A bulb is a short stem with fleshy leaves or leaf bases. The leaves often function as food storage organs during dormancy.

A bulb's leaf bases, known as scales, generally do not support leaves, but contain food reserves to enable the plant to survive adverse conditions. At the center of the bulb is a vegetative growing point or an unexpanded flowering shoot. The base is formed by a stem, 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. Tunicate bulbs have dry, membranous outer scales that protect the continuous lamina of fleshy scales. Species in the genera Allium, Hippeastrum, Narcissus, and Tulipa all have tunicate bulbs. Non-tunicate bulbs, such as Lilium and Fritillaria species, lack the protective tunic and have looser scales.

Other types of storage organs (such as corms, rhizomes, and tubers) are sometimes erroneously referred to as bulbs. The technical 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.

Nearly all plants that form true bulbs are monocotyledons, and include:

  • Onion, garlic, and other Allia, family Alliaceae.
  • Lily, tulip, and many other members of the lily family Liliaceae.
  • Amaryllis, Hippeastrum, Narcissus, and several other members of the amaryllis family Amaryllidaceae.
  • Two groups of Iris species, family Iridaceae: subgenus Xiphium (the "Dutch" irises) and subgenus Hermodactyloides (the miniature "rock garden" irises).

Oxalis, in the family Oxalidaceae, is the only dicotyledon genus that produces true bulbs.

Bulbous plant species cycle through vegetative and reproductive growth stages; the bulb grows to flowering size during the vegetative stage and the plant flowers during the reproductive stage. Certain environmental conditions are needed to trigger the transition from one stage to the next, such as the shift from a cold winter to spring.

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Famous quotes containing the word bulb:

    ... until the shopkeeper plants his boot in our eyes,
    and unties our bone and is finished with the case,
    and turns to the next customer, forgetting our face
    or how we knelt at the yellow bulb with sighs
    like moth wings for a short while in a small place.
    Anne Sexton (1928–1974)