plu·mule /ˈplu(ˌ)mju(ə)l/
  幼芽,棉毛
  Plu·mu·la n.; pl. L. Plumule E.-las
  1. Bot. A plumule.
  2. Zool. A down feather.
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  Plu·mule n.
  1. Bot. The first bud, or gemmule, of a young plant; the bud, or growing point, of the embryo, above the cotyledons. See Illust. of Radicle.
  2. Zool. (a) A down feather. (b) The aftershaft of a feather. See Illust. under Feather. (c) One of the featherlike scales of certain male butterflies.
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  plumule
       n : down feather of young birds; persists in some adult birds