Flatstem pondweed, potamot zostériforme

Potamogeton zosterifolius Schumacher subsp. zosteriformis (Fernald) Hultén; P. zosterifolius var. americanus A. Bennett

Rhizomes absent. Cauline stems conspicuously flattened ("wing-flattened"), without spots, 60--120 cm; glands absent or rarely present, when present, gold, 0.3 mm diam. Turions common, terminal or lateral, 4--7.5 ´ 2--4.5 cm, firm; leaves ± 2-ranked; outer leaves 3--4 per side, base not corrugate, apex acute; inner leaves undifferentiated. Leaves submersed, ± spirally arranged, sessile, rigid; stipules persistent, conspicuous, convolute, free from blade, white, not ligulate, 1.5--3.5 cm, fibrous, shredding at tip, apex obtuse to acuminate; blade light green, linear, not arcuate, 10--20 cm ´ 2--5 mm, base rounded, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, blunt, acuminate, or bristle-tipped, lacunae absent; veins 15--35. Inflorescences unbranched, emersed; peduncles not dimorphic, terminal, erect to ascending, occasionally recurved, cylindric, 2--5 cm; spikes not dimorphic, cylindric, 15--30 mm. Fruits light green to olive-green, quadrate-oblong or nearly orbicular, turgid, sessile, with abaxially keeled, not laterally keeled, 4--5 ´ 3--3.5 mm, keel winglike; beak erect, 0.6--1 mm; sides without basal tubercles; embryo with 1 full spiral. 2n = 52.

Flowering summer--fall. Lakes, ponds, and slow streams; 0--15003000 m; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Calif., Conn., Idaho, Ill., Ind., Iowa, Kans., Maine, Mass., Mich., Minn., Mont., Nebr., Nev., N.H., N.J., N.Y., N.Dak., Ohio, Oreg., Pa., S.Dak., Utah, Vt., Wash., Wis., Wyo.

One hybrid, Potamogeton zosteriformis ´ P. strictifolius, has been described and has been given the name P. ×haynesii Hellquist & G. E. Crow and is known from northern Michigan, Minnesota, Vermont, and southern Canada.

SELECTED REFERENCE

Hellquist, C. B. and G. E. Crow. 1986. Potamogeton ´ haynesii (Potamogetonaceae), a new species from northeastern North America. Brittonia 38: 415--419.