Corms, rhizomes and tubers are examples of underground stems.
Underground stems are plant structures that are found below the soil surface and are used for food storage and vegetative reproduction. Corms, rhizomes and tubers are all modified forms of underground stems that have adapted to serve different functions.
Corms are short, solid, swollen stems that are used for food storage in some plants.
Rhizomes are elongated, horizontal stems that grow underground and can produce new shoots and roots, allowing the plant to spread and reproduce vegetatively.
Tubers are swollen, fleshy stems that also serve as food storage structures. They are typically rounded or potato-shaped and grow underground attached to a short stem or stolon.
So, to summarize, corms, rhizomes, and tubers are all different types of underground stems that have adapted to serve as food storage structures in plants.