The Collaborative International Dictionary of English v.0.48:

Caudex \Cau"dex\, n.; pl. L. Caudices, E. Caudexes. [L.]
   (Bot.)
   The stem of a tree., esp. a stem without a branch, as of a
   palm or a tree fern; also, the perennial rootstock of an
   herbaceous plant.
   [1913 Webster] Caudicle

WordNet (r) 3.0 (2006):

caudex
    n 1: persistent thickened stem of a herbaceous perennial plant
         [syn: stock, caudex]
    2: woody stem of palms and tree ferns