When the West was being opened, pioneers cut down great stands of virgin timber without thinking or possibly without knowing the damage they were doing. Lumbermen later contributed heavily to this destruction. It soon became evident that the tree had an important function other than being a source of lumber.
Clearing out whole forests without replanting or leaving some trees standing lef t the ground exposed to the elements, and soil erosion and floods resulted. Now that we realize the importance of conserving our source of lumber and of protecting our soil against erosion, this damage has been greatly slowed.
The tree and its food
A tree has three main parts: the roots, the trunk, and the crown. The roots absorb water and foods in solution from the ground. The water and food pass up through the trunk of the tree, out into the branches, and to the leaves. The leaves convert the food into a form that can be used by the tree to continue its life and growth.
This entire process is similar to that which takes place in the human body. Here food is taken in by the mouth, it passes into the stomach, and then to other organs where it is converted into a form that can be assimilated. The blood passes through these organs and to the heart, from which it goes to the various parts of the body through small tubes known as arteries. The arteries do work similar to that of the inner bark of a tree.
When the water and food have been assimilated by the tree, the waste passes off into the atmosphere as oxygen and water vapor. Evaporation from a large tree may be as much as 250 gallons per day. To evaporate this amount of water, the tree must take out of the ground more than 250 gallons.
Thus a large stand of trees can absorb a tremendous amount of water. Surface water, which is the result of spring thaws and rain, causes floods. The growing tree helps to control this surplus water. As will be seen later, the structure of the tree is such that the more water there is available, the more it can use. When the water content of the ground is reduced, there is a corresponding reduction in the absorption by the tree; this shows up in the cell structure.
Structure of the tree
In order to work wood properly, to pre-vent undue waste, and to produce articles of lasting quality, you should learn something of the tree's structure. As you work with wood you will soon discover that certain parts of the wood can be worked in one manner, but other parts must be treated differently. You will find an example of this when you take up squaring and another when you study finishing processes.
Trees belong to two great groups of plants angiosperms and gymnosperms. The angiosperm trees are known as deciduous trees or hardwoods. They lose their leaves during the fall and winter, then burst into foliage in the spring. The gymnosperm trees are commonly known as conifers or softwoods. They are often spoken of as evergreens.
The terms hardwoods and softwoods are commonly used in industry; but they are likely to be misleading and confusing to the beginner because they are not descriptive of relative hardness. Basswood, poplar, aspen, and cottonwood are deciduous trees, shedding their leaves in the fall; therefore they come under the general grouping of hardwood.
But actually they are among the softest of woods. On the other hand, longleaf pine is as hard as the average hardwood; but since it is of the evergreen or conifer group it is known as softwood. While it would be better to use the proper botanical names gymnosperm and angiosperm, or the simpler terms conifer and deciduous, the terms softwood and hardwood have been in use for so many years that they have become part of the woodworker's vocabulary.
Cells of a tree
As the tree grows the cells increase in number, adding more wood to the cross section or diameter (as well as to the height) of the tree. This growth from season to season produces what are known as annual rings. They can be seen at the end of a piece of wood or on the end of a log, where they appear as concentric circles. It takes one year to pro-duce a ring; therefore, by counting the number of rings at the end of the log, the age of the tree from which the log was cut can be determined.
These so-called rings are not lines, but small cells and fibers produced during the summer when the growth of the tree slackens. The greatest growth of the tree during any one season takes place in the spring. At this time the cells and fibers grow to a relatively large size. As the season progresses the speed of the growth slackens, with the result that the size of the cells is reduced. It is this difference in size between the large spring cells and the small summer cells that produces the "lines" that form the annual rings. Annual rings are also visible on the face and edge of a piece of wood. On these surfaces they appear as straight or curved lines.
Both hardwoods and softwoods have horizontal fibers that extend across the vertical fibers; these are known as medullary rays. The medullary rays radiate from the center of the tree and appear as radial lines. Their function is to store the food material of the tree and to distribute it horizontally.
When a log is sawed so as to expose the medullary rays, this surface is commonly called edge grain when applied to softwoods and quartersawed when it refers to hardwoods. In some hardwoods the surface produced by quartersawing is covered with large " flakes" formed by the medullary rays leaving and entering the surface. This is most notable in quartersawed oak and is valued in the manufacture of furniture because of its attractive pattern.
Heartwood and sapwood
As the tree grows and increases in diameter, the cells at the center no longer conduct sap through the tree. In this change, the living cells die; they become infiltrated with various substances, some of which darken the wood. With some species these infiltrations are known to add appreciably to the weight of the wood. The portion of the tree that undergoes this change is known as heartwood.
The light-colored sapwood is next to the heartwood. It contains the living cells and fibers that function as conductors of food and water from the roots to the leaves. In its green state, sapwood contains more moisture than heartwood; its weight per cubic foot is therefore higher. But when the wood is seasoned this moisture is removed from the sapwood, bringing the weight of the sapwood down to that of the heartwood.
While it is of ten said that heartwood is stronger than sapwood, this is not always true. In thousands of tests made on various woods grown in the United States there has been found in most species no effect on the properties of the wood due to this change. Such trees as the redwood and the black locust have been found to produce stronger heartwood than sapwood, but these are the exception. In some respects heartwood is superior to sapwood.
Heartwood is less subject to attack by stain, by mold-producing fungi, and by certain insects. Because of the dark color of some heartwood, it is considered more ornamental for some uses. Because of its light color, sapwood is used for such things as floors and furniture with a bleached finish.
Between the sapwood and the inner bark is a layer of cellular tissue known as the cambium. This layer is made up of two parts; one always remains the cambium, and the other forms the new cells and fibers. It is due to the constant developing of this tissue that the tree is capable of increasing in size.
Outside the cambium layer, and covering it, is the bark. This thick outside covering of the tree is its protective coat. The bark has two parts, the inner bark or bast, and the outer bark or cortex.
In some trees the bast is the thin hairy material on the inner surface of the bark; it is sometimes stripped off and used in the manufacture of rope and cloth. The cortex is also important commercially; cork, tannic acid, and certain medicines are obtained from the cortex of some trees.