18 FARM FORESTRY 



The outer layers of a tree thus form a shell of living tissue 

 over the dead inner portions of the roots, stem and branches. 

 Often trees are seen growing vigorously with a hollow trunk, 

 showing that the central portion of the tree has nothing to do 

 with the life of the tree. 



DIAMETER GROWTH IN TREES 



Annual Rings. — A careful examination of the bark, sap- 

 wood and heartwood will show each to be made up of layers 

 or concentric rings, one of which is produced each year. It is 

 possible to tell the age of a tree by counting the rings in the 

 wood. During the spring and early summer the cells in the 

 cambium layer divide and produce new cells. Wood cells are 

 produced on the inside of the cambium layer and bark cells 

 on the outside. The amount of bark produced is usually much 

 less than the amount of wood tissue. Over the whole tree — 

 roots, trunk and branches — is thus added each year a new 

 layer of wood and bark. 



Spring and Summer Wood. — If an annual ring of oak 

 wood is examined it will be found to be made up of two layers, 

 a soft, open, lighter colored layer, the spring wood, and a dense, 

 hard, darker colored layer, the summer wood. In the spring 

 of the year a large amount of water is needed by most trees 

 to produce the new leaves and to supply the growing parts of 

 the tree. As a result the cells produced in the early part of 

 the growing season are large with thin walls. As soon as the 

 leaves have developed, a much less amount of water is needed 

 and the cells produced later in the season are smaller and 

 have thicker walls. This gives rise to what are known as 

 spring and summer wood. This difference in the texture of 

 the spring and summer wood produces the grain in wood and 

 separates one year's growth from another. With tropical trees 

 this difference in the spring and summer wood does not exist. 

 There is no rest period in the growth of trees similar to our 

 winter, so there is continuous growth in the wood and no an- 



