Bulb, Corm, tuber, Rhizome! What does it all mean?

The term “bulb” is commonly used to refer to the thickened underground storage organ produced by some plants. However, many of what we call bulbs may be corms, tubers, tuberous roots or rhizomes.

You might think of a bulb as you would a chicken egg. The outside of a “true bulb” is a compressed stem (basal plate) bearing a growing point or flower bud and enclosed by thick, fleshy scales called bulb scales. Much like the egg’s shell protects the growing chick enclosed inside the egg.

Some true bulbs such as narcissus, amaryllis and tulip are protected from drying and mechanical injury by dry and membranous outer scales called a tunic. Other true bulbs such as lilies are called non-truncate or scaly because their outer scales are succulent and separate, giving the bulb a scaly appearance.

A corm is a solid mass of stem tissue with a basal plate on the bottom and a terminal shoot bud on top. In addition to the terminal shoot bud, axillary or lateral buds are produced at each of the nodes along the sides of the corm. In the event that the terminal bud should be injured or otherwise prevented from growing, these lateral buds are capable of producing a shoot. The solid stem structure of the corm is protected against injury and water loss by dry leaf bases that are similar to the tunic that enclose true bulbs. Gladiolus and watsonia are typical cormous plants.

Tubers such as caladium and gloriosa are thickened underground stems with many buds (eyes) present in regular order over their surfaces. Unlike true bulbs and corms which are covered with dried leaves or scales, tubers are covered with a tough skin. Tuberous roots such as dahlia and ranunculus are true roots and lack nodes and internodes. Buds are present only at the crown or stem end of the root.

Rhizomes are thickened horizontal stems growing along or below the surface of the ground. Underground rhizomes such as canna and calla produce roots on their lower surface and send shoots above ground.

The term bulb will probably always be used when referring to plants that produce underground storage organs.