The primary function of these underground storage structures is to store nutrient reserves to ensure the plants survival.
Bulbs or bulb like plants are usually perennials. They have a period of growth and flowering. This is followed by a period of dormancy where they die back to ground level at the end of each growing season. For spring bulbs, the end of the growing season is in late spring or early summer. Spring bulbs start to grow again in the fall and flower the following growing season.
Bulbs can be broken down into five types of storage structures. corms, tubers, tuberous roots and rhizomes. A sixth category of fleshy roots has been added here for the purpose of showing the structure. Daylilies and peonies, which are popular plants with gardeners, are examples of this type.
The true bulb has five major parts. It contains the basal plate (bottom of bulb from which roots grow), fleshy scales (primary storage tissue), tunic (skin-like covering that protects the fleshy scales), the shoot (consisting of developing flower and leaf buds), and lateral buds (develop into bulblets or offsets).
True bulbs are divided into tunicate bulbs and imbricate bulbs. A tunicate bulb has a paper-like covering or tunic that protects the scales from drying and from mechanical injury.
Good examples of tunicate bulbs include: tulips, daffodils, hyacinths, grape hyacinths (muscari), and alliums.
Many plants such as daffodils form new bulbs around the original bulb. These bulbs, called offsets, develop from buds within the base of the mother bulb and produce new plants. When these bulbs become overcrowded, the flowers start to diminish in size. This is an indication that it is time to dig up and divide the bulbs.
Imbricate bulbs must be kept constantly moist before planting so they are not injured by the scales drying out.
Lilies can be propagated from bulbils that develop in the leaf axils of the plant. They can also bepropagated from bulblets that develop at the base of fleshy lily scales if maintained in a moist sand medium. It will take more than one year for the bulbils or bulbets to become flower size.
Corms
A corm is a swollen stem base that is modified into a mass of storage tissue. A corm does not have visible storage rings when cut in half. This distinguishes it from a true bulb.
The corm contains a basal plate (bottom of bulb from which roots develop), thin tunic and a growing point. Examples of plants that develop from corms include gladiolus, crocus, and autumn crocus.
When gladiolus corms are dug in the fall, they should be separated into well developed corms, to be stored for replanting, and poorly developed corms which the gardener may want to discard. The newly dug bulbs will have cormels that are peasize formed around the top of the old corm. The remains of the old corm will be directly beneath the newly formed bulb. When the bulb is cleaned up and the old stem removed, the growing point of the bulb will be evident. The cormels can be saved and replanted in the back of the garden until they reach flowering size.
Examples of plants that develop from corms include gladiolus, crocus and autumn crocus.
A tuber differs from the true bulb and the corm by not having a basal plant from which roots develop and not having a protective tunic covering.
The caladium tuber has buds scattered over the tuber surface from which shoots and roots develop. Examples of plants that develop from tubers include caladiums, oxalis and anemones, and the common vegetable, the potato. (The potato does flower.)
The tuberous root differs from other root structures by the nutrient reserves being stored in an actual root instead of an enlarged stem.
The dahlia reproduces from buds at the top end of the root or base of the stem.
Rhizomes differ from other storage structures by growing horizontally under the surface of the soil. On some plants, this type of rooting structure can be very invasive.
Plants that have fleshy roots store nutrient reserves in the fleshy roots.
