Many popular garden flowers, such as daffodils and lilies, grow from bulbs. The descriptive definition of a bulb is a mass at the root of a plant that grows into a flower, usually on an annual basis. Looking closer at the structure of bulbs, however, reveals the purpose behind these structures and their usefulness to the gardener.
Although gardeners tend to identify bulbs based on what they observe of the plant's root structure, from the botanist's standpoint, a bulb is any underground structure that serves primarily to store nutrients for plant growth. When spring arrives and conditions become favorable for growth, the plant develops and blooms using the food stored in the bulb.
At the bottom of the bulb is the basal plate. Looking at a bulb, you can identify the basal plate because the thin, shaggy roots grow from it. The bulb itself consists of fleshy scales, and some species have an additional papery covering called a tunic. Onions provide excellent examples of both the scales, which form as visible layers, and the papery covering that protects them. Cutting open a bulb reveals that it contains a developing flower bud and leaves underneath the scales. Finally, lateral buds form at the base of the bulb and develop into budlets or offsets, which in turn mature into bulbs that form new plants.
The parts of the bulb allow it to function as a nutrient storage structure, protect the developing plant and promote asexual reproduction. The bulb's fleshy scales hold most of its nutrient content and feed the young plant enveloped within. The tunic protects the scales from damage and, especially, drying. For this reason, imbricate bulbs--bulbs that lack the tunic--must be kept moist before planting, whereas tunicate bulbs do not require this. Although flowers reproduce sexually, structures like the lateral buds also permit vegetative reproduction, when the parent plant grows identical copies of itself. This ensures a degree of reproductive success, even if sexual reproduction fails.
Differences in structure indicate different types of bulbs. True bulbs contain all of the structures described above. Corms are swellings at the base of the stem that store nutrients, much like a true bulb, but lacking the visible scales. Crocus and gladiolus develop from corms. Tubers lack a basal plate and tunic. Perhaps the most familiar tuber is the potato, which left long enough in a pantry will sometimes form small green sprouts. Rhizomes act as storage structures but spread horizontally under the soil. Finally, some plants, such as daylilies, store nutrients in a fleshy root.
The structure of the bulb confers many advantages to the gardener. Because bulbs act as both nutritional and protective structures, most can be easily transported and stored. Many are hardy, giving rise to the flowers in some regions before the last snows have even melted. Because bulbs reproduce vegetatively, the gardener who plants a few scattered bulbs may, in a few years, find herself with a garden flush with blossoms.