knap·weed /-ˌwid/
  矢車菊屬的一種
  Knap·weed n. Bot. The black centaury (Centaurea nigra); -- so called from the knoblike heads of flowers. Called also bullweed.
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  knapweed
       n : any of various plants of the genus Centaurea having purple
           thistlelike flowers