Wood Anatomy

Wood Anatomy

Woody trees such as the giant sequoia of California and the blue gum of Australia are among the world's largest organisms. Despite the tremendous bulk of their stems, only a relatively thin layer of tissue between the bark and the wood is actually alive and continues to produce new layers of wood throughout the life of the tree. How the meristematic tissue that produces these layers remains active for hundreds or even thousands of years is one of the major unanswered questions of plant biology.

Meristematic Regions and Growth Patterns of Woody Plants

Plant embryos inside the seed are tiny rudimentary individuals that have the potential to produce the entire adult plant. This potential for growth rests in specialized areas of the embryo called the apical meristems . Upon seed germination, the root apical meristem at the basal end of the embryo starts to grow and to produce the root system of the plant. The shoot apical meristem at the opposite end of the embryo produces the shoot system. These apical meristems extend the length of the stem-root axis and, through cell proliferation, produce all the primary tissues of the plant body (epidermis, vascular tissues, and ground tissues). Plants belonging to the monocot group have only apical meristems. While some monocot stems become thick through primary thickening growth and become large trees like the date palm, monocots are never truly woody plants because they lack a vascular cambium.

In contrast, almost all plants belonging to the dicot group have two types of lateral meristems that increase the girth of the stems and roots: vascular cambium and cork cambium. The vascular cambium arises within the tissues of the stem and root. It first appears between the xylem and phloem of the vascular bundles and then extends between the vascular bundles to form a continuous sheet of cells around the stem. The defining feature of the vascular cambium is its distinctive plane of cell division: vascular cambium cells called cambial initials divide periclinally (that is, in a plane that is parallel with the surface of the stem and root). Cells derived from divisions of the cambial initials are called derivatives. Derivatives formed toward the inside of the stem and root become xylem cells, while derivatives formed toward the outside become phloem cells. The tissues formed by a lateral meristem are called secondary tissues, so the vascular cambium produces secondary xylem (wood) and secondary phloem.

The cells of secondary xylem tissue have hard, rigid cell walls and do not compress easily. Therefore, as the cambial initials produce increasingly more secondary xylem derivatives and the stem or root increase in girth, the cambium and all other tissues to the exterior begin to be stretched. This is often the signal for the formation of the second kind of lateral meristem, the cork cambium. In the stem, the cork cambium usually arises just under the epidermis, while in the root the cork cambium arises closer to the vascular tissue, from the pericycle. In each case, the initials of the cork cambium divide periclinally, like the vascular cambium, but the cork cambium produces derivative cells only toward the outside. These derivatives mature as cork tissue, a compact, waterproof, and airtight layer that protects internal cells.

Many common wild and garden plants such as roses, sunflowers, and asters have both vascular and cork cambiums that produce secondary vascular and cork tissues. These plants live for a relatively short time and their lateral meristems produce only a moderate amount of secondary tissue. Other plants live for decades or centuries, and their lateral meristems produce secondary tissues year after year, building up massive volumes of wood and bark. These are the true woody plants, and most produce a large main stem or trunk, with smaller lateral branches.

Anatomy of a Woody Stem

Many features of wood and bark can be identified in a tree stump without a microscope's magnification. Growth rings reflect the annual activity of the vascular cambium. Early in the growing season, the cambium produces xylem derivatives that mature as wide, thin-walled cells, forming a visible light layer of wood called the earlywood. Later in the growing season, xylem derivatives mature as narrower, thicker-walled cells, forming a darker layer in the wood called latewood. In temperate climates, each pair of earlywood and latewood layers form a recognizable annual ring. In years when conditions for tree growth are good, the vascular cambium divides actively and produces a wide annual ring. When growth conditions are poor, the cambium divides slowly, and the annual ring is narrow. The distinctive pattern of wide and narrow annual rings in a tree trunk provides clues about tree growth rates in the past, often for periods that extend for hundreds of years. Dendrochronologists use this information to make deductions about past climatic conditions and, in some cases, can determine the exact dates that a piece of wood from an ancient building was part of a living tree.

Many trees have a darker region of wood at the center of the trunk or root called heartwood. The coloration arises from tannins and other substances that retard decay created by xylem parenchyma cells before they die. Since the conducting cells (vessel elements and tracheids ) and the supporting cells (sclerenchyma fibers) are already dead, the entire heartwood is nonliving. The lighter wood toward the outside of the trunk is called sap-wood. Sapwood contains living parenchyma cells that function in storage and to recover nutrients from the sap. Although the entire sapwood region is moist, usually only the outer growth rings nearest the vascular cambium actually transport water from the roots to the leaves.

Another conspicuous feature of woody stems and roots are the panels of parenchyma tissue called rays that extend radially from the center of the heartwood, across the cambium, and into the bark. The ray parenchyma cells are produced by specialized cambium initials called ray initials and function as other xylem parenchyma cells. In some kinds of wood such as oak, the rays are very wide; in others such as pine, the rays are very narrow.

The bark found at the exterior of woody stems and roots is a composite structure. The secondary phloem that conducts the products of photosynthesis from leaves to roots is located directly adjacent to the vascular cambium. In most trees, the sieve elements of the secondary phloem are able to translocate for only one year. As the phloem ages, it becomes nonfunctional. Before all the phloem cells die, some of the parenchyma cells give rise to a new cork cambium that produces a new layer of cork tissue. Thus, bark is composed of alternating layers of dead phloem and cork tissues, with the only living cells found toward the inside. Thus woody stems have a thick insulating layer that protects the delicate vascular cambium within.

Differences Between Hardwood and Softwood

Hardwood is the term used for the strong, dense wood of angiosperm trees such as maple, oak, and mahogany. Usually more than 50 percent of the volume of the wood is composed of sclerenchyma fibers, cells with extremely thick, lignin-impregnated walls, which give the wood its great physical strength. The remainder of the wood consists of conducting cells, the vessel elements and tracheids, and parenchyma cells. Hardwood trees vary in the arrangement of these cells within the annual ring. Some, like oak and elm, have wide, thin-walled vessel elements in the earlywood and much narrower vessel elements in the latewood, accentuating the differences between the two parts of the annual growth ring. This pattern is referred to as ring porous wood. Other hardwoods such as maple and willow have vessel elements of more uniform diameter scattered across the growth ring. This pattern is called diffuse porous wood.

The term softwood is used for the wood of conifers such as pines, firs, and spruces. The wood tends to be softer and less dense because it lacks the specialized sclerenchyma fibers of the hardwoods. Most of the volume of conifer wood is occupied by tracheids, cells that both conduct water and provide mechanical support. Because they carry out both functions, tracheids have relatively thin cell walls. The parenchyma tissue of softwoods often contains resin-filled ducts; these are part of the tree's defense system against insects and fungal diseases.

see also Anatomy of Plants; Conifers; Cork; Dendrochronology; Trees; Vascular Tissues; Wood Products.

Nancy G. Dengler

Bibliography

Carlquist, Sherwin. Ecological Strategies of Xylem Evolution. Los Angeles: University of California Press, 1975.

Esau, Katherine. Anatomy of Seed Plants. New York: John Wiley & Sons, 1977.

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

Zimmerman, Martin H. Xylem Structure and the Ascent of Sap. New York: Springer-Verlag, 1983.