Many people use the general term "bulb" in reference to all geophytes, but not all geophytes have bulbous storage organs. There are many other types of storage organs. Each organ is defined partially by the plant part from which it originated and by the tissue serving as the primary storage tissue. Still, all storage organs serve the same purpose - storage of reserve substances that allow rapid growth when environmental conditions are favorable. The types of storage organs most relevant to horticulturists are true bulbs, corms, tubers, rhizomes, tuberous roots, and enlarged hypocotyls.
This graphic was taken fromThe Physiology of Flower Bulbs, 1993.
A true bulb is comprised of a compressed stem, or basal plate, and modified leaves called scales serve as the primary storage tissue. Examples include tulip, lily, allium (e.g. onion), Dutch iris, hyacinth, daffodils and others. A bulb is referred to as tunicated if the outermost scale is dried out and papery, serving as protection for the fleshy scales beneath. Tulip bulbs are tunicated. Bulbs which lack the papery outer covering, such as lily bulbs are described as non-tunicated.
This graphic was taken fromThe Physiology of Flower Bulbs, 1993.
Like a true bulb, the corm is a modified stem with a basal plate, but the primary storage tissue is the stem tissue itself, rather than leaf tissue; so, corms are frequently described as"solid bulbs". These organs also may be tunicated or non-tunicated, and they have nodes from which meristems originate. Gladiolus, freesia, crocus, and ixia are some examples.
A tuber is a thickened underground stem, and the stem tissue serves as the primary storage tissue; but unlike a corm, it has no basal plate. Meristems occur on the tuber and are commonly called "eyes" on a potato. Caladium is the most common floricultural tuber.
This graphic was taken fromThe Physiology of Flower Bulbs, 1993.
A rhizome is a modified stem, and the stem tissue itself is the primary storage tissue. The rhizome, however, is unique in that it grows horizontally through soil. Some iris have rhizomes as well as lily-of-the-valley, calla lily, oxalis, and achimenes.
This graphic was taken fromThe Physiology of Flower Bulbs, 1993.
A tuberous root consists of enlarged fleshy root tissue. Roots are fleshy; because, they are the primary storage tissue. Growth arises from buds at the top (crown) of the root mass. Examples include dahlia, anemone, and ranunculus.
This graphic was taken fromThe Physiology of Flower Bulbs, 1993.
The hypocotyl of a seedling is the portion of the stem below the cotyledon and above the roots. In some plants, the hypocotyl enlarges, becoming a fleshy storage site as the plant develops. Cyclamen and gloxinia are examples of plants with enlarged hypocotyls. Also, "tuberous" begonias are not actually tuberous but have enlarged hypocotyls.
This graphic was taken fromThe Physiology of Flower Bulbs, 1993.