Corms look a lot like bulbs on the outside but they are quite different. They have the same type of protective covering and a basal plate like the bulb does, but do not grow in layers.
Instead, the corm is the actual base for the flower stem and has a solid texture. As the flower grows, the corm actually shrivels as the nutrients are used up. Essentially the corm dies, but it does produce new corms right next to or above the dead corm. It has contractile roots that bring down the corms as they rise up to the surface of the soil which is why the flowers come back year after year. Depending on the type of flower, it may take a couple years to reach blooming size.
A corm does not have visible storage rings when cut in half. This distinguishes it from a true bulb.
Many corms produce two different types of roots. Those growing from the bottom of the corm are normal fibrous roots, they are formed as the shoots grow, and are produced from the basal area at the bottom of the corm. The second type of roots are thicker layered roots that form as the new corms are growing, they are called contractile roots and they pull the corm deeper into the soil. They are produced in response to fluctuating soil temperatures and light levels. Once the corm is deep enough within the soil where the temperature is more uniform and there is no light, the contractile roots no longer grow and the corm is no longer pulled deeper into the soil.
The newly dug corms will have cormels that are pea size formed around the top of the old corm. The remains of the old corm will be directly beneath the newly formed corms. When the corm is cleaned up and the old stem removed, the growing point of the corm will be evident. The cormels can be saved and replanted in the back of the garden until they reach flowering size.
Examples of Hardy Corms – Crocus. Arisaema, Crocosmia, Liatris
Examples of Tender Corms: Gladiolus, Colocasia, Alocasia, Bananas (Musa), Ensete, Taro, Xanthosoma