Elodea longivaginata H. St. John

Leaves sometimes in 2s, spreading, linear to narrowly elliptic, 4.7--24.8 ´ (0.8--)1.8--4.3 mm, flat. Inflorescences: staminate spathes cylindric, 10.2--42 mm; peduncles abscissing following anthesis; pistillate spathes linear, 9--67 mm. Flowers unisexual; staminate flowers: stamens 7--9; pedicels remaining briefly following anthesis; inner 3 filaments connate proximally, forming column; anthers 3--4.5 mm; pollen in monads; pistillate flowers: styles 2.3--3 mm. Seeds ellipsoid, 2.8--3 mm, densely covered with long hairs.

Flowering summer--fall. Waters of rivers and reservoirs; 100--2500 m; Alta., B.C., Man., Sask.; Colo., Idaho, Kans., Minn., Mont., N.Mex., N.Dak., Oreg., S.Dak., Utah, Wyo.