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Discoid
Discoid means circular, like a disc. In the Composite family (Asteraceae), a discoid flower head is one that contains only rayless, disc florets. That is, ray florets and ligulate florets are absent.
 

Figure A.   Discoid flower heads of Erechtites hieraciifolius.
 

Figure B.   Discoid flower heads of Ageratina altissima.
(All florets shown are disc florets, each with 5 corolla lobes.)
 

Figure C.  Discoid flower head of Cirsium vulgare.

 

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