Source : Webster's Revised Unabridged Dictionary (1913)

Xylem \Xy"lem\, n. [Gr. xy`lon wood.] (Bot.)
   That portion of a fibrovascular bundle which has developed,
   or will develop, into wood cells; -- distinguished from
   phlo["e]m.

Source : WordNet®

xylem
     n : the woody part of plants: the supporting and
         water-conducting tissue, consisting primarily of
         tracheids and vessels