Bulbs are structures that provide security for plants. They store food, protect delicate buds and weather harsh conditions that wither green leaves. True bulbs are only found in the division of plants called monocots, or monocotyledons. These contain the lilies, tulips and daffodils so familiar to gardeners, each of which evolved by using its bulb to survive.
A bulb is technically a very short stem with thickened, fleshy leaves surrounding a dormant bud. Specialized stem tissue forms the base of the bulb, the basal plate, that will sprout roots under favorable conditions. If you slice an onion in half from top to bottom, you will see a typical bulb structure. In some bulbs, such as lilies, the leaves form overlapping scales.
Some similar structures are commonly called bulbs, but are technically corms or tubers. A corm is solid rather than layered, but has a basal plate and is also formed from stem tissue. Crocus and gladiolus bulbs are actually corms.
Tubers are thickened rhizomes, horizontal stems that grow beneath the surface of the soil. Cyclamen bulbs and potatoes are both tubers.
The modified leaves and stem tissue that surround the root and leaf buds provide food for their first growth as well as protection. As a bulb sprouts, it gradually becomes smaller, transferring all its carbohydrates to the new shoot. After the plant flowers, it begins to make a new bulb and, as the leaves die, it sends as much food as possible to the swelling bulb. It gradually withdraws its energy from the above-ground portion of the plant, readying itself for dormancy.
As underground storage organs, bulbs free a plant from dependence on rain, warmth and sunlight, allowing it to survive drought, low temperatures, high winds, deep snow or intense heat in a dormant state. Tulips, for instance, are found wild throughout western Asia, an area with hot dry summers and cold winters. Without the bulb, tulips would never have been able to colonize such a harsh environment.
Bulbs seem to be associated with other characteristics such as large, bright flowers, wind-dispersed seeds and narrow leaves, all factors that favor survival and propagation in meadows and other open areas with pronounced seasons. It is thought that these traits evolved as monocots moved from their ancestral forests to the harsher conditions of the plains.
Bulbs in the Garden
As bulbs have been brought into cultivation, they have evolved in ways that make them more adaptable to garden conditions. For instance, a wild tulip that requires perfect drainage and a dry, hot summer may have seedlings that survive a somewhat cooler, more moist summer and these will be selected for further propagation.