The Collaborative International Dictionary of English v.0.48:

Ligneous \Lig"ne*ous\ (l[i^]g"n[-e]*[u^]s), a. [L. ligneus, fr.
   lignum wood. Cf. Lignous.]
   Made of wood; consisting of wood; of the nature of, or
   resembling, wood; woody.
   [1913 Webster]

         It should be tried with shoots of vines and roots of
         red roses; for it may be they, being of a moreligneous
         nature, will incorporate with the tree itself. --Bacon.
   [1913 Webster]

   Ligneous marble, wood coated or prepared so as to resemble
      marble.
      [1913 Webster]

WordNet (r) 3.0 (2006):

ligneous
    adj 1: consisting of or containing lignin or xylem; "ligneous
           (or woody) tissue"