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.
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