Rhizomes present. Cauline stems terete, without spots, to 250 cm; nodal glands absent. Turions absent. Leaves submersed, ± spirally arranged, sessile, lax; stipules deteriorating into fibers and deciduous, inconspicuous (absent on proximal portion of stem), convolute, free from blade, light brown to green, not ligulate, 3.5--6.5 cm, not fibrous, not shredding at apex; blade olive-green, broadly lanceolate, orbiculate, or ovate, not arcuate, 0.9--7.6(--9.7) cm ´ 7--40 mm, base rounded, without basal lobes, clasping, margins entire, often crispate, apex not hoodlike, not splitting when pressed, round, rarely acute; veins 3--25. Inflorescences unbranched, emersed; peduncles not dimorphic, terminal or axillary, erect to rarely recurved, cylindric, 1--7.3 cm; spike not dimorphic, cylindric, 0.4--4.8 cm. Fruits sessile, greenish brown to olive-green, obovoid, turgid or rarely concave, not abaxially or laterally keeled, 1.6--3 ´ 1.3--2.2 mm; beak erect, 0.4--0.6 mm; sides without basal tubercles; embryo with 1 full spiral. 2n = 52.
Flowering summer--fall. Waters, often brackish, of lakes, streams, rivers, and bays; 0--100 m; Greenland; St. Pierre and Miquelon; N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que.; Ala., Conn., Del., D.C., Fla., La., Maine, Md., Mass., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., Vt., Va.; Central America (Guatemala); Eurasia; Africa; Australia.
Four hybrids, Potamogeton perfoliatus ´ P. praelongus (= P. ´ cognatus Ascherson & Graebner), P. perfoliatus ´ P. pusillus (= P. ´ mysticus Morong), P. gramineus ´ P. perfoliatus (= P. ´ nitens Weber [P. ´ subnitens Hagström]), and P. alpinus ´ P. perfoliatus (= P. ´ prussicus Hagström), have been described.
Stem slender, to 2.5 mm, often much branched; internodes mostly 1-3 cm; lvs all submersed, ovate to suborbicular, 1-5(-9) cm נ5-25(-40) mm, mostly broadly rounded or obtuse, sessile, with cordate-clasping, often perfoliate base, delicately 3-25-veined, 3(5) veins stronger; stipules axillary, free, evanescent; peduncles 1-7 cm, about as thick as the stem, not clavate; spikes dense, cylindric, 1-4.5 cm; frs obliquely obovoid, 1.6-3 mm (including the short beak), very shallowly pitted, with 3 obscure dorsal keels, or keelless; 2n=52. Ponds and slow streams; irregularly cosmopolitan, in Amer. from Nf. and Ont. to N.C. and O., disjunct along the Gulf Coast.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.