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.
Distribution
Nearly worldwide.
Discussion
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.
Subspecies 3 (3 in the flora).
Selected References
None.
Lower Taxa
Key
| 1 | Leaf blade 1-veined, subulate, 0.2–0.7 mm wide; New England and s Quebec. | Potamogeton pusillus subsp. gemmiparus |
| 1 | Leaf blade 1–5-veined, acute to obtuse, 0.2–2.5 mm wide; widespread throughout North America. | > 2 |
| 2 | Mature fruits obovoid, sides concave; beak toward adaxial edge, rarely median; peduncles filiform to cylindric, 1–3 per plant; inflorescences interrupted; leaf blade with 0–2 rows of lacunae along midrib, apex acute, rarely apiculate, rarely with bristle; stipules connate. | Potamogeton pusillus subsp. pusillus |
| 2 | Mature fruits widest at middle or ovoid, sides rounded; beak median, not toward adaxial edge; peduncles cylindric, more than 3 per plant; inflorescences continuous; leaf blade with 1–5 rows of lacunae along midrib, apex acute to obtuse; stipules convolute. | Potamogeton pusillus subsp. tenuissimus |