Wood, also known as secondary xylem, is a composite of tissues found in trees. Secondary xylem is composed primarily of cells, called vessel elements in angiosperms, or of slightly different cells in gymnosperms called tracheids. These cells of secondary xylem, along with specialized cells of a type called parenchyma, are made by a meristematic tissue called the vascular cambium. As the vascular cambium generates new cells, secondary xylem accumulates on its inside, and the tree increases in diameter.
Newly made vessel elements and tracheids are water conduits from the roots of plants to their leaves. When first made, vessel elements and tracheids are alive but once they mature and become functional, they die. The functional vessel elements or tracheids occur in a few cell layers behind the vascular cambium, in a water-conducting section of the secondary xylem known as sapwood.
The parenchyma are made by the vascular cambium along with the vessels or tracheids, and are located at certain points along the perimeter of the vascular cambium. As the tree expands through growth, these narrow columns of parenchyma cells, called xylem rays, become longer, and ultimately extend from the vascular cambium to very near the center of the tree trunk. The function of xylem rays is to transfer aqueous material horizontally along the diameter of the tree, at a right angle to the flow of water in vessel elements and tracheids. The parenchyma cells of the xylem rays are alive in their mature, functional state.
As newer vessel elements or tracheids are made, older ones become buried under successive layers of more recently formed xylem. As the tree gets progressively larger in diameter, older secondary xylem tissues no longer conduct water. After this happens, these nonconducting cells are used to store waste products, such as resins. The xylem rays function to conduct wastes from actively functioning cells near the vascular cambium, to the non-functioning xylem cells. This waste-filled secondary xylem is called heartwood. By the time that a tree is larger than about 4-8 in (10-20 cm) in diameter, most of its biomass is composed of heartwood. New sapwood is created during each growing season but, within two to three years these cells become part of the heartwood. It is the heartwood of trees that is harvested to manufacture the lumber and paper used by people.
Wood of different species varies in density and strength, due to the size and density of the vessel elements or tracheids in the secondary xylem. For example, heartwood of the Brazilian ironwood (Caesalpinia ferrea) has very tiny vessel elements and is extremely dense. At the opposite extreme, the heartwood of balsa (Ochroma pyramidale) has very large vessel elements, and is correspondingly light in density. The wood of typical gymnosperms is generally soft and light in density, because tracheids do not fit together as closely as the vessel elements in the xylem of most angiosperms.
The size of tracheids and vessel elements also varies within a single tree, according to the season of the year that they were laid down during growth. In spring, when air temperatures are cool and soil moisture is typically plentiful, the vascular cambium of trees makes large diameter xylem cells. As the moisture wanes and temperatures increase in the summer, the vascular cambium makes smaller diameter cells. In the winter, no new cells are made, because of the cold temperatures. This cycle repeats itself every year and makes visible growth rings in the tree (except in the tropics). These rings are evident because spring wood, with larger diameter cells, is relatively dark in appearance, while summer wood is lighter in color. This annual repetition of differing cell sizes in growth rings is useful in ecological studies through dendrochronology. Because the size of vessel elements or tracheids is dependent on both air temperature and water, dendrochronologists can determine past periods of drought, flood, and unseasonal cold or heat, by studying variations in the width of growth rings.
Vessel elements and tracheids differ in length between angiosperms and gymnosperms. Although the length of individual cells makes little difference in the ability of trees to conduct water, the length of cells is of great importance to the paper industry. The length of these cells corresponds to the fiber length of pulp that is to turned into paper, and influences the quality of paper that can be produced. Short fibers make fine grade papers, while longer fibers make coarser grade papers.
Bell, P. R. Green Plants, Their Origin and Diversity. Portland: Dioscorides Press, 1992.
Fahn, A. Plant Anatomy. 5th ed. New York: Pergamon Press, 1990.
Lewington, A. Plants for People. New York: Oxford University Press, 1990.
Stephen R. Johnson