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.
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duckweed
n : any small or minute aquatic plant of the family Lemnaceae
that float on or near the surface of shallow ponds