The Collaborative International Dictionary of English v.0.48:

Lemna \Lemna\ n.
   A genus of minute acquatic herbs floating on or below the
   water surface of still water consisting of a leaflike frond
   or plant body and single root.

   Syn: genus Lemna.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Lemna
    n 1: minute aquatic herbs floating on or below the water surface
         of still water consisting of a leaflike frond or plant body
         and single root [syn: Lemna, genus Lemna]