Bulbs and rhizomes
Bulbs and rhizomes are specialized plant structures that play crucial roles in asexual reproduction, a process where plants produce genetically identical clones of themselves. Bulbs are thickened underground stem bases that store starch, enabling the plant to survive dormant periods and support rapid growth when conditions are favorable. Examples of bulbs include onions and daffodils, which may have protective layers of dried leaves. Corms and tubers are similar, with corms being wider and shorter and tubers developing from stem structures called stolons. Both can propagate new plants from their parts.
Rhizomes, on the other hand, are horizontal underground stems that facilitate the spread of certain plants, such as ferns, by producing new shoots as they grow. Stolons, or runners, are above-ground stems that can also develop into new plants when they root away from the parent. Fleshy roots serve to store energy and can support asexual propagation when adequate stem tissue is present. This diverse array of structures illustrates the various strategies plants use to reproduce and thrive in their environments.
Bulbs and rhizomes
Categories: Anatomy; physiology; reproduction and life cycles
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. Scalelike 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 aboveground 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 aboveground 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. Belowground, 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 belowground. 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 new tuber. 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 form at 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 offspring 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.
Bibliography
Hartmann, Hudson T., and Dale E. Kester. Plant Propagation. 6th ed. Englewood Cliffs, N.J.: Prentice Hall, 1997. A classic text covering plant propagation, asexual, and sexual reproduction. Illustrated.
Hartmann, Hudson T., A. M. Kofranek, V. E. Rubatzky, and William J. Flocker. Plant Science: Growth, Development, and Utilization of Cultivated Plants. 2d ed. Englewood Cliffs, N.J.: Prentice Hall, 1988. A readable introductory botany text that places plant growth in context with the biology of plants. The chapter on propagation of plants describes asexual reproduction. Heavily illustrated with black-and-white drawings and photographs.
Kaufman, Peter B., et al. Plants: Their Biology and Importance. San Francisco: Harper & Row, 1989. Illustrated with black-and-white and color drawings and photographs, this botany text is easily understood. The chapter on modifications of plant organs provides a good review of bulbs but is not as well illustrated as it could be.
Mogie, Michael. The Evolution of Asexual Reproduction in Plants . New York: Chapman and Hall, 1992. A study of the “cost of sex” in plant evolution, concluding that plants that reproduce asexually never developed sexual reproduction, rather than having developed sexual reproduction and then selecting against it. Includes bibliography and index.
Raven, Peter H., and George B. Johnson. Biology. 5th ed. Boston: WCB/McGraw-Hill, 1999. Beautifully illustrated with many color drawings and photographs, this college-level biology text places plant reproduction in context of the biology of plants and that of the plants in context of the living and nonliving world. Chapter 40, “Sex and Reproduction,” provides a good explanation of the consequences of asexual reproduction.
Richards, A. J. Plant Breeding Systems. 2d ed. New York: Chapman and Hall, 1997. A unified and comprehensive treatment of plant reproduction. Mostly focuses on sexual reproduction but considers asexual modes as well. Includes glossary, references, and index.