Rhizome

In botany, a rhizome is a classically underground, horizontal stem of a plant that often sends out roots and shoots from its nodes. Though a number species of plants have above ground rhizomes or rhizomes that sit at the soil surface with some Iris species. Rhizomes may also be referred to as creeping rootstalks, or rootstocks. A stolon is like to a rhizome, sprouting from an existing stem with long internodes and generating new shoots at the end. Rhizomes have short internodes noramly with papery leaves at the nodes, they root at the underside and form green shoots at the apex.
Many plants have rhizomes that serve to spread the plant by vegetative reproduction. Examples of plants that do this are asparagus, ginger, irises, Lily of the Valley, Cannas and sympodial orchids. The spreading stems of ferns are also called rhizomes.
A tuber is a thickened part of a stolon or root that has been enlarged for use as a storage organ.They are classically high in starch. An example of a tuber is the common potatoa modified stolon. Thought the term tuber is frequently used impresicly and some times applied to plants with rhizomes.