The Collaborative International Dictionary of English v.0.48:

Quincuncial \Quin*cun"cial\, [L. quincuncialis, from quincunx.
   See Quincunx.]
   [1913 Webster]
   1. Having the form of a quincunx.
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   2. (Bot.) Having the leaves of a pentamerous calyx or corolla
      so imbricated that two are exterior, two are interior, and
      the other has one edge exterior and one interior; as,
      quincuncial [ae]stivation.
      [1913 Webster]

   Quincuncial phyllotaxy (Bot.), an arrangement of five
      leaves in a spiral, each leaf two fifths of a circle from
      the next.
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