2. All wood used in the United States for building construction and finishing comes from what are known as exogenous trees, or those which increase in size by the formation of new wood each year on its outer surface.
The wood of these trees is made up of bundles of long tubes, cells or fibres, with their long axis generally parallel to the stem of the tree. Crossing these fibres in a radial direction from the pith to the bark are other fibres which are known as pith fibres or medullary rays, and which serve the purpose of binding the whole together. Besides these wood fibres there are resin ducts scattered through the wood of the pines and spruces, and in the wood of the broad-leafed trees hollow ducts or vessels.
The various fibres and vessels above mentioned differ in both their chape and disposition in different kinds of trees, and consequently give rise to differences in the structure of the wood, and it is often only by a microscopical examination of the structure that the different varieties of the same kind of woods can be determined.
The structure of the wood determines to a large extent its appearance when finished, and also has a marked influence upon its physical and mechanical properties.
Fig. 1 shows a bundle of wood fibres, a b, highly magnified, with the pith ormedullary rays, c d, running at right angles to them.
Fig. 2 shows a block of oak, not magnified, in which the medullary rays are very prominent. These medullary rays occur in all woods, but in the soft woods they are not generally noticed. They give the peculiar silver-mottled effect seen in quarter-sawed oak, and add much to the appearance of most of the hard woods. They also form a large part of the wood of all trees - in pine over 15,000 of these pith rays occur on a square inch of tangential section, and even in oak the very large rays, which are readily visible to the eye, represent scarcely a hundredth part of the number which the microscope reveals.
Besides affecting the appearance of the wood the medullary rays also greatly affect the shrinkage and checking of the wood in seasoning, and have much to do with the strength of the wood.
The process of growth of exogenous trees in a temperate climate is as follows:
"In the spring the roots absorb juices from the soil, which are converted into sap and ascend through the cellular tubes to form the leaves. At the upper surface of the leaves the sap gives off moisture, absorbs carbon from the air and becomes denser; after the leaves are full grown vegetation is suspended until autumn, when the sap in its altered state descends chiefly between the wood and the bark, where it deposits a layer of new wood (the annual ring for that year), a portion at the same time being absorbed by the bark. The new wood thus formed covers all parts of the stem and branches."
As the tree increases with age the inner layers become choked or filled with the secretionary substance peculiar to the tree and fall out of use, except as they serve the mechanical function of keeping the tree from breaking under its own weight or from the force of the wind. This process of growth, therefore, produces two kinds of wood in the same tree, viz., the sapwood and the heartwood.
Practically speaking, the sapwood of trees is that portion of the wood where the cells are open to the upward passage of the sap. It varies in width and in the number of rings which it contains, even in different parts of the same tree. It also varies considerably in different kinds of trees; it is small in the hard woods and in the long leaf and white pines, and great in loblolly and Norway pines. The sapwood possesses but little strength and is subject to rapid decay, owing to the great quantity of fermentable matter contained in it.