According the University of Illinois Extension, the definition of a bulb is any plant that stores its complete life cycle in an underground storage structure. The main function of these structures is to store nutrients for the plants' survival. Bulbs have a period of growth where the plant sprouts, grows, blooms, and then dies back. This period is followed by a period of dormancy where the plant collects nutrients and then rests before blooming again.
Parts of a Bulb
A bulb contains five main parts: the roots or basal plate; fleshy scales (primary storage tissue); a tunic (a covering that protects the fleshy scales), the shoot, which is the developing leaves and flowers; and the lateral buds or offsets. Bulbs are divided into two types: tunicate and imbricate. Tunicate bulbs have a thin covering that protects the scales from drying out. Good examples of this type of bulbs are tulips, daffodils, and hyacinth. The imbricate bulbs do not have the covering and must be kept continuously moist before they are planted. The lily is an imbricate bulb.
The term bulb is used loosely to include corms, tubers, tuberous roots, and rhizomes, as well as true bulbs.
A corm is merely the swollen stem base altered into a mass of storage tissue. It is not a true bulb because it does not have visible storage rings when cut in half. A corm does have roots and the thin covering for moisture. Plants that grow from corms are gladiolus and crocus.
The tuber is different than a bulb or corm because it is lacking the basal plate where roots develop. It also does not have the protective covering. Tubers have buds that spread out over the tuber surface where shoots and roots develop. Tubers are actually swollen portions of underground stems and have nodes. Buds sprout at the nodes.
Tuberous Roots and Fleshy Roots
Tuberous roots look much like tubers but are swollen and have nutrient-storing root tissue spreading out in length. These spreading root tissues take up water and nutrients to feed the plant. They grow in clusters, branching out from a central point. Dahlias are an example of a tuberous root.
Roots that are bigger in diameter and softer to the touch are called fleshy roots. Fleshy roots normally store more nutrients for the plant than other root types. Peonies and daylilies have fleshy roots.
The difference in rhizomes is that they grow horizontally underground. This can be considered invasive unless many of one plant are desired. The lily-of-the-valley is an example of a rhizome.