Potamogeton obtusifolius

Rhizomes absent. Cauline stems slightly compressed, without spots, 35--90 cm; glands yellow-green to gold, 0.2--1 mm diam. Turions terminal, abundant, 3.5--7.8 cm ´ 2.3--5.1 mm, soft; leaves ± 2-ranked; outer leaves 3--4 per side, base not corrugate, apex apiculate to obtuse; inner leaves undifferentiated. Leaves submersed, ± spirally arranged, sessile, flaccid; stipules persistent, inconspicuous, convolute, free from blade, white, not ligulate, 0.6--1.8 cm, fibrous, rarely shredding at tip, apex obtuse; blade light green to somewhat reddish, linear, not arcuate, 3--8.2 cm ´ 1--3.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, obtuse or round-apiculate, lacunae in 1--3 rows each side of midrib; veins 3. Inflorescences unbranched, emersed; peduncles not dimorphic, axillary, erect, rarely recurved, cylindric, 0.8--1.9(--4.2) cm; spike not dimorphic, cylindric, 8--13 mm. Fruits sessile, olive-green to brown, obovoid, turgid, abaxially keeled or not, laterally keeled or not, 2.5--3.6 ´ 1.7--2.4 mm, lateral keels when present without points; beak erect, 0.8--1 mm; sides without basal tubercles; embryo with 1 full spiral. 2n = 26.

Flowering and fruiting summer--fall. Medium- to low-alkaline waters of lakes and slow-flowing streams; 50--2000 m; Alta., B.C., Man., N.B., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Alaska, Conn., Idaho, Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Y., Pa., R.I., Vt., Wash., Wis., Wyo.; Eurasia.

Potamogeton obtusifolius is a distinctive linear-leaved species with the leaf blades round at the apex, especially when fruiting inflorescences 5--7 mm wide are present. This is unusually wide for one of the linear-leaved species.

Two hybrids, Potamogeton obtusifolius ´ P. pusillus (= P. ´ saxonicus Hagström) and P. friesii ´ obtusifolius (= P. ´ semifructus A. Bennett ex Ascherson & Graebner), have been described.

Stems very slender, to 1 m, much branched, commonly with paired glands at the nodes; rhizome scarcely developed; lvs all submersed, often suffused with red, linear, 3-10 cm נ1-3.5 mm, obtuse or rounded (sometimes minutely cuspidate) at the tip, the lateral veins 2(4), very delicate, the midvein flanked by 1-4 rows of lacunar cells on each side; stipular sheaths axillary, free, 1-2 cm, very thin and scarious, the margins merely overlapping; winter-buds commonly produced, 3.5-8 cm, but the lvs not strongly differentiated; peduncles axillary, 1-2(-4) cm; spikes 8-15 mm, thick-cylindric, with mostly 3 closely set whorls of fls; body of the fr obovoid, slightly compressed, with or without a low dorsal ridge-keel, 2.5-3.5 mm, the beak 0.6-0.7 mm; 2n=26. In ponds and streams; Que. and N.S. to Mackenzie R. and B.C., s. to n. N.J., Minn., and Wyo.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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