By: The Lone Gardener
A bulb is a modified shoot or flower bud that usually forms underground. It has swollen leaf bases or thick scales that protect the bud and store food to nourish it during a rest period in which the plant's top growth dies back. The bulb contains nearly everything the embryonic bud will need to grow and bloom. The bud rests in the center of the bulb. It is surrounded by scales, which resemble the layers of an onion in bulbs such as narcissus and tulips, or are like the cloves of a garlic bulb in the bulbs of lilies. The scales are anchored to a tough basal plate (the flat end of the bulb) from which the roots will grow.
Some bulbs are called "tunicate," which means the scales are covered by a thin skin known as a tunic. Bulbs reproduce by means of underground offsets called bulblets, or in some lilies, by small, round bulbils that develop in the leaf axils. Both will eventually grow into full-size bulbs if planted. Daffodils and narcissus, tulips, hyacinths, and lilies all grow from true bulbs.
In addition to true bulbs, there are a number of other underground storage organs which in informal usage are also thought of as bulbs because the plants grow in much the same way. These other structures are corms, rhizomes, tubers, and tuberous roots.
A corm is the swollen underground base of a stem, which stores food in the same way a bulb does, although corms store most of it in the basal plate instead of in scales. Corms are usually flatter than bulbs. The corms reproduce by forming small cormels or cormlets at the base of the parent plant, or, less commonly, on aboveground stems. Crocus and gladiolus are two common plants that grow from corms.
Rhizomes are swollen underground stems that grow horizontally through the soil instead of vertically down into the ground. They can produce new roots and shoots, and many plants spread by means of their creeping rhizomes. The rhizomes can be divided or cut up to propagate new plants. Bearded iris is probably the best-known rhizomatous plant.
Tubers are swollen stems that are also located underground in most cases. Tubers contain buds or "eyes" from which new plants can grow.
Tuberous roots are swollen roots, like those of dahlias, which have become modified into storage organs. Tuberous-rooted plants have the growth buds in the crown, the base of the plant where stems and roots meet.
Bulbs and the other bulblike structures can be hardy and able to withstand below-freezing temperatures, or they may be tender, and damaged or killed by frost. The bulbs that bloom in spring – crocuses, daffodils, scilla, tulips, hyacinths, and the rest – are hardy. Many summer bulbs – gladiolus, tuberous begonias, dahlias, and Peruvian daffodils, for example – are tender. But not all summer-blooming bulbs are tender. Many lilies, for example, are quite hardy.
