The wood nearest to the center of the tree where the pith is located is considerably harder and denser, as well as darker in color, than that which is on the outside nearer the bark. This wood is called heartwood to distinguish it from the other and softer wood which is called sapwood. The reason why the heartwood is harder and denser than the sapwood is that it is older and has been com pressed more and more each year as the tree has increased in size, so that the pores have gradually become filled up. The sapwood is soft and of a lighter color than the heartwood showing that it has heen more recently formed. The time required to transform the wood from sapwood to heartwood varies from nine to thirty-five years, according to the nature of the tree, and those trees which perform this hardening in the shortest time usually yield the most durable timber. It is not certainly known whether the change from sapwood to heartwood takes place ring by ring and year by year or whether sections of the trunk consisting of a number of rings change at the same time, but it is probable that the latter process is what really takes place, indeed there seems to be evidence to show that not even the whole of each ring changes at one time, but that part of a ring may remain sapwood after the remainder has become heartwood.
In addition to the annual rings, there are to be seen on the cross section of any log other lines which run from the center toward the bark at right angles to the annual rings. These are called medullary rays. Usually they do not extend to the bark, but alternate with others which start at the bark and run in toward the center but are lost before they reach the pith. This is shown at E and F in the figure. The medullary rays are much more pronounced and the structure of the wood is much more complicated in the broad-leaved trees than in the conifers, the structure of which is comparatively simple with most of the fibers running up and down in the direction of the growth of the tree. Thus, the wood of the pines and other conifers splits very much more easily than that of the oaks, chestnuts, and other broad-leaved trees.
rays are sometimes called pith rays and are caused by fibers or of fibers which run at right angles to the others. It is the pith rays which appear as smooth, shiny spots or blotches in woods which have.been quarter sawed. This will be explained later when dealing with the conversion of timber from its natural state into planks and other shapes ready for the market.
Details of Wood Structure. If a piece of wood were to be examined carefully under a microscope it would be seen that it was a composite substance, made up of a great number of very small fibers, and that these fibers were not solid but were so many little tubes or cells arranged together in a more or less complicated manner according to the kind of wood. Thus a piece cut from one of the needle-leaved trees would be seen to be much more simple and regular in arrangement than a piece cut from one of the broad-leaved trees.
Both kinds of wood are composed of bundles of these fibers or tubes running parallel to the stem of the tree which are crossed by other fibers running at right angles to the first ones and binding the whole together. The cross fibers are much more numerous in the wood of the broad-leaved trees than in that of the conifers, and it is these fibers which appear on the cross section of a log as pith rays. There are also to be seen through the microscope a few resin ducts and other special fibers scattered through the wood. It is said that in pike more than 15,000 fibers occur on a square inch of section so that each one is very small and they can not be distinguished without the aid of a powerful microscope. The general arrangement is shown in Fig. 2, in which AA are the fibers parallel to the trunk of the tree and BB are the cross fibers. It will he noticed in this figure that the more numerous the cross fibers, the more thoroughly the wood will be tied to gether, and the harder and tougher it will be; also that it will split much more readily if there are a few cross fibers than it will if there are many.
Thus the most important characteristics of tim ber are directly dependent on the structure of the wood.
Grain. The arrangement of the fibers which go to make up a piece of timber give to it certain characteristics which are described as different conditions of the "grain" of the wood, the word "grain" being used as a substitute for the word fiber. Thus "across the grain," means at right angles to the general direction of the fibers; "along the grain," means parallel to the direction of the fibers. In like manner woods are said to be "fine grained," "coarse grained," "cross grained," or "straight grained," these terms being used to indi cate the relation of the fibers to each other and to the general direction of the growth of the tree. The wood is said to be fine grained when the annual rings are relatively narrow so as to show a large number of fine lines on a cross section of the log, and it is said to be coarse grained when the rings are wider so as to show a smaller number of coarser lines on the cross section of the log. Woods which are fine grained are generally harder and denser than those which are coarse grained and they can he made to take a high polish, while with the others, as a rule, this is not possible. Fine-grained woods are also said to be close grained. When the fibers are straight and parallel to the direction of the trunk of the tree, the wood is said to be straight grained, but if they are twisted so as to be spiral in form, not growing straight but following around the trunk of the tree, the wood is said to be cross grained. In Fig. 3, are shown three pieces of timber of which A is absolutely cross grained, B is Partially cross grained, and C is straight grained. As examples, it may be mentioned that hemlock is coarse grained and usually cross grained, while white pine is close grained, although soft, and is usually straight grained.