Yenissei River pondweed, potamot de l’Ilenissei

Potamogeton porsildiorum Fernald

Rhizome absent. Cauline stems compressed-filiform, without spots, to 50 cm; glands white, 0.3--0.5 mm diam. Turions lateral, common, 3.5--9.5 cm ´ 2--5 mm, soft; leaves ± 2-ranked; outer leaves 3--4 per side, base not corrugate; inner leaves undifferentiated. Leaves submersed, ± spirally arranged, sessile, flaccid; stipules deliquescent, inconspicuous, convolute, free from blade, pale brown, not ligulate, 1--2 cm, not fibrous, not shredding, apex obtuse; blade dark green, linear, not arcuate, 3.5--9.5 cm ´ 1.5--2 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, rounded or nearly acute to mucronate, lacunae in 1--2 rows each side of midvein; veins 9--17. Inflorescences unbranched, emersed; peduncles not dimorphic, axillary, erect, cylindric, 1.7--3.5 cm; spikes not dimorphic, cylindric, 10--30 mm. Fruits sessile, reddish brown, oblong-obovoid, compressed, abaxially ridged, not laterally keeled, 3--4 ´ 1.5--2 mm; beak nearly erect, 0.3--0.5 mm; sides without basal tubercles; embryo with 1 full coil.

Fruiting early summer--late summer. Shallow water of ponds and lakes; 0--915 m; Man., N.W.T., Ont., Nunavut, Que., Yukon; Alaska; Siberia.