This section is from the book "An Illustrated Flora Of The Northern United States, Canada And The British Possessions Vol1", by Nathaniel Lord Britton, Addison Brown. Also available from Amazon: An Illustrated Flora of the Northern United States, Canada and the British Possessions. 3 Volume Set..
Thallus disc-shaped, 7-12-nerved. The thinly-capped rootlets as well as the nerves are provided with a single bundle of vascular tissue. Spathe sac-like. Anthers 2-celled. The ovary produces two anatropous ovules. Fruit rounded, wing-margined. [Greek, in allusion to the cluster of rootlets.]
Lemna polyrhiza L. Sp. PI. 970. 1753.
Spirodela polyrhiza Schleid. Linnaea 13: 392. 1839.
Thallus round-obovate, 2"-5" long, thick, flat and dark green above, slightly convex and purple beneath, palmately 5-11-nerved. Each thallus bears a cluster of from 5-11 elongated rootlets. Rootcap pointed.
In rivers, ponds, pools and shallow lakes, Nova Scotia to British Columbia, south to South Carolina, Texas, northern Mexico and Nevada. Also in Jamaica. Widely distributed in the Old World and in tropical America. Seldom collected in flower.