Rhizome:

In botany, a rhizome (from Greek: ῥίζωμα, rhizoma, "root-stalk") is a characteristically horizontal stem of a plant that is usually found underground, often sending out roots and shootsfrom its nodes.

Rhizomes may also be referred to as creeping rootstalks, or rootstocks. In general, rhizomes have short internodes; they send out roots from the bottom of the nodes and new upward-growing shoots from the top of the nodes. It is a method of reproduction for plants.