Stems stout, commonly 2-3 mm thick, bearing a pair of opposite, usually unequal lvs subtending the umbel, often extended and bearing additional umbels; lvs subrotund, 5-15 cm long and wide; fls heterostylic; cor bright yellow, 2-2.5 cm, its lobes somewhat fringed below; anthers 4-5 mm; fr 12-25 mm, strongly beaked; seeds flat, the margin ciliate and narrowly winged; 2n=54. Native of Europe, intr. in quiet waters here and there in our range. June-Sept.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.