22.
Potamogeton natans
Linnaeus, Sp. Pl. 1: 126. 1753.
Floating-leaf pondweed, potamot flottat
Rhizomes present. Cauline stems terete, often rust-spotted, 30--90 cm; nodal glands absent. Turions absent. Leaves both submersed and floating, ± spirally arranged. Submersed leaves sessile, rigid; stipules persistent, conspicuous, convolute, free from blade, whitish, not ligulate, 4.5--1 cm, fibrous, not shredding at tip, apex obtuse; blade light to dark green, phyllodial, not arcuate, 9--20 cm ´ 0.7--2.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, obtuse, lacunae absent; veins 3--5, obscure. Floating leaves: petioles lighter green immediately proximal to apex, 5.5--29 cm; blade adaxially light green, elliptic to ovate, 3.5--11 cm ´ 15--60 mm, base cordate, apex acute to rounded; veins 17--37. Inflorescences unbranched, emersed; peduncles not dimorphic, terminal, erect to ascending, cylindric, 4.5--9.5 cm; spikes not dimorphic, cylindric, 25--50 mm. Fruits sessile, green to greenish brown, obovoid, turgid, not abaxially or laterally keeled, 3.5--5 ´ 2--3 mm; beak erect to apically recurved, 0.4--0.8 mm; sides without basal tubercles; embryo with 1 full spiral. 2n = 52.
Flowering summer--fall. Quiet or slow-flowing waters of ponds, lakes, and streams; 0--3100 m; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Calif., Colo., Conn., Idaho, Ill., Ind., Iowa, Kans., Maine, Mass., Mich., Minn., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.Dak., Ohio, Oreg., Pa., R.I., S.Dak., Utah, Vt., Wash., W.Va., Wis., Wyo.; Eurasia.
Potamogeton natans is the common floating-leaved pondweed of the north temperate ure areas. It is essentially circumboreal and can easily be identified by floating leaves that are almost always cordate at the base of the blade, the petiole with a short band of light tissue at its apex, and the submersed phyllodial leaves. Also, the apex of the petiole usually is bent so that the blade appears oriented in the opposite direction from which the petiole appears to be oriented.
One hybrid, Potamogeton natans ´ P. nodosus (= P. ´ schreberi Fischer [P. ´ perplexus A. Bennett]), has been described.
