Answer: She called it a rhizome because a rhizome has nodes and inter nodes and is a modified stem that helps in propagation of the plant.
Explanation:
The roots of irises plant is called as rhizome because it has nodes and inter nodes. They have the ability to run inside the ground in a horizontal way.
As it is a stem it contains nodes and inter nodes in it. The nodes of the stem contains the axillary buds.
It is different from normal roots in the respect that normal roots does not contains nodes and inter nodes.