Meristems

Meristems are regions of active cell division within a plant. In general there are two types of meristems: apical meristems and lateral meristems. Apical meristems are located at the tip (or apex) of the shoot and the root, as well as at the tips of their branches. These meristems occur in all plants and are responsible for growth in length. By contrast, lateral meristems are found mainly in plants that increase significantly in diameter, such as trees and woody shrubs. Lateral meristems are located along the sides of the stem, root, and their branches; are found just inside the outer layer; and are responsible for growth in diameter.

The term meristem comes from the Greek word meaning "divisible," which emphasizes the fundamental role played by mitotic cell division in these tissues. Meristematic cells are those that divide repeatedly and in a self-perpetuating manner; that is, when a meristematic cell divides, one of the daughter cells remains meristematic. Meristems, however, may not be constantly active. For example, in temperate climates meristematic cells stop dividing during the winter but then begin dividing again in the spring.

Apical Meristems

Both root and shoot apical meristems consist of a group of two types of cells: initials and their immediate derivatives. Initials are the true meristematic cells in that they divide almost continuously throughout the growing season. When an initial divides it forms two daughter cells, one a new initial and the other a derivative that soon stops dividing and eventually differentiates into part of the mature tissues of the plant. In many cases the older derivatives elongate, and it is this process that pushes the initials of the shoot apical meristem higher into the air and the initials of the root apical meristem deeper into the soil. All tissues produced by an apical meristem are called primary tissues.

In most plants the root apical meristem is covered by a protective root cap and consists of a group of relatively small, roughly spherical cells, each having a dense cytoplasm and a large nucleus but no apparent vacuole. The derivatives of certain apical initials give rise to additional root cap cells, thus replacing those that were lost as the root cap rubbed against soil particles. The derivatives of other initials give rise to the mature tissues of the main body of the root, such as xylem, phloem, cortex, and epidermis. In the center of the root apex is a cluster of cells that divides very infrequently. These cells comprise the quiescent center, whose apparent function is to serve as a source of cells should the initials become damaged.

In angiosperms, the shoot apical meristem is not covered by a protective cap and has additional features that distinguish it from the root apex. For example, the lateral appendages of the stem—the leaves and lateral buds—are produced at the shoot apex. Leaves arise as small protuberances (called leaf primordia) slightly to the side of the apical-most cells. As they elongate, the resulting leaves cover and protect the apical meristem. Buds develop in the angle between the stem and each leaf primordium, a location called the leaf axil. In a plant growing vegetatively, these axillary (or lateral) buds contain meristems that can develop into branches. When the plant reproduces sexually, the shoot apical meristem produces flowers instead of leaves. The various flower parts—petals, sepals, stamens, and carpels —are modified leaves and are produced in a manner similar to that of leaf primordia.

The apical meristem of most angiosperms has a tunica-corpus arrangement of cells. The tunica consists of two or more layers of cells, and the corpus is a mass of cells underneath. Cells of the tunica and corpus give rise to the leaves, buds, and mature tissues of the stem.

Lateral Meristems

Two types of lateral meristems, also called cambia (singular: cambium), are found in plants: the vascular cambium and the cork cambium. Each type consists of a hollow, vertical cylinder of cells that contribute to the thickness of woody plants. As with apical meristems, lateral meristems consist of initials and their immediate derivatives. All tissues produced by a lateral meristem are called secondary tissues.

The vascular cambium contains two kinds of initials: fusiform initials and ray initials, both of which have large vacuoles. Each type of initial produces derivatives toward the inside that develop into xylem cells and derivatives toward the outside that develop into phloem cells. The fusiform initials are long, tapering cells that are vertically oriented. They give rise to xylem vessel elements and phloem sieve-tube members; these cells are involved in the vertical transport of materials through the plant. The ray initials are cube-shaped cells that give rise to xylem parenchyma and phloem parenchyma and together constitute the vascular rays. Rays are involved in the lateral transport of materials. Both the fusiform and ray initials produce many more xylem cells than phloem cells. The accumulating xylem cells push the vascular cambium increasingly farther away from the center of the root, stem, or branch, and as a result the organ increases in diameter.

In response to this increase in thickness the epidermis and other cells exterior to the vascular cambium stretch and eventually break. Before cracks occur, a cork cambium differentiates from cells of the cortex. The cork cambium (or phellogen) produces cork cells (phellem) toward the outside and phelloderm toward the inside. Together, these three tissues constitute the periderm. Cork cells have a flattened shape, and their walls become filled with suberin, a fatty material that makes these cells an impermeable barrier to water, gases, and pathogens. Although the cork cambium and phelloderm are alive at maturity, cork cells are dead. The cork thus provides an effective seal that replaces the epidermis. As the plant organ continues to increase in diameter, the cork cells themselves crack, and additional cork cambia differentiate from underlying tissues as replacements.

Robert C. Evans

Bibliography

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Mauseth, James D. Plant Anatomy. Menlo Park, CA: Benjamin/Cummings Publishing Co., 1988.

Moore, Randy, W. Dennis Clark, and Darrell S. Vodopich. Botany, 2nd ed., New York: McGraw-Hill, 1998.

Raven, Peter H., Ray F. Evert, and Susan E. Eichhorn. Biology of Plants, 6th ed. New York: W. H. Freeman and Company, 1999.