frogbit

from WordNet (r) 3.0 (2006)

frogbit
    n 1: European floating plant with roundish heart-shaped leaves
         and white flowers [syn: {frogbit}, {frog's-bit},
         {Hydrocharis morsus-ranae}]

from The Collaborative International Dictionary of English v.0.48

frogbit \frog"bit`\, frog's-bit \frog's-bit\, n. (Bot.)
      (a) A European plant ({Hydrocharis Morsus-ran[ae]}),
          floating on still water and propagating itself by
          runners. It has roundish heart-shaped leaves and small
          white flowers.
      (b) An American plant ({Limnobium Spongia}), with similar
          habits.
          [1913 Webster]