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.]

Two American species and S. oligorhiza, a native of southern Asia, Australia and the Fiji Islands. The following is the type of the genus.

1. Spirodela Polyrhiza (L.) Schleid. Greater Duckweed

Fig. 1120

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.

1 Spirodela Polyrhiza L Schleid Greater Duckweed 1120