Examine the end of a log cut from a tree such as the oak; we shall find that the center, which in the young tree was soft, has become hard and dry, and that upon it are marked a series of concentric rings—rings having a common center. These rings are known as annual rings because one is added each year.
Usually, about three-quarters of the rings from the center outward will be found to have a different color from the remaining ones. These inner rings form what is called heartwood. The wood of the remaining rings will be found softer and to contain a larger proportion of sap. This part is called sapwood. Young trees are composed mainly of sapwood. As the tree grows older more of it is changed to heartwood, the heartwood becoming greater in proportion to the sapwood with age.
Fig. 195.
Upon examining these rings each will be found to be made up of two layers; one a light, soft, open, rapid growth formed in the spring, the other, a dark, hard, close, slow growth formed in the summer.
Fig. 196.
Frequently, the center of the annual rings is not in the center of the log. [Fig. 196]. This is due to the action of the sun in attracting more nourishment to one side than to the other.
Surrounding the sapwood is the bark. The inner part of the bark is called bast and is of a stringy or fibrous nature. Bark is largely dead matter formed from bast, [Fig. 195]. Its function is to protect the living tissues.
Between the bast and the last ring of the woody tissue is a thin layer called the cambium. This layer is the living and growing part of the tree. Its cells multiply by division and form new wood cells on the inside and new bast cells on the outside.