3. Stuckenia filiformis (Persoon) Börner, Flora fur das deutsche Volk. 713. 1912.
Potamogeton filiformis Persoon, Syn. Pl. 1: 152. 1805
Stems freely branching proximally, sparsely branching distally, subterete, (10--)20--60(--100) cm. Leaves: length and width of those on main stem only slightly larger than those on branches; stipules with stipular sheaths often inflated on proximal portion of stem, 1--4(--9.5) cm, summit of midstem stipules tight to stem, ca. ± same width as stem, ligule 2--20 mm, distinct, especially on distal stipules; blade filiform or slenderly linear, 1--15 cm ´ 0.2--2(--3.7) mm, apex notched, blunt, or short-apiculate; veins 1--3. Inflorescences: peduncles terminal, erect, filiform to slender, 2--10(--15) cm; spikes cylindric to moniliform, 5--55 mm; verticels 2--6(--9). Fruits dark brown, obovoid, 2--3 ´ 1.5--2.4 mm; beak inconspicuous.
Subspecies ca. 5 (3 in the flora): nearly worldwide.
Three distinct subspecies seemingly apparently occur in North America. They are separated mainly by the size of the plants and the peduncle characteristics. Stuckenia filiformis subsp. occidentalis typically grows in cold deep water, standing or with a strong current. This variety tends to become robust and is easily confused with Stuckenia vaginata and S. striata. Stuckenia filiformis subsp. alpina is a much smaller plant typically growing in standing waters. Stuckenia filiformis subsp. filiformis is restricted to the far north. In the intermountain region of western United States is a more robust form that is quite similar to S. filiformis subsp. alpina but although it has previously been recognized as Potamogeton [Stuckenia] filiformis var. macounii Morong (J. L. Reveal 1977b).