See J. E. Bryan, Bulbs (1989).
In botany, the resting stage of certain seed plants, 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 corms of the crocus and gladiolus and the elongated rhizomes of some irises are not bulbs.
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A bulb's leaf bases generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. The leaf bases may resemble scales, or they may overlap and surround the center of the bulb as with the onion. A modified stem forms the base of the bulb, and plant growth occurs from this basal plate. Roots emerge from the underside of the base, and new stems and leaves from the upper side.
Other types of storage organs (such as corms, rhizomes, and tubers) are sometimes erroneously referred to as bulbs. The correct term for plants that form underground storage organs, including bulbs as well as tubers and corms, is geophyte. Some epiphytic orchids (family Orchidaceae) form above-ground storage organs called pseudobulbs, that superficially resemble bulbs.
Some ferns, such as Hen and Chicken Fern grow offshoots on top of their fronds, which are also referred to as bulbils.