Rhizome of Ginger is a type of modified underground stem which grows horizontally underground and bears nodes, internodes and scaly leaves and buds, which gives rise to aerial shoots.
The adventitious root arises from the lower surface of nodes. It is not a true root because root does not have nodes and internodes. The rhizome does not perform the function of anchorage and absorption, rather serve as reservoir for food storage. All these characteristics support the fact that ginger is a stem and not a root.