Elsewhere in this section, we use the term "bulb" to describe all bulbous plants, including tubers, rhizomes and corms as well as true bulbs. All bulbous plants contain a swollen food storage organ that enables them to survive when dormant or when conditions are unsuitable for growth.

Bulbs

True bulbs are formed from fleshy leaves or leaf bases, and often consist of concentric rings of scales attached to a modified stem, or basal plate. The leaf bases may overlap and surround the centre of the bulb as with lilies, or may completely surround the inner regions of the bulb, as with the onion. Roots emerge from the underside of the base, and new stems and leaves from the upper side.

The outer scales often form a dry, protective skin or tunic, as found in daffodils, tulips and Reticulata irises. With some lilies (Lilium) and Fritillaria species, the scales are separate and no tunic is formed. Some lilies also form small bulbs called bulbils in their leaf axils, whereas several members of the onion family, Alliaceae (including Allium sativum - garlic), form bulbils in their flower heads.

Rhizomes

Rhizomes are swollen, horizontal underground stems that often send out roots and shoots from their nodes. They are typically found in the Iridaceae, notably in irises, and in Liliaceae.

Corms

Corms are short, vertical swollen bases of stems, typically surrounded by protective skins or tunics formed from the previous year's leaf bases. Although externally they look very similar to bulbs, they are distinguished internally by their solid tissue, as opposed to a bulb's visible layers.

Corms are common in the family Iridaceae, which includes gladioli, crocuses, Watsonia and Romulea; and in the Liliaceae and related families, with such genera as Brodiaea and Colchicum.

Tubers

Tuberous is a term applied to many plants with swollen, often irregularly shaped stems or roots used for food storage. Unlike a bulb, a tuber persists for one season only; new tubers developing on a plant in the following year are formed in different places.

True tubers are found in some orchids (such as Dactylorrhiza), in Corydalis, in cyclamen species (although these are often called corms), and in plants such as Ranunculus asiaticus, where they are lobed or clustered together.