In botany, the resting stage of certain seed plant s, particularly perennial monocotyledons (see cotyledon ), consisting of a relatively large, usually globe-shaped, underground bud with membranous or fleshy overlapping leaves arising from a short stem.
The fleshy leaves function as food reserves that enable a plant to lie dormant when water is unavailable (during winter or drought) and to resume active growth when favourable conditions again prevail. There are two main types of bulbs. One, typified by the onion , has a thin papery covering protecting its fleshy leaves. The other, the scaly bulb, as seen in true lilies, has naked storage leaves, with no papery covering, making the bulb appear to consist of angular scales. Bulbs enable many common ornamentals, such as the narcissus , tulip , and hyacinth , to flower rapidly in early spring when growing conditions are favourable. Other bulb-producing plants bloom in the summer (e.g., lilies) or fall (e.g., the autumn crocus ). The solid corm s of the crocus and gladiolus and the elongated rhizome s of some irises are not bulbs.