Source : Webster's Revised Unabridged Dictionary (1913)

Sapwood \Sap"wood`\, n. (Bot.)
   The alburnum, or part of the wood of any exogenous tree next
   to the bark, being that portion of the tree through which the
   sap flows most freely; -- distinguished from {heartwood}.

Source : WordNet®

sapwood
     n : newly formed outer wood lying between the cambium and the
         heartwood of a tree or woody plant; usually light
         colored; active in water conduction