The Collaborative International Dictionary of English v.0.48:

Lignify \Lig"ni*fy\, v. i. (Bot.)
   To become wood.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lignify \Lig"ni*fy\ (l[i^]g"n[i^]*f[imac]), v. t. [imp. & p. p.
   Lignified (l[i^]g"n[i^]*f[imac]d); p. pr. & vb. n.
   Lignifying (l[i^]g"n[i^]*f[imac]`[i^]ng).] [L. lignum wood
   + -fy: cf. F. lignifier.] (Bot.)
   To convert into wood or into a ligneous substance.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lignify
    v 1: convert into wood or cause to become woody