Rhizome of Ginger is a type of underground modification of stem which grows horizontally underground and bear nodes, internodes and scaly leaves and buds , which gives rise to aerial shoots.
The adentitious root arises from the lower surface of nodes. It is not a root because root does not have nodes and internodes . Further the rhizome donot perform the function of roots, i.e., anchorage and absorption , rather serves as resevoir for storage of food. All these characteristics support the fact that ginger is a stem and not a root.
