The Collaborative International Dictionary of English v.0.48:

Rootstock \Root"stock`\, n. (Bot.)
   A perennial underground stem, producing leafly stems or
   flower stems from year to year; a rhizome.
   [1913 Webster]

WordNet (r) 3.0 (2006):

rootstock
    n 1: a horizontal plant stem with shoots above and roots below
         serving as a reproductive structure [syn: rhizome,
         rootstock, rootstalk]
    2: root or part of a root used for plant propagation; especially
       that part of a grafted plant that supplies the roots