Bulbs have adapted morphologically to periodically favourable and unfavourable circumstances due to seasonal climatic changes, and consequently to the different growing conditions during a year. This adaptation has resulted in a subterranean storage organ and in an alternating resting and growing period.
The inclusive term for all bulbs is geophyte: "a perennial plant which survives unfavourable conditions by means of subterranean parts". In the storage organ the surplus food (starch, sugars and protein) is stored to enable the plant to survive the unfavourable periods. In or on the storage organ the main bud is situated which will develop during the next growing season into a complete plant with roots, leaves and flowers. At the beginning of the rest period, the parts of the plant above soil level will usually die off, as well as all, or a part of, the roots.
In a true bulb, the surplus food is stored in the modified, thickened, fleshy base of the leaves. The leaf stem is reduced to a basal plate. If the leaflike parts enclose the bulb completely and the tips have grown together they are called rings; examples are Tulipa and Allium. If the leaflike parts enclose the bulb only partly, they are called scales, as is the case in Fritillaria and Lillium.
Most true bulbs have a typical development where the main bud or apex only forms the flowers after the leaves have been produced. The next growing season, the vegetative growth is continued by a new main bud which develops from a lateral bud, usually the one closest to the old main bud. This new bud normally starts developing in the generative phase of the old main bud or during flowering.
There are annual and perennial bulbs. Annual bulbs are found in e.g. Tulipa and perennial ones in e.g. Narcissus, Hippeastrum and Lillium.
A rhizome is a subterranean creeping, branching stem with scaly leaves and buds. The food supply is stored in this stem. The rhizome is solid and forms a bud at the tip, from which the leaves and flowers will grow. Rhizomes usually grow horizontally with roots only on the under side. Examples of a rhizome are Iris germanica and Agapanthus.
There are two types of corms: stem-corms and root-corms. In stem-corms, a part of the bottom of the stem is modified to a corm. The bud is situated on top of the stem-corm. At the end of the growing season, the old corm usually dies off and a new corm develops on top of the old one. Stem-corms are mostly annuals and examples are Gladiolus and Crocus. In root-corms, the surplus food is stored in thickened roots.
Tubers are only found in Dicotyledons. The other forms of bulbs described above are found in the Monocotyledons. A tuber is a subterranean stem, usually rounded and fleshy and sometimes covered with scaly leaves concentrated on the top part of the tuber. A tuber normally has more buds that can grow simultaneously in one growing season. Examples of tubers are Begonia, Dahlia and Sinningia.