Bulbs and Rhizomes

Bulbs and rhizomes are modified stems, stem bases, or other underground organs used by plants for food (or energy) storage and in asexual reproduction.

Plants reproduce both sexually and asexually. Although sexual reproduction is part of the typical life cycle of plants, for a variety of reasons a plant may reproduce asexually. Exact duplicates of a plant, called clones, are formed by asexual reproduction.

Asexual Reproduction

Asexual reproduction involves the production of offspring through the formation of propagules by mitosis (the process of nuclear cell division). Because genetic recombination does not occur in mitosis, the offspring are genetically identical to the parent plant.

Asexual reproduction does not occur in all plants; some reproduce asexually only when humans intervene. Asexual reproduction occurs when a single plant produces a vegetative propagule that develops into a separate free-living plant. Many of the propagules that support asexual reproduction are actually highly modified branches. Others are modified roots.

In rare instances, the tissues of leaves may be modified by nature to support asexual reproduction. The propagules of asexual reproduction vary enormously. They are often found in catalogs describing “bulbs,” but technically they include true bulbs, corms, stolons, tubers, rhizomes, turions, pseudobulbs, and fleshy roots.

True Bulbs

Bulbs, corms, stolons, tubers, rhizomes, and turions are all modified stems. Bulbs are modified stem bases that develop underground. The stem is shortened and thickened to produce a mass of tissue shaped like a coin or like a child’s toy top. Scale like leaves with thickened bases are attached to the base of the bulb.

Starch is stored in the thickened bases, a food supply that allows the bulb to survive through a dormant season and to produce adventitious roots. Roots are often absent when the bulb is dormant. The starch can also support a period of rapid stem and leaf growth in the growing season and may support the flowering and fruiting of the plant.

In tunicate bulbs, a cloak of dried leaves surrounds the outside of the bulb. These dried leaves provide a barrier to desiccation and allow the tunicate bulbs to be stored above ground for weeks or even months.

Onions (Allium cepa) and daffodils (Narcissus) are examples of tunicate bulbs. Other bulbs have no cloak and usually have shorter, less cylindrical leaves. These scaly bulbs dry quickly when kept above ground and usually develop flowers only after a more normal, aerial branch system forms. Lilies (Lilium) have scaly bulbs.

Stems of both tunicate and scaly bulbs can branch. Below ground, branches appear at first as miniature bulbs (bulbils or bulblets). Bulblets take their energy from the parent bulb but eventually produce aerial stems or leaves and can be separated from the parent.

Profuse branching can be stimulated by wounding the stem of the parent bulb. All the bulbs produced by this technique are clones, identical to the parent in their genetic makeup and physical characteristics.

Corms and Tubers

Corms are similar to bulbs in many ways. They have a disk-shaped or top-shaped stem mass that is shorter and wider than most typical stems. They are often cloaked in a tunic of dried leaves that are thinner and smaller than those on bulbs.

Corms do not store significant amounts of starch; it is instead stored primarily in the basal plate of the stem. Branches of the corm stem produce new, miniature corms (cormels). Wounding the parent stem stimulates greater branching. Gladiolus and crocus are two common garden plants that produce corms.

Tubers are thick, starchy stems that form usually at the tip of a stolon, runner, or tiller. Tubers may form on the soil surface or below ground. A familiar example is the white or Irish potato (Solanum tuberosum). The leaves on most tubers are much smaller than the leaves on other parts of the stem, but above each leaf on the tuber is a well developed axillary bud, commonly called an eye.

The axillary bud has the potential to elongate, forming a complete and fully developed branch. If the stolon connecting the tuber to the parent plant dies, the branch from the eye of the tuber becomes an independent clone of the parent plant.

Most tubers contain many eyes. If the tuber is cut into smaller pieces, each containing an eye, each piece develops a rhizome, which in turn develops a newtuber. By this technique, a significant increase in the number of plants can be obtained.

The cut pieces of tuber are initially prone to decay, but after they have dried for a few days, they heal over with a layer of callus which protects them like a skin. Cutting tubers into small “seed” pieces is a common method for propagating tuber-forming species.

Rhizomes, Stolons, and Fleshy Roots

Rhizomes are specialized, underground stems. Unlike most areal stems, the rhizomes are normally oriented horizontally. Just as pieces of tuber can provide the tissue and energy source for the formation of a new plant, so too can pieces of a rhizome.

Many ferns and fern allies propagate naturally by rhizomes. Large stands of these plants can form from a single individual as the rhizomes grow and branch. Eventually, older pieces of the rhizome die, leaving a population of individuals that all have identical genetic characteristics.

Stolons are long, thin, horizontal stems, also called runners or tillers, which grow along the surface of the ground. When the stolon has grown far enough from the parent plant, the growth pattern changes, and a crown, or tuber, forms. A crown is a compressed stem mass with leaves arranged close to one another, also called a rosette. Within the crown, roots format the points of attachment of the leaves to the stems.

If the stolon is broken or dies, the crown becomes an independent clone of the parent plant. In this way, a large number of off-spring can be produced from a single plant. This is a mechanism of reproduction of the strawberry (Fragaria) and is also a common reproductive mechanism for grasses, including crab grass (Digitaria sanguinalis) and quack grass (Agropyron repens).

Fleshy roots store energy that can be useful for asexual propagation. Most require at least a small amount of stem tissue to support cell growth and differentiation. The true yam (Dioscorea) is a tuber, but the sweet potato (Ipomoea batatas) is a fleshy root which can easily be propagated asexually. Many buttercups (Ranunculus) are also propagated by breaking up the clumps of their fleshy roots.