E-Flora BC: Electronic Atlas of the Flora of British Columbia

General:
Perennial, aquatic or semi-terrestrial herb from strong rhizomes; stems up to 150 cm long, freely branched, nearly circular in cross-section, tapered.

Leaves:
Submersed leaves narrowly lanceolate, thin, flat, green, 2-7 cm long, 0.5-2.5 cm wide, unstalked, not clasping the stem, the tips rounded or pointed, the stipules 0.5-3 cm long; floating leaves markedly different from the submersed ones, lanceolate to elliptical, thick and leathery, 2-10 cm long, 1-4 cm wide, long-stalked, the tips pointed, tapering at the bases.

Flowers:
Inflorescence spikelike, the spikes 1-2.5 cm long, with 5 to 10 whorls of flowers, the stalks thin.

Fruits:
Achenes, almost globe-shaped, 1.7-2.5 mm long, 1.6-2 mm wide, the beaks short, the keels sharp and prominent.

Notes:
Stem anatomical characters Interlacunar bundles present in 1 circle, subepidermal bundles present or absent, pseudoepidermis absent or 1 cell thick, endodermis of U-cells. A highly variable species with several varieties described in N America based on stem branching and size of submersed leaves (e.g. var. maximus Morong ex A. Benn., var. myriophyllus Robbins). Their taxonomic recognition is questionable. Potamogeton gramineus hybridizes with P. natans and without examining stem anatomical characters, this hybrid (P. sparganiifolius Laest. ex Fries) can easily be misidentified as P. nodosus.