The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sapwood
    n 1: newly formed outer wood lying between the cambium and the
         heartwood of a tree or woody plant; usually light colored;
         active in water conduction