• Importance

  • Botanical aspects
    CHAPTER 16 IN THE TEXT, 378 ff.

    Wood is the most widely used building material in the U.S. The forests that provide wood still cover about 20% of the earth's surface and represent about 90% of the total terrestrial biomass. By 2000, it is estimated that this will have shrunken considerably.
    Wood has many desirable properties. It is relatively inexpensive, available, is esthetically attractive, a poor conductor of noise, heat, and electrical current, easily worked, and endures for long periods. Wood is also used to manufacture many other substances, e.g., paper, cork, and rayon.
    Much wood is used as plywoods and veneers. Woods can be identified by keys. What is wood?

    Secondary xylem and phloem are produced by the vascular cambium. The accumulation of xylem in plants is called wood. These cells are impregnated with lignin and, as they mature, are dead and cease to function in conductance. See the diagrams on pages 383 and 384.

    The annual production of new xylem cells leads to the production of rings. The old phloem gets crushed as the plant grows. The region of the wood that is still active in conduction of water is called sapwood. The older non-functional wood is called heartwood. Heartwood is often darker because it is impregnated with other substances.
    The vascular cambium also makes other types of cells. Some of these are conducting parenchyma cells known as ray cells. Around these tissues is another cambium system called bark (See the diagram on pages 381 and 382).
    Part of the bark is dead phloem cells. Most is periderm, a tissue that replaces the epidermis as a protective layer. One part of the periderm is the phellogen, another meristomatic tissue that makes phellem or cork. The periderm must constantly change to accomodate growth of the trunk and branches. It repeatedly makes new phellogen and sloughs off the old, cracked layers of cork.

    Hardwood is derived from dicotyledonous angiosperms. Most dicotyledonous plants have vessels or short water conducting cells that conduct water through openings in the ends of the cells. See the picture on page 383.
    Hardwoods have tracheids, vessels, fibers, ray tracheids, and xylem parenchyma. The cell walls are about 70% cellulose and hemicellulose and about 20-35% lignin. Lignin is a polymeric phenylpropanoid material that helps to stiffen the plant.

    Softwood is derived from gymnosperms. The xylem of gymnosperms is composed mostly of tracheids. Gymnosperms do not have vessels. Resin canals are common in gymnosperms (p. 383 and 380).
    Softwoods are simpler than hardwoods; they tend to be more uniform. They have tracheids, resin canals, but no true fibers or vessels. This makes the wood apparently non-porous. Monocotyledonous angiosperms do not have secondary growth and hence do not make wood.

    Tree rings
    As mentioned above, these are produced by differential growth through the growing season. They are usually associated with rapid growth in spring or the wet season and slower growth later in the year.

    Characteristics of woods
    Woods differ in color, porosity, grain and figure. See the illustrations on page 384. Porosity is mainly a function of dicot woods. This term refers to how the vessels are distributed. Grain refers to the alignment of the xylem cells.
    Figure is determined by many factors: rays, porosity, grain, and arrangement of rings. The presence of knots or burls may also be involved. The figure also differs greatly depending on how the wood is cut. Woods also differ in density and a number of mechanical properties (see pg. 384).
    Density is the weight per unit size.
    Compared to water, woods that have a density greater than one will sink. Lignum vitae has a density of 1.23. Balsa has a density of 0.13. Pine has a density of 0.35-0.50. Oak is about 0.60. Mechanical properties are illustrated on page 384.
    How the lumber is cut determines what it looks like to a great extent. Three major cuts of wood are found: transverse or cross sections (quartersawn), longitudinal (plain sawn) or radial sections, and tangential sections. These sections are the most common.
    Durability is also important. This is more of a problem in the tropics.

    Fuel uses of wood
    One of the major uses of wood is as a fuel. It is burned directly or first converted to charcoal. Charcoal is produced by burning the wood under conditions of reduced oxygen. In some parts of the U.S., increased wood burning has lead to major pollution problems.
    In general, woods that are dense make the best firewood and charcoal. In the U.S., wood is used about 9% for fuel, about 30% in Europe, but about 90% in Latin America and Africa, about 75% in Asia.

    Wood is harvested and milled. The exact way varies. In general, however, the value of wood is such that almost none of the wood is wasted. See the diagram on page 388. The fresh wood is allowed to season or dry, often under conditions of controlled humidity. Air drying can be hastened in kilns.
    Drying time can be reduced from about a year to several days. Lumber is often treated with preservatives. Some wood is used directly as posts, poles, mine timbers etc.

    Veneers are thin sheets of wood of uniform thickness that are usually affixed to another surface, often another wood or a composition board. The thin sheets can be made by "peeling" a continuously revolving log or shaving veneer off in linear sheets. The rotary method yields much larger pieces and is most commonly used, especially on less expensive woods. Veneer varies from 1/10 inch thick upward, but most are about 1/10 inch thick.
    Slicing wastes much less than sawing. Part of the reason that veneers are used is because they are cheaper, but they are also often superior in terms of strength and durability. A former limitation of the uses of veneer was the quality of the adhesives used to affix the veneer to the other material. In recent years, this problem has largely been solved.
    Veneer is an ancient product. Furniture items made this way have been found in Egyptian tombs as old as 3,500 years.

    Plywood is a veneer with several sheets of wood glued together at right angles. The number of plies is odd, i.e., 3-ply, 5-ply, etc. The outer layers are usually a better quality than the inner layers.
    Fiberboard and particle board Fiberboard is made from wood fibers rather than small pieces of wood. The fibers are xylem elements that have been separated from each other by chemical solutions that dissolve out the pectins. The fibers are then mixed together with resins to give strength and bind things together. Other materials can be added for insect resistance and additional desired properties.
    Some important tropical hardwoods:
    Purpleheart (Peltogyne paniculata, Fabaceae) and rosewood (Dalbergia nigra, Fabaceae) are from Brazil.
    Satinwood (Zanthoxylum flavum, Rutaceae) is from the West Indies.
    Primavera (Cybistax donnell-smithii, Bignoniaceae) is from Mexico.
    Macassar ebony (Diospyros ebanum, Ebenaceae) is from Asia.
    Teak (Tectona grandis,Verbenaceae) is from S.E. Asia, but is widely cultivated.
    Spanish cedar (Cedrela odorata, Meliaceae).
    Mahogany (Swietenia mahogani and S. macrophylla, Meliaceae).
    Circassian walnut or English walnut (Juglans regia, Juglandaceae).
    Sycamore (Platanus occidentalis, Platanaceae) called lacewood when quarter sawed.

    Rayon, cellophane, and acetate
    Rayon and cellophane are the same product in different form. Both are made from cellulose from wood. Chips of wood are digested with caustic soda. The resultant mass is allowed to oxidize in air and carbon disulfide added. This produces cellulose xanthate. More caustic soda is added.
    The mixture becomes viscous and is then exuded as fine sheets or as fibers. These are drawn through an acid bath to get rid of the base. Rayon makes a fiber that is a lot like cotton. It accepts dyes well.

    Acetates are made from wood or cotton fibers and acetic acid, acetic anhydride and a catalytic agent. The acetate is then chilled in water which causes flakes of the material to precipitate. These are dissolved in acetone. They may be dyed and the dyes are very permanent.
    The triacetates are made by dissolving triacetate in alcohol and methylene chloride and spinning it. Acetate fibers dry rapidly, are soft, resist wrinkling, and are not attacked by moths and molds.

    Cork is not wood, but part of the phellogen derived material. Cork comes from Quercus suber (Fagaceae) and is native to the area around the Mediterranean Sea. Cork is an excellent insulating material because it has numerous air-filled cells.
    Cork is lightweight and floats well. The bark can be stripped from the trees when they are about 25 years old. They can be stripped each 9-10 years after that.

    Bamboo is not wood because it doesn't contain any secondary growth. In much of the tropical third world bamboo replaces wood. This name applies to a number of species of the subfamily Bambusoideae, of the Poaceae. New growth is produced only by apical meristems. This material is both light weight and strong.
    Even scaffolding for very tall buildings is made from bamboo.

    Future supplies of wood
    Previously wood was largely wild-harvested. Today, in Europe, North America and Japan, most of the wood harvested is planted and managed as most other cultivated plants. Consumption of forestry products is high and currently increasing.
    In most of the rest of the world that still has forests (for example, Amazonian Brazil, S.E. Asia and much of west Africa, the forests are being wild harvested and we do not even know how to reforest them. Widescale deforestation has very serious economic and ecological results.
    In some cases, e.g., for paper manufacture, herbaceous plants can replace trees. Kenaf (Hibiscus cannabinus, Malvaceae) seems to be quite promising and one acre can replace 5 acres of pine trees.

    Major forests of the world
    Softwoods: Conifers are the most important gymnosperms. These trees, which generally favor cool climates, make up more than 60% of the forest lands in the U.S. Pines, spruces, firs, Douglas fir, larches, hemlock, kauri-agathis, araucarias, cedars, junipers, and cypress are the most important. Conifers often grow in pure stands.
    Hardwoods: These trees are usually much more diverse and do not occur in pure stands. They may be divided into two types: temperate and tropical. Many are deciduous. More than one half the world's remaining forests are tropical hardwood types. These are rarely managed. Slash and burn agriculture is important.