Bulbs are not categorized as simply as it would often seem. Relatively few of the flowers called bulbs are technically bulbs; the type of flowers referenced by this title include many corms, tubers, tuberous roots and rhizomes. All are similar in function in that they are large parts of the plant that have been grown underground and are capable of generating new plants on their own. Many bulb-type plants are grown as food, and many are used to beautify houses and gardens.
A true bulb consists of an embryonic plant, containing leaves, flower buds and stems. It is contained within an underground base, or bulb-shaped object. You can recognize these from the outside from their scales, like those of an onion. Their scales (also called layers) are much larger than in corms (another type of plant referred to as a bulb). Flowers grown from true bulbs include daffodils, belladonna lilies and tulips. True bulbs are also rounder than corms, their closest relatives.
Corms are flatter than true bulbs and have smaller scales. They do not contain leaves, flower buds or stems, and the plant's food is contained in the base rather than in the scales, as is the case with true bulbs. Corms grow roots from the basal plate (a flat surface on the bottom) and a stem from the top. Common corms include crocuses and gladioli.
Tubers and Tuberous Roots
Tubers, unlike corms and true bulbs, do not have basal plates. They also do not have scales. Roots can grow from many points on the tuber's surface, and there are many growth points for stems as well. Tubers are starchy so that they can hold food for the winter. One example is the potato. Tubers used in flower gardens include begonias, cyclamen, lotus and mint. Dahlias and day lilies are examples of tuberous roots, which are different from tubers in that they are actually roots. True bulbs, corms and tubers are not roots; they are stem bases.
A rhizome is an underground stem. They tend to grow horizontally, and the plants that grow them send them out to form newer plants as a means of propagation. Bamboo is an example of how invasive rhizomes can be if they are not kept under check. The bearded iris is a rhizome, as is the lily-of-the-valley.