Zannichellia palustris L.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Rhizome scarcely differentiated from the very slender and fragile leafy stem; lvs 3–10 cm × 0.5 mm, 1-nerved, or partly 3-nerved in robust plants; achenes keeled on both the outer and inner sides, the body 2–3 mm, the style 1 mm; 2n=24, 48, 32, 36. In fresh (often eutrophic) or brackish water almost throughout N. Amer., and widespread in the Old World.