113. Structure.—For convenience, tree structure is usually studied (1) in transverse or cross section, (2) radially, (3) tangentially.
A transverse section is obtained by cutting a log at right angles to its length; a radial section by cutting it along the radius; a tangential section by making a cut at right angles to a radius.
If we should cut transversely a young tree, a sprout, or branch of an oak or similar tree, we should find it composed of three layers of tissue (1) pith or medulla, (2) wood, (3) bark, Fig. 194. These tissues, if magnified, would be found composed of little closed tubes or cells. Fig. 195. Examine the end of a log cut from a tree such as the oak; we shall find that the center, which in the young soft, has become hard and dry, and that upon it are marked a series of concentric rings—rings having a common center. These rings are known as annual rings because one is added each year.
Usually, about three-quarters o£ the rings from the center outward will be found to have a different color from the remaining ones.
These inner rings form what is called heartwood. The wood of the remaining rings will be found softer and to contain a larger proportion of sap. This part is called sapwood. Young trees are composed mainly of sapwood.
As the tree grows older more of it is changed to heartwood, the heartwood becoming greater in proportion to the sapwood with age.
Upon examining these rings each will be found to be made up of two layers; one a light, soft, open, rapid growth formed in the spring, the other, a dark, hard, close, slow growth formed in the summer.
Frequently, the center of the annual rings is not in the center of the log. Fig. 196. This is due to the action of the sun in attracting more nourishment to one side than to the other.
Surrounding the sapwood is the bark. The inner part of the bark is called bast and is of a stringy or fibrous nature. Bark is largely dead matter formed from bast, Fig. 195. Its function is to protect the living tissues.
Between the bast and the last ring of the woody tissue is a thin layer called the cambium. This layer is the living and growing part of the tree. Its cells multiply by division and form new wood cells on the inside and new bast cells on the outside.
Heartwood is dead so far as any change in its cells is concerned. Its purpose is merely to stiffen and support the weight of the tree.
Sapwood, on the other hand, has many active cells which assist in the life processes of the tree, tho only in the outer layer of cells the cambium, does the actual growing or increasing process take place.
Again examining the end of the log, we shall find bright lines radiating from the center. They are composed of the same substance as the pith or medulla and are called pith or medullary rays. These rays are present in all trees which grow by adding ring upon ring but in some they are hardly visible. The purpose of these horizontal cells is to bind the vertical cells together and to assist in distributing and storing up plant food.
Fig. 197 shows a log cut longitudinally or lengthwise. The lines we call grain, it will be seen, are the edges of the annual rings, the light streaks being an edge view of the spring layer and the dark streaks an edge view of the summer or autumn wood.
Knots are formed by the massing or knotting of the fibers of the tree through the growth of a branch. Fig. 198 shows the manner in which the fibers are turned. This packing of the fibers is what causes a knot to be so much harder than the rest of the wood.
114. Growth.—Sap is the life blood of the tree. In the winter when most of the trees are bare of leaves there is but very little circulation of the sap. The coming of spring, with its increase of heat and light, causes the tree to begin to take on new life; that is, the sap begins to circulate. This movement of sap causes the roots to absorb from the soil certain elements such as hydrogen, oxygen, nitrogen and carbon, also mineral salts in solution. The liquid thus absorbed works its way upward, mainly by way of the sap-wood and medullary cells. Upon reaching the cambium layer, the nourishment which it provides causes the cells to expand, divide and generate new cells. It also causes the buds to take the form of leaves.
When the sap reaches the leaves a chemical change takes place. This change takes place only in the presence of heat and light, and is caused by the action of a substance called chlorophyll. The importance of the work performed by chlorophyll cannot be overestimated. Nearly all plant life depends upon it to change mineral substance into food. Animals find food in plant life because of this change.
Assimilation is the process of taking up and breaking up, by the leaves, of carbonic acid gas with which the cells containing chlorophyll come in contact. Carbon, one of the elements, is retained, but oxygen, the other element, is returned to the air.
Carbon is combined with the oxygen and hydrogen of the water, which came up from the roots, to form new chemical compounds. Nitrogen and earthy parts, which came with the water, are also present.
Chlorophyll gives to leaves and young bark their green color.
The roots of the trees are constantly drinking plant food in the daytime of spring and early summer. From midsummer until the end of summer the amount of moisture taken in is very small so that the flow of sap almost ceases.
The leaves, however, are full of sap which, not being further thinned by the upward flow, becomes thickened thru the addition of carbonic acid gas and the loss of oxygen.
Toward the end of summer this thickened sap sinks to the under side of the leaf and gradually flows out of the leaf and down thru the bast of the branch and trunk, where another process of digestion takes place. One part of this descending sap which has been partly digested in the leaves and partly in the living tissues of root, trunk and branch, spreads over the wood formed in the spring and forms the summer wood. The second part is changed to bark. What is not used at once is stored until needed.
The leaves upon losing their sap change color, wither and drop off. By the end of autumn the downward flow of changed sap from the leaves is completed and the tree has prepared itself for the coming winter.
It must be remembered that the foregoing changes are made gradually. After the first movement of the sap in the early spring has nourished the buds into leaves of a size sufficient to perform work, there begins a downward movement of food materials—slight at first, to be sure, but ever increasing in volume until the leaves are doing full duty. We may say, therefore, that the upward movement of the sap thru the sapwood and the downward flow of food materials thru the bast take place at the same time, their changes being of relative volumes rather than of time.
115. Respiration and Transpiration.—Plants, like animals, breathe; like animals they breathe in oxygen and breathe out carbonic acid gas. Respiration, which is but another name for breathing, goes on day and night, but is far less active than assimilation, which takes place only during the day. Consequently more carbonic acid gas is taken in than is given out except at night when, to a slight extent, the reverse takes place, small quantities of carbonic acid gas being given off and oxygen taken in.
Very small openings in the bark, called lenticels, furnish breathing places. Oxygen is also taken in thru the leaves.
Transpiration is the evaporation of water from all parts of the tree above ground, principally from the leaves.
The amount of water absorbed by the roots is greatly in excess of what is needed. That fresh supplies of earthy matter may reach the leaves, the excess of water must be got rid of. In trees with very thick bark, transpiration takes place thru the lenticels in the bottom of the deep cracks.
116.Moisture.—Water is present in all wood. It may be found (1) in the cavities of the lifeless cells, fibers and vessels; (2) in the cell walls; and (3) in the living cells of which it forms over ninety per cent. Sapwood contains more water than heartwood.
Water-filled wood lacks the strength of wood from which the greater part of the moisture has been expelled by evaporation.
117.Shrinkage.—Water in the cell walls—it makes no difference whether the cells are filled or empty—causes their enlargement and consequently an increase in the volume of the block or plank.
The removal of this water by evaporation causes the walls to shrink; the plank becomes smaller and lighter. Thick walled cells shrink more than thin ones and summer wood more than spring wood. Cell walls do not shrink lengthwise and since the length of a cell is often a hundred or more times as great as its diameter the small shrinkage in the thickness of the cell walls at A and B, in Fig. 199, is not sufficient to make any noticeable change in the length of the timber.
Since the cells of the pith or medullary rays extend at right angles to the main body, Fig. 200, their smaller shrinkage along the radius c: the log opposes the shrinkage of the longitudinal fibcr. This is one reason why a log shrinks more circumferentially, that is along the rings, than it does radially or along the radii. A second cause lies in the fact that greatly shrinking bands of summer wood are interrupted, along the radii, by as many bands of slower shrinking spring wood, while they are continuous along the rings.
This tendency of the log to shrink more tangentially or along the rings than along the radii leads to permanent checks. Fig. 201A. It causes logs sawed into boards to take forms as shown in Fig. 201B.
Warping is caused by uneven shrinkage. Sapwood, as a rule, shrinks more than heartwood of the same weight. The wood of pine, spruce, cypress, etc., shrinks less than the wood 01 trees such as the oak.
118. Weight.—Wood substance is 1.6 times as heavy as water; it matters not whether it be wood of oak, pine or poplar. Wood placed in water floats because of the air enclosed in its cells; when the cells become filled with water it sinks.
The weight of any given piece of wood is determined (1) by the wood substance—this is always the same, (2) by the amount of water enclosed in its cells—this varies. Some kinds of woods are heavier than others similarly seasoned because they contain more wood substance in a given volume.
Weight of wood is an important quality. To a large extent, strength is measured by weight; a heavy piece of oak will be stronger than a light one of the same species.
Lightness, strength and stiffness are properties which recommend wood for different uses.
119. Other Properties.—Strength, elasticity, hardness, toughness and cleavability, as applied to timber, have their usual meanings.
120. Grain.—Wood fibers generally extend parallel to the axis of the trunk or branch which they form. In this case the wood is said to be straight grained.
Frequently the fibers grow around the tree as in Fig. 202, or several layers may grow obliquely in one direction and the next series grow obliquely in the opposite direction, Fig. 203. Boards cut from such trees will be cross grained or twisted.
The surface of the wood under the bark is seldom smooth. Usually these hollows are filled even by the addition of one or two new rings of growth. However, in some woods as maple, the unevennesses are maintained, the high places being added to as are the low. Fig. 204. Dormant buds frequently cause small cone-shaped elevations, which tho covered with successive layers of new wood, retain their shape. Cross sections of these cones will appear in the sawed boards as irregular circles with a dark speck in the center, known as "birds' eyes."