Woodworker's Journal 2008-32-5, страница 39
smaller branches and twigs on which the leaves grow. The leaves absorb carbon dioxide from the air, give off oxygen and by photosynthesis enrich the sap with sugars which are passed down the inner bark and used to promote growth. (Italicized words appear in the Glossary on page 41, for ease of reference or, in some cases, expanded explanation.) Wood Formation Just below the bark is a microscopically thin layer of living cells called the cambium that sheathes the tree from ground to crown. The cambium cells grow and divide. One half of the cells make either wood or bark; the other half remain in the cambium to grow and divide again. New cells on the inside of the cambium become one of the woody elements. Cells on the outside become bark, which is divided into two layers. The inner bark carries the sugar-rich sap down from the leaves to feed the cambium and roots. The outer bark protects the fragile cambium from invasion by insects, fungi, animals and extremes of heat and cold. Variable Growth Despite the woodworker's ardent hopes to the contrary, trees do not exist to provide us with useable, straight-grained flat material — and many of its growing characteristics attest to this contrariness. For instance, due to a host of factors, including soil, weather and the proximity of other trees, trees do not grow at the same rate. Some trees grow in a spiral form. You can observe this in the surface cracks on a wooden utility pole. The rate of spiraling varies. Many large tropical trees will spiral in one direction for, say, six growth periods, then spiral in the opposite direction for several more. Laminating the trunk in this way must surely increase its resistance to stress, but it also poses problems for the woodworker when machining the converted board. If for some reason the felled tree shows asymmetric growth — the heart being closer to one edge rather than centered — it is usually discarded. Boards made from it are prone to sudden breaking, and it distorts beyond use as it dries. Examples of Variable Growth food materials are passed down the inner bark to feed the cambium. Sap: water and nutrients are taken up to the leaves by the sapwood. By a process called photosynthesis, leaves convert nutrients brought up by the sapwood into sugar-rich food, which is then passed down the tree by the inner bark. Bark Cambium Sapwood Heartwood Water and soil nutrients Differential growth rate: In the photo above, the bottom piece of oak grew one inch in four years; the top piece took 10 years to grow the same amount. One result is a very different grain pattern on the surface. The bottom piece will display lines wide apart; the upper piece close together. 10 Years Reaction Wood: compression/tension wood. In hardwoods, tension wood formed in the large area away from the off-center heart shrinks in length and machines leaving a wooly surface. In softwoods, this same area makes compression wood. It also shrinks considerably (10 - 20 times normal) and is dangerously brittle. Off-center Heart Woodworker's Journal October 2008 39 |
