Rhizome

This kind of bulb does not always grow entirely below the ground

A rhizome is actually a thickened stem growing partially or entirely below ground. Roots grow directly from its underside. The primary growing point is at one end of the rhizome; additional growing points form along its sides. Bearded iris is probably the best-known rhizomatous plant; others are calla lily (Zantedeschia) and canna.

New plants are produced from the growing points, so a planting that starts with a single rhizome can spread horizontally into the surrounding soil.

Printed from:
http://www.sunset.com/garden/flowers-plants/rhizome-00400000012477/