CLASSES OF TREES There are in general four kinds of trees from which timber suitable for structural purposes may be obtained, which differ from each other in their manner of growth and in the details of their struc ture, as well as in their adaptability to building work, but of these only two, the so-called broad-leaved trees and the needle-leaved trees, yield timber used in any great quantity for building. The other two are suitable for structural work but for one reason or another have not been extensively utilized as yet except in the immediate neighborhood of the places where they grow. This is especially true of the bamboos, which grow in abundance in China and the Philippine Islands and are there used extensively for building purposes, but which have never as yet been introduced into other countries, although the wood has certain characteristics which might make it very suitable for use in some locations, and the tree could probably be made to grow in any warm climate such as that of the southern states. There is another class of tree of which the palms are the most well-known representatives, but the use of the lumber cut from these trees is very limited.
Manner of Growth. There is a marked difference between the four classes of trees mentioned above in regard to their manner of growth. The palms and bamboos are somewhat similar and are known as endogenous trees, differing from the broad-leaved trees and the conifers which are known as exogenous trees. The endogens, to which family also belong cornstalks and certain kinds of grasses, increase from the inside and do not usually have a covering of bark. The wood is soft in the center of the trunk and becomes hard toward the side. The soft interior of the stem times is found to be missing entirely, leaving a hollow sort of tube, but this is true of the bamboos only, the palms being solid. The wood of these trees is composed of a multitude of cells or pockets like that of the exogenous trees, but the end of a log which has been cut does not show the rings which we see at the end of a log cut from a broad-leaved tree or a conifer, Fig. 1. Instead we see a series of dots of a darker color than the general surface, the difference being due to the different ways in which the two kinds of trees grow.
The exogenous trees, to which class belong the broad-leaved trees and the conifers, increase from year to year both in height and in size of trunk. The increase in height and in the length of
the branches is the result of a sort of extension process which takes place at the ends of all the small offshoots as well as at the extreme end of the main trunk of the tree. A bud is first formed at each of these places and speedily develops into a small twig, at first quite soft and with a covering of thin skin. In the course of time the skin gets harder and darker in color and the woody tissue inside gets firmer, while the extension process continues to take place at the end. Thus the branch or trunk of the tree becomes each year a little longer but any particular point on the branch remains in the same position with relation to the ground or to the parent trunk or branch. While the lengthening process is going on, another and a different kind of growth is taking place. The fluid known as sap is continually passing up and down between the roots of the tree and the leaves, and each year a new layer of wood is formed on the outside of the trunk and branches underneath the bark. Thus a cross section of the trunk of an exogenous tree presents a series of rings beginning at the center, where there is a small, whitish substance called pith, and extending to the outside where there is a covering of bark. In Fig. 1, A is the pith, B is the woody part of the tree, and C is the bark. The arrangement of the wood in concentric rings is due to the fact that it was formed gradually, one layer being added each year, and for this reason the rings or layers are called annual rings. It is interesting to note that the age of the tree may usually be deterMined with a fair degree of accuracy by counting the number of layers which appear on the cross section. The width of the annual rings varies from one-fiftieth of an inch to one-eighth of an inch according to the character of the tree and the position of the ring with relation to the center. In general, it may be said that the widest rings are to be found nearest the center or pith and that they grow regularly narrower as they approach the outside or bark. They are also wider at the bottom of the tree than at the top. The rings are very seldom circular or regular in form, but follow the contour of the tree trunk.