rhizome

An underground stem that grows horizontally and, through branching, acts as an agent of vegetative propagation. In some plants the rhizomes are cordlike,   as   in   nettle   (Urtica   dioica), while in others, e.g. Solomon's seal (Polygonatum multiflorum), they are fleshy and also serve as organs of perennation. Compare corm , rootstock , tuber.