PLANTING AND CARE OF TREES 43 



ber of years. The parts of a twig as furnishing a means of iden- 

 tification is further discussed in Part II. 



,A general idea of the internal structure of an older stem may 

 be obtained by the study of a block of Oak wood (fig. 22). Aside 

 from the relatively narrow pith in the center, the trunk of a tree is 

 made up of three general regions : — the protective covering of 

 harh outside, the thick cylinder of wood within and the thin sheet 

 of delicate tissue forming the camhium between these two regions. 

 The cambium layer is of special interest since it is the only part 

 that remains alive throughout the growth of the trees. In making 

 a whistle, the boy finds in this layer an easy place of separation 

 of wood from the bark. Each year its outer surface forms a new 

 layer of the bark and its inner surface a new layer of wood. By 

 adding an annual ring of wood the cambium itself is pushed out 

 by just so much each season away from the center of the tree but 

 always remains a narrow layer. The bark, as well as the wood, is 

 thus increased in thickness each year, but unlike the wood, it is 

 subjected to the pressure of growth from within and in most spe- 

 cies eventually dies, cracks and wears away on the outside as rap- 

 idly as it is renewed on its inner surface. 



The annual rings usually give an accurate record of the age and 

 rapidity of growth of a tree, since defoliation by insect injury or 

 other influences that would cause a double period of growth in a 

 single year are of rare occurrence. Trees like the Oak and Ash 

 produce in the early part of the growing season large tubular cells 

 which extend vertically up and down the trunk and later form cells 

 of smaller dimensions. In such trees, therefore, the porous spring 

 wood can be readily distinguished from the denser summer ivood. 



At the end of a log a cr-oss section of the trunk is exposed, and 

 on it can be seen fine lines radiating out from the center. These 

 are cut edges of the medullary rays or pith rays, as these narrow 

 sheets of tissue are sometimes called. A section cut lengthwise of 

 the log and parallel to the annual rings is called a tangential sec- 

 tion and shows the extent up and down of the medullary rays as 

 seen in end view. The radial section is made lengthwise of the 

 log and parallel to the medullary rays. If extended, it would pass 

 through the center of the tree. This is the direction in which 

 lumber is sawed in making "quartered oak." The shiny streaks 

 ■ ^ Oak wood cut in this fashion are called the "silver" and are in 



