Stems decumbent and prostrate or rooting at base, often colonial, the floriferous branches erect, 1-4 dm; submersed lvs, if present, 2-4 cm, ovate or broadly oblong, deeply pinnatisect into narrow segments; emersed lvs linear-oblong to linear-oblanceolate, 2-6(-8) cm, serrate, tapering to the base; fr ovoid-pyramidal, 2-5 mm long and wide. Swamps, marshes, wet shores, and shallow water; Que. and N.S. to w. Ont. and Minn., s. to Cuba and Guatemala. July, Aug. The widespread var. crebra Fernald & Griscom has the fr 2-4 mm wide, acutely angled but not winged, the sides nearly flat. Var. palustris (P. platycarpa), on the coastal plain from Mass. to La., has the frs 3.5-5 mm wide, sharply angled, or narrowly winged above or throughout, with concave sides. Var. amblyogona Fernald, chiefly Ozarkian, extending e. to Ind., has frs 2-4 mm wide, plump, obtusely angled with convex sides.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.