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 outside. The soft interior of the stem sometimes 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.

Fig. 1. Log Section of Conifer Showing Age Rings

Fig. 1. Log Section of Conifer Showing Age Rings.

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 compressed 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 been 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.

Medullary rays are sometimes called pith rays and are caused by fibers or bundles 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.