Many bulbs require a period of low-temperature exposure before they begin to send up the new plant. Both the temperature and length of this treatment are of critical importance to commercial growers who force bulbs for holidays. Corms are not the same as bulbs. They have shapes similar to bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem whose scales have been reduced to a dry, leaflike covering.
Some plants produce a modified stem that is referred to as a tuberous stem.
Examples are tuberous begonia and cyclamen. The stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the crown and fibrous roots are found on the bottom of the tuberous stem. In addition, some plants such as the dahlia and the sweet potato produce an underground storage organ called a tuberous root, which is often confused with bulbs and tubers. However, these are roots, not stems, and have neither nodes nor internodes.