Source : Webster's Revised Unabridged Dictionary (1913)

Fastigiate \Fas*tig"i*ate\, Fastigiated \Fas*tig"i*a`ted\, a.
   [L. fastigium gable end, top, height, summit.]
   1. Narrowing towards the top.

   2. (Bot.) Clustered, parallel, and upright, as the branches
      of the Lombardy poplar; pointed.

   3. (Zo["o]l.) United into a conical bundle, or into a bundle
      with an enlarged head, like a sheaf of wheat.

Source : WordNet®

fastigiate
     adj : having clusters of erect branches (often appearing to form a
           single column)