Bulbs, Corms, Tubers and Rhizomes

Bulbs, Corms, Tubers and Rhizomes are actually underground stems, not roots. These underground stems are all storage facilities for the plant, storing starches, proteins and other nutrients. In other perennials this function would be carried out over winter by the root system.

The differences between Bulbs, Corms, Tubers and Rhizomes

Bulbs

Bulbs are a collection of underground leaves that store food and water at the center of the bulb. These leaves are visible as layers. Due to this layered nature, bulbs are often called true bulbs. Bulbs are usually rounded or egg-shaped. They have a pointed end, which grows up forming the stem.

Tulips, onions, daffodils, alliums, hyacinths, and lilies are some examples of bulbs.

Corms 

Corms are similar to bulbs but do not have fleshy scales.  Corms are modified stems with a tunic. They are a type of stem tissue differentiated to store nutrients. Corms are either rounded or slightly flattened at the top. The basal plate occurs at the bottom of the corm and a corm may have several growing points, which appear as bumps or nodes, called cormels or cormlets. The parent corm dies back and the Cormels can be divided from the parent corm and planted separately.

Some examples of corm-developing plants are gladiolus, freesia, and crocus.

Tubers 

Tubers are the undifferentiated stem. They lack a basal plate. Most roots develop from the bottom of the tuber. Some tubers have organized growing points called eyes at the top. Tubers do not produce an offspring but, they grow in size in every year. Some tubers have fleshy roots, which look like fat fingers. The two types of tubers are stem tubers and root tubers.

Stem tubers such as potatoes are underground stems. Root tubers such as beets are underground roots.

Rhizomes 

Rhizomes are the swollen underground stems, which grow horizontally under the soil. New growing points occur along the surface of the rhizomes. Some underground rhizomes are ginger, turmeric, common iris, lily-of-the-valley, canna and calla lily.