Go To Keys To The Genera Of Lemnaceae
Go To The List Of Species Of Lemnaceae
Go To The List Of .GIF Images Of Lemnaceae
Go To The WAYNE'S WORD Home Page