Stems submersed, elongate, branched; lvs submersed, ternately or binately dissected into filiform segments, soft and collapsing when withdrawn from water, 2-3 cm, the free petiole mostly about equaling the glabrous stipular part; fls at the water-surface, white, 1-1.5 cm wide; stamens mostly 10-25, achenes 10-20(-40), with a minute beak only 0.2-0.5 mm; 2n=32. Ponds and slow streams; circumboreal, but not at the highest latitudes; in Amer. s. to N.J., Pa., Minn., and the western cordillera. June-Aug. We have two vars. Var. trichophyllus, with essentially the range of the sp., has the receptacle hirsutulous, with tufted hairs. (R. aquatilis var. capillaceus) Var. calvescens W. B. Drew, from Que. to Minn., and Pa., chiefly in N. Engl., has the receptable glabrous or with a few scattered hairs.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.