This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
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 pine 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 A A are the fibers parallel to the trunk of the tree and BB are the cross fibers. It will be noticed in this figure that the more numerous the cross fibers, the more thoroughly the wood will be tied together, 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 timber are directly dependent on the structure of the wood.