Rhizomes absent. Cauline stems terete to slightly compressed, without spots, 18--150 cm; glands present on at least some nodes, green, gold, brown, or rarely white, to 0.5 mm diam. Turions common, soft, lateral or terminal, 0.9--3.2 cm ´ 0.3--1.8 mm, soft; leaves ± 2-ranked; outer leaves 1--3 per side, base not corrugate, apex subulate to obtuse; inner leaves rolled into hardened fusiform structure. Leaves submersed, ± spirally arranged, sessile, delicate to coarse; stipules persistent, inconspicuous, connate or convolute, free from base of blade, brown to green or white, not ligulate, 0.31--0.92 cm, rarely appearing fibrous, not shredding at tip, apex obtuse; blade pale green to olive-green, rarely somewhat reddish, linear, not arcuate, 0.9--6.5 cm ´ 0.2--2.5 mm, base slightly tapering, without basal lobes, not clasping, margins entire, not crispate, apex not hoodlike, subulate to obtuse, lacunae absent or present, in 0--5 rows each side of midrib; veins 1--3(--5). Inflorescences unbranched, submersed or emersed; peduncles not dimorphic, axillary or terminal, erect, rarely recurved, filiform to slightly clavate, 0.5--6.2(--6.6) cm; spikes not dimorphic, capitate to cylindric, 1.5--10.1 mm. Fruits sessile, green to brown, ovoid to obovoid, turgid to concave, not abaxially or laterally keeled, 1.5--2.2 ´ 1.2--1.6 mm; beak erect, 0.1--0.6; sides without basal tubercles; embryo with less than 1 full spiral. Three hybrids, Potamogeton perfoliatus ´ P. pusillus (= P. ´ mysticus Morong), P. friesii ´ P. pusillus (= P. ´ pusilliformis Fisher [P. ´ intermedius Fischer]), and P. obtusifolius ´ P. pusillus (= P. ´ saxonicus Hagström), have been described.
Stems very slender, to 1.5 m, with numerous branches distally, with or without nodal glands; rhizome scarcely developed; lvs all submersed, narrowly linear, 1-7 cm נ0.2-2.5 mm, 1- or 3(5)-veined, with 0-5 rows of lacunar cells on each side of the midvein; stipular sheaths axillary, free, to 1 cm, delicate, the margins connate or merely overlapping; winter-buds commonly produced,
1-3 cm, the inner lvs rolled into a hard, fusiform structure; peduncles filiform to slightly clavate, axillary or
terminal, mostly erect, 0.5-6 cm; spikes capitate to cylindric, 2-10 mm, with 1-4 whorls of fls; body of the fr
ovoid to obovoid, usually plump, not keeled, 1.5-2.2 mm, the beak 0.1-0.6 mm; 2n=26. Common and
widespread in both acid and alkaline waters from Nf. to Alas., s. to Fla. and Mex. Highly variable. (P. berchtoldii; P. panormitanus; P. tenuissimus) The most narrow-lvd phase, with subulate, 1-nerved lvs 0.2-0.6 mm wide, considered to be restricted to N. Engl. and s. Que., has been called var. gemmiparus Robbins. (P. gemmiparus)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.