tuber

n. 1 thick rounded part of a stem or rhizome, usu. Found underground and covered with modified buds, e.g. In a potato. 2 similar root of a dahlia etc. [latin, = hump, swelling]

n. A fleshy, rounded stem or root, usually containing starchy matter, as the potato or arrowroot; a thickened root-stock. See Illust. of Tuberous.