corm

A corm is a short, vertical, underground plant stem that stores nutrients. It is typically solid and surrounded by a tunic, a protective covering, rather than by concentric layers of storage leaves as in a true bulb. A corm has a basal plate from which roots grow downward and buds or nodes on its upper surface from which shoots may arise. Unlike bulbs, most of the tissue in a corm is stem tissue rather than layered leaves.

In perennial geophytes, corms form anew each growing season. The old corm may wither after fueling a new shoot, while a new corm develops above or beside it. Some species also produce smaller corms, or cormels, that can be separated and replanted. When conditions are favorable, the shoot grows, leaves unfold, and the cycle renews in the next season.

Common examples of cormous plants include the saffron crocus (Crocus sativus) and Gladiolus species. Corms are often lifted from the ground and dried for storage in winter in temperate regions. Propagation is usually by separating mature corms or by planting offsets and cormels.

Corms are distinguished from bulbs and tubers by structure. Bulbs consist of layered leaves surrounding a central stem, as in onions or tulips. Tubers are swollen underground storage organs derived from roots or stolons, as in potatoes. Corms store food in stem tissue and have a tunic, buds, and a basal plate, rather than concentric leaf scales or true roots as primary storage.