Paragraph 51. While it is impossible to understand just how, and why, the tree plant produces a large and strong body, yet by careful study men have been able to find out much about how a tree grows. We cannot at this time make a full explanation of the tree and its manner of growing. The references regarding trees will help you to an understanding of these matters. We are concerned now in thinking of trees from the standpoint of lumber. In order that we may thoroughly understand the nature of lumber, we must remember that the tree grows by adding a layer of woody fiber to its trunk or stem every year. A tree grows very rapidly during the early spring, and continues its growth throughout the summer while the leaves are on. When the leaves have fallen off it indicates that the tree has practically ceased its growth for the season, and through the cold weather it does not add to its woody fiber as it does through the spring and summer. This will help you to understand how it is that a tree produces a ring of growth around its body every year. When the tree is cut down the end of the log shows a great number of rings, each ring representing the growth of one year. By counting these rings one can determine the approximate age of the tree.
It is important that we learn something about these rings because they have much to do with the nature of wood. When a log is sawed into boards these rings form the figure which we often speak of as the grain. In quarter sawed material there is another very beautiful kind of figure in the grain, which is caused by the rows of cells known as medullary rays. They extend from the heart to the bark of the tree.