rootstock \ˈ ̷ ̷ˌ ̷ ̷\noun 1. a.: a rhizomatous underground part of a plant b.: a stock for grafting consisting of a root or a piece of root; broadly: stock 2.: hydrorhiza 3.: a source of offshoots : origin < going back to one of the rootstocks of economic thought — Times Literary Supplement >