A bulb's leaf bases, also 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).
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. Once the flowering period is over, the plant enters a foliage period of about six weeks during which time the plant absorbs nutrients from the soil and energy from the sun for setting flowers for the next year. Bulbs dug up before the foliage period is completed will not bloom the following year but then should flower normally in subsequent years.
After the foliage period is completed, bulbs may be dug up for replanting elsewhere. Any surface moisture should be dried, then the bulbs may be stored up to about 4 months for a fall planting. Storing them much longer than that may cause the bulbs to dry out inside and become nonviable.
Some lilies form small bulbs, called bulbils in their leaf axils. Several members of the onion family, Alliaceae, including Allium sativum (garlic), form bulbils in their flower heads, sometimes as the flowers fade, or even instead of the flowers. The so-called Tree onion (Allium cepa var. proliferum) forms small onions which are large enough for pickling.
Some ferns, such as Hen and Chicken Fern produce new plants at the tips of the fronds' pinnae, which are sometimes referred to as bulbils.
Bulbils form in the leaf axils of Lilium lancifolium
Wild garlic (Allium vineale) bulbils sprouting
"Tree onions" form clusters of small bulbs instead of flowers
- Bell, A.D. 1997. Plant form: an illustrated guide to flowering plant morphology. Oxford University Press, Oxford, U.K.
- Mishra, S.R. (2005). Plant Reproduction. Discovery Publishing House. pp. 120–125. ISBN 978-81-7141-955-5.
- Ellis, Barbara W. (2001). Bulbs. Houghton Mifflin Harcourt. p. 3. ISBN 978-0-618-06890-6.
- Hartmann, Hudson Thomas; Dale E. Kester (2002). Hartmann and Kester's Plant Propagation: Principles and Practices (7 ed.). Prentice Hall. p. 561. ISBN 978-0-13-679235-2.