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Plants perennial, in fresh water. Rhizome slender to slightly robust, usually densely branched, with apical dormant buds. Stems terete, 1-2 mm in diam., usually densely branched, sometimes sparsely branched. Leaves dimorphic; stipules axillary, convolute, conspicuous, 6-35 mm, herbaceous or membranous, amplexicaul. Submerged leaves sessile, translucent, linear-oblong to oblanceolate, 3-5 cm × 5-12 mm, herbaceous, (3-)5-9(-13)-veined, base cuneate, margin ± minutely denticulate, apex mucronate. Floating leaves present or absent, petiolate; petiole usually longer than blade; blade opaque, elliptic or ovate-elliptic to elliptic-lanceolate, (7-)11-21(-23)-veined, leathery, base cuneate or rounded, margin entire, apex obtuse. Spikes cylindric, 15-40 mm, densely flowered, with many whorls of opposite flowers; peduncles 4-7 cm, thickened upward. Carpels 4. Fruit obovoid, 2.4-3.1 mm, abaxial keel obtuse, with a short beak at tip. Fl. and fr. Jul-Sep. 2n = 52.
Stems slender, somewhat compressed, much branched, 3–7 dm; submersed lvs sessile, linear or lance-linear to oblanceolate, 3–8 cm × 3–10 mm, acute to cuspidate,3–7-veined; floating lvs narrowly to broadly elliptic, 2–5 cm, a fourth to half as wide, 11–19-nerved, obtuse to rounded or subcordate at base, the slender petiole often exceeding the blade; stipules axillary, free, 1–3 cm, obtuse; peduncles 2–4(–15) cm, thicker than the stem; spikes dense, cylindric, 1.5–3 cm; frs obovoid, 2–2.5 mm, the sharp dorsal keel and the obscure lateral ones developed chiefly beyond the middle, the sides shallowly pitted; 2n=52. A highly variable circumboreal sp., in Amer. extending s. to N.Y., Io., and Calif. (P. heterophyllus, misapplied) Putative hybrids with various other spp. have been called P. ×hagstroemii A. Benn., P. ×spathulaeformis Morong, P. ×subnitens Hagstr., and P. ×varians Morong)
