A
corm is a short, vertical, swollen underground
plant stem that serves as a
storage organ used by some plants to survive winter or other adverse conditions such as summer drought and heat (
estivation). A corm consists of one or more
internodes with at least one growing point, with protective leaves modified into skins or tunics. The thin tunic leaves are dry papery, dead petiole sheaths, formed from the leaves produced the year before which acts as a covering that protects the corm from insects and water loss. Internally a corm is mostly made of starch-containing
parenchyma cells above a circular basal node that grows roots.
Corms are sometimes confused with true bulbs; they are often similar in appearance to bulbs externally, and thus erroneously called bulbs. Corms are stems that are internally structured with solid tissues, which distinguishes them from bulbs, which are mostly made up of layered fleshy scales that are modified leaves. As a result, when a corm is cut in half it is solid, but when a true bulb is cut in half it is made up of layers. Corms are structurally plant stems, with nodes and internodes with buds and produce adventitious roots. On the top of the corm, one or a few buds grow into shoots that produce normal leaves and flowers.
Corms can form many small cormlets called cormels, from the basal areas of the new growing corms, especially when the main growing point is damaged. They are used to propagate corm forming plants. Corms of a number of species of plants are replaced every year by the plant with growth of a new corm; this process starts after the shoot has developed fully expanded leaves. The new corm forms at the shoot base just above the old corm. As the new corm is growing, short stolons are produced that end with the newly growing small cormels. As the plants grow and flower, the old corm is used up and shrivels away. The new corm that replaces the old corm grows in size, especially after flowering is done.
When cormels are produced, the old corm produces the greatest number of cormels when it is close to the soil surface. The small cormels normally take one or two more years of growth before they are large enough to flower.
