Source : Webster's Revised Unabridged Dictionary (1913)

Rootstock \Root"stock`\, n. (Bot.)
   A perennial underground stem, producing leafly s?ems or
   flower stems from year to year; a rhizome.

Source : WordNet®

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