Potamot oblong

Rhizome present. Cauline stems terete, without spots, 5--30 cm; nodal glands absent. Turions absent. Leaves both submersed and floating or only submersed, ± spirally arranged. Submersed leaves petiolate, lax; stipules persistent, conspicuous, convolute, free from blade, greenish, not ligulate, to 3 cm, not fibrous, not shredding at tip, apex acute; petioles 1--3(--8) cm; blade dark green, lanceolate to occasionally ovate, occasionally arcuate, 3--13 cm ´ 4--15(--44) mm, base acute, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, acute, lacunae in 2--4 rows each side of midvein; veins 7--11. Floating leaves: petioles continuous in color to apex, 1--17 cm; blade adaxially reddish brown, ovate, 3--8(--10) cm ´ 10--46 mm, base rounded or slightly cordate, apex obtuse, acute, or somewhat apiculate; veins (11--)15--19(--21). Inflorescences unbranched, emersed; peduncles not dimorphic, erect to slightly recurved, cylindric, 4--12 cm; spikes not dimorphic, cylindric, 20--40 mm. Fruits sessile, dark reddish brown, obovoid to orbicularoid, compressed, not abaxially or laterally keeled, (1.6--)2--2.5 ´ (1.2--)1.5--2.1 mm; beak minute or obsolete, recurved; sides without basal tubercles; embryo with 1 full spiral. 2n = 26.

Flowering July--August. Acid to neutral waters of ponds, lakes, and occasionally streams; 10--100 m; introduced; St. Pierre and Miquelon; Nfld. and Labr. (Nfld.), N.S.; Europe; n Africa; Atlantic Islands (Azores).