Spirodela polyrhiza (L.) Schleid.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.
Species Description - Thallus broadly oval to obovate, anthocyanic beneath, (3–)5–8(–10) mm, ± evidently 5–15-nerved, forming turions especially in the fall; roots mostly 5–12; spathe open only at the top; seeds 1 or 2, smooth or minutely reticulate; 2n=30, 40, 50. Quiet water; nearly cosmopolitan, and throughout our range.