Exercise B Common Types of Wood

The beauty and strength of wood lies in the orientation of its cclls and tissues. The vessels, trachcids, and fibers are oriented along the long axis, w hile the rays of parenchyma arc oriented along the radius. Annual growth rings arc composed of alternating concentric layers of softer earlywood and denser late wood. When combined, all these cclls help make wood an immensely strong material for its weight. That is why wood is an excellent material for constructing buildings, making furniture, and carving into sculptures.

Woods vary in texture, color, growth pattern, smell, feel, and even taste. The appearance of a cut surface can be described .as rough, smooth, dull, lustrous,

Semi Ring Pourous Cross Section

Wood yellowish-white in cross section: Abies (fir)

Hoartwood purplish lo brownish-rod; wood fine-textured with strong aromatic odor. Thuja (cedar)

Hoartwood purplish lo brownish-rod; wood fine-textured with strong aromatic odor. Thuja (cedar)

Types Wood ParenchymaDifferent Type Cross Cut Saw

Large rays easily visible with unaldod oyo. broad and conspicuous, and very line rays Just visiblo with hand lens; wood hard; pale sapwood and cream lo doop brown hoartwood: Quorcus (oak)

Rays not visiblo without a lens; pores in latowood only slightly smaller than earlywood (semi-ring porous); wood very hard with hint of rod color: Carya (hickory)

Semi Ring Pourous Cross Section

or greasy. When milled, the orientation of the saw cut yields different grain patterns depending on the direction of the fibers and the three-dimensional configuration of the carlywood and late wood. We exploit these patterns when selecting wood for furniture. The orientation of the saw cur can also affect how a board responds to the forces of compression, tension, and shearing.

Each year a tree grows, it lays down more wood (secondary xylem). The vessel elements, tracheitis, and fibers arc dead during the first year they are produced. In contrast, the parenchyma cells in die xylem rays arc living. As the years pass, the parenchyma in the inner regions of the wood die. This inner core, the heartwood, and is no longer living tissue, yet it continues to give the tree support against gravity and wind. The wood on the outside, the sapwood, maintains the function of water conduction. As sapwood becomes heartwood, darkening often occurs. The dark color is due to deposition of various chemicals, including tannins, dyes, and oils. These chemicals come from the dead parenchyma cells of the rays. Often these parenchyma cells also form tyloses in the adjacent vessels. The change from sapwood to heartwood adds to the beauty of die wood. In some species, the sapwcx>d docs not transform into heartwood. Thus, wc can use the presence or absence of heartwood to distinguish types of wood.

In diis exercise, you will use your new knowledge of die cells and tissues of wcxxl to identify* different types of wood. You will be able to see most of the characteristics widiout any magnification. At most, you may need a hand lens or dissecting scope to see some of the characteristics.

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