Bulb, in common usage, an underground stem bearing scales (modified leaves) and one or more buds. Bulbs contain stored foods and can give rise to new plants. Scales arise from the stem and surround one or more buds in the bulb. The underside of a bulb produces roots; the bud, or upper side, produces leaves. Corms, tubers, and rhizomes, like true bulbs, are underground stems and are commonly called bulbs. Tuberous roots are also commonly called bulbs.
A true bulb has a very small disc-shaped stem and large, thick scales. A corm consists mainly of an enlarged stem base with a few scales. A tuber consists mostly of an enlarged stem tip with small surface scales. A rhizome consists of a stem of varying thickness with small scales on the stem surface.
also Stem, subtitle Modified Stems: Underground Modified Stems.