Flora Boreali-Americana 1: 20. 1803.
Plants perennial, of fresh waters. Rhizomes and stolons absent. Erect stems rooted in substrate, branched or unbranched, elongate. Leaves cauline, whorled, 3–7 at each node, or leaves opposite at 4+ proximalmost nodes, submersed, sessile; blade linear to linear-lanceolate, apex acute; midvein without lacunae along side(s), blade uniform in color throughout;; abaxial surfacely without prickles or aerenchyma; intravaginal squamules entire. Inflorescences solitary, sessile; spathes not winged. Flowers unisexual, staminate and pistillate on different plants, or rarely bisexual, usually projected to surface of water by elongate floral tube base, sessile; petals white. Staminate flowers: filaments distinct or 3 inner connate ½ their length; anthers oval; pollen in monads or tetrads. Pistillate flowers: ovary 1-locular; styles 3, not 2-fid. Fruits ovoid to lance-ellipsoid, smooth, dehiscing irregularly. Seeds cylindric to fusiform, glabrous to hirsute.
North America, Central America, South America, Europe.
An additional name, Elodea schweinitzii (G. Planchon) Caspary, has appeared in North American literature. Although it was accepted by some (H. St. John 1965), others stated, "Elodea schweinitzii is something of a taxonomist's nightmare" (C. D. K. Cook and K. Urmi-König 1985). The few collections referable to the name are exceedingly variable, so variable in fact that the authors added, "the main character is variability itself." I am following Cook and Urmi-König in not accepting the taxon.
Species 5 (3 in the flora).
John, H. 1962. Monograph of the genus Elodea (Hydrocharitaceae): Part I. The species found in the Great Plains, the Rocky Mountains, and the Pacific states and provinces of North America. Res. Stud. State Coll. Wash. 30(2): 19--44. St.
|1||Staminate spathes 4 mm or less; styles usually 2 mm or less; leaves usually less than 1.7 mm wide.||Elodea nuttallii|
|1||Staminate spathes 6 mm or more; styles usually more than 2 mm; leaves usually more than 1.8 mm wide.||> 2|
|2||Pollen in tetrads; staminate pedicels detaching before or during anthesis; anthers 3 mm or less; leaves mostly in 3s; seeds 4.5–5.7 mm or more.||Elodea canadensis|
|2||Pollen in monads; staminate pedicels remaining briefly following anthesis; anthers 3–4.5 mm; some leaves in 2s; seeds 4 mm or less.||Elodea bifoliata|