Rhizomes absent; cauline stems terete to slightly compressed, without dark spots, 18–150 cm long. Turions common, terminal, 0.9–3.2 cm long, 30–180 mm wide; outer leaves 1–3 per side, the base without corrugations, apex aristate or obtuse; inner leaves differentiated, rolled into hardened structure, parallel to outer leaves. Nodal glands present or absent, green or gold or rarely white, mm diameter to 0.5. Leaves submersed, alternate, more or less spiraling. Submersed leaves sessile; ligules inconspicuous, free from blade, light greenish brown to rarely white, not fibrous or rarely fibrous, not shredding apically, persistent, connate or convolute, 31–92 cm long, the apex obtuse; leaf blade lax to coarse, light green to dark green or rarely rufescent, linear, not arcuate, 0.9–6.5 cm long, 0.2–2.5 mm wide, with 1–3(–5) veins, 0–5 rows of lacunae each side of midrib, the base parallel-sided, not clasping, without basal lobes, the margins entire, the apex acuminate to obtuse, without a bristle tip, not cucullate. Inflorescences unbranched, distal only; distal peduncles slightly clavate to filiform, turgid, axillary or terminal, erect to rarely recurved, 0.5–6.2(–6.6) cm long; spikes emersed or submersed, capitate to cylindrical, 1.5–10.1 mm long, the nodes 3, internodes 2.5–5 mm long. Flowers whorled; perianth spathulate, without claw, the blade 1.5–3 mm long, 2.2–2.5 mm wide; anthers 0.4–1 mm long; carpels 4. Fruiting gynophore absent. Fruits obovate to ovate, pale green to brown, compressed to turgid, abaxially rounded, laterally rounded, 1.5–2.2 mm long, 1.2–1.6 mm wide; beak erect, 0.1–0.6 mm long; tubercules absent; embryo less than one full spiral. 2n= 26.

Subspecies 3 (2 in flora): Near worldwide.

Three hybrids, Potamogeton perfoliatus ´ P. pusillus = P. ´ mysticus T. Morong, P. friesii ´ P. pusillus = P. ´ pusilliformis F. E. L. Fisher (P. ´ intermedius F. E. L. Fischer), and P. obtusifolius ´ P. pusillus = P. ´ saxonicus J. O. Hagström, have been described.

1. Mature fruit obovate, sides concave beak mostly toward adaxial edge, rarely median; peduncle filiform to cylindrical, usually 1--3 per plant; inflorescence usually interrupted; leaves with up to 2 rows of lacunae along midrib, apex acute,rarely apiculate or with a bristle; stipules mostly

connate………………………………………………………………. 4a. Potamogeton pusillus spp. pusillus

2. Mature fruit mostly widest at middle or ovate, sides rounded, beak mostly median, toward adaxial edge; peduncles cylindrical, usually more than 3 per plant; inflorescence continuous; leaves with 1--5 rows of lacunae along midrib, apex acute to obtuse; stipules mostly

convolute. …………………………….…………………………..4b. Potamogeton pusillus spp. tenuissimus

4a. Potamogeton pusillus C. Linnaeus spp. pusillus

Potamogeton panormitanus A. Bivona-Bernardi; Potamogeton pusillus C. Linnaeus var. minor (A. Bivona-Bernardi) M. L. Fernald & B. Schubert

Leaves: stipules usually connate; blades 1--3-veined, 1.4--6.5 cm ´ 0.5--1.9 mm, apex acute or rarely apiculate, rarely with a bristle, lacunae present or absent, 0--2 rows each side of midrib. Inflorescences: peduncles 1--3 per plant, filiform to cylindrical; spikes cylindric, usually interrupted. Fruits obovate, sides usually centrally concave; beak toward adaxial edge, rarely median. 2n = 26 (Eurasia) [O. Palmgren 1939]

Flowering and fruiting spring--fall. Streams, lakes, or marshes; 0--2500 m; Alta., B.C., Man., N.B., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Miss., Mich., Minn., Mo., Mont., Nebr., Nev., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo.; Eurasia; Africa; South America.

Potamogeton pusillus subsp. pusillus is a near cosmopolitan taxon. It is an easily determined taxon whenever in fruit as its inflorescence is interrupted. This inflorescence character combined with narrow, linear leaves having 1--3 veins makes this taxon easily recognized. The nodal glands are green, essentially the color of the stems. They often appear only as bumps on the stem at the nodes. Consequently, they often are difficult to see. Also, they frequently occur at only a few nodes per plant. Thus, one can easily overlook these glands.

4b. Potamogeton pusillus C. Linnaeus spp. tenuissimus (F. K. Mertens & W. D. J. Koch in J. C. Röhling) R. R. Haynes & C. B. Hellquist, Novon 6: 370. 1996.

Potamogeton pusillus var. tenuissimus Mertens & Koch in Röhlings, Deutschl. Fl. 1: 857. 1823. P. berchtoldii F. X. Fieber; P. berchtoldii F. X. Fieber var. tenuissimus (F. K. Mertens & W. D. J. Koch in J. C. Röhling) M. L. Fernald; P. berchtoldii F. X. Fieber var. polyphyllus (T. Morong) M. L. Fernald; P. berchtoldii F. X. Fieber var. lacunatus (J. O. Hagström) M. L. Fernald; P. berchtoldii F. X. Fieber var. colpophilus (M. L. Fernald) M. L. Fernald

Leaves: stipules usually convolute; blades 1--3(--5) veined, 0.9--5.4 cm ´ 0.2--2.5 mm; apex acute to obtuse, lacunae present, 1--5 rows each side of midrib. Inflorescences: peduncles mostly more than 3 per plant, cylindric to slightly clavate; spikes capitate to cylindric, continuous. Fruits ovate, sides mostly rounded, rarely concave; beak mostly median, rarely towards adaxial edge. 2n = 26 [R. L. Taylor & G. A. Mulligan 1968; A. Löve & D. Löve 1981]

Flowering and fruiting summer--autumn. Shallow waters of lakes and streams; 50--2700 m; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Fla., Idaho, Ill., Ind., Iowa, La., Maine, Md., Mass., Mich., Minn., Miss., Mont., Nev., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., Tex., Vt., Va., Wash., Wis., Wyo.; Eurasia.

This subspecies is the most common linear-leaved species in temperate North America. Whenever one finds a linear-leaved pondweed with 1--5 rows of lacunae each side of midvein, the chances are that it is this taxon. The only other taxon that it could be is Potamogeton obtusifolius, which can be separated by having a cylindric inflorescence, whereas ssp. tenuissimus has a capitate inflorescence.