The Collaborative International Dictionary of English v.0.48:

Perfoliate \Per*fo"li*ate\, a. [Pref. per- + L. folium leaf.]
   1. (Bot.) Having the basal part produced around the stem; --
      said of leaves which the stem apparently passes directory
      through.
      [1913 Webster]

   2. (Zool.) Surrounded by a circle of hairs, or projections of
      any kind.
      [1913 Webster]

WordNet (r) 3.0 (2006):

perfoliate
    adj 1: (of a leaf) having the base united around (and apparently
           pierced by) the stem