duck·weed /ˈdʌkˌwid/
  水萍
  Duck·weed n. Bot. A genus (Lemna) of small plants, seen floating in great quantity on the surface of stagnant pools fresh water, and supposed to furnish food for ducks; -- called also duckmeat.
  ◄ ►
  duckweed
       n : any small or minute aquatic plant of the family Lemnaceae
           that float on or near the surface of shallow ponds