Like a small version of no. 1 [Spirodela polyrhiza (L.) Schleid.]; thallus 2.5-5 mm, half to 3/4 as wide, obscurely 3-5-nerved; roots 2-5; seed solitary; 2n=40, 50. Quiet water; widely scattered in warm regions, rare and irregular with us. (S. oligorhiza)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.