OUR FORESTS 



The annual rings as a rule may be clearly seen on a cross-section of 

 the tree trunk. In the center is the pith. Around the pith is the 

 ring formed the first year ; around the first year's growth is the ring 

 formed the second year, and so on. The wood nearest the bark is 

 often lighter than that in the center. This lighter wood is known as 

 the sapwood, because it is the living wood through which the water 

 taken up by the roots passes on its way to the 'crown. After it has 

 served for a number of years carrying on these life processes, the sap- 

 wood gradually changes to heartwood. Through infiltration of 

 chemical substance and certain changes in the character of the cell 

 walls, the wood becomes darker in color and wholly lifeless. The one 



Outer bark or corky layer is composed 

 of dry dead tissue. General protec- 

 tion against external injuries. 



Inner bark or bast is moist and soft. 

 Carries prepared food from leaves to 

 all growing parts of tree. 



Cambium layer (microscopic) is inside 

 of inner bark and forms wood and 

 bark cells. 



Sapwood carries sap from roots to 

 leaves. 



Heartwood (inactive) gives the tree 

 strength. 



Pith is the soft tissue about which the 

 first wood growth takes place in the 

 newly forming twigs. 



Pith rays connect the various layers 

 from pith to bark for storage and 

 transference of food. 



Annual ring is usually a well-defined 

 layer of light and dark wood, one 

 season's growth. Lighter and more 

 porous parts is spring wood. Darker 

 part is summer wood. 



Figure 2. — How the Tree Trunk Grows. 



Annual rings are formed around the pithy growth of the first year, and in most trees 

 can be plainly seen. Usually they show a lighter color for the spring growth and a 

 darker color for the summer growth. The growing takes places in the cambium layer, 

 where the cells develop and divide, part forming the sapwood and part the inner bark. 

 As the tree increases in size, the sapwood nearest the center changes and becomes 

 heartwood. and the cells of the inner bark harden and become dry, adding to the 

 thickness of the outer bark. 



function of the heartwood is mechanical; it serves only to support 

 the living parts of the tree. This is why hollow trees may still 

 flourish and bear fruit. 



The cells between the last layer of the sapwood and the bark make 

 up what is known as the cambium layer. It is here that new growth 

 takes place. The inner side of the cambium layer forms new wood 

 and the outer side new bark. In addition to the true cambium, which 

 forms both wood and bark, there is another cambium which makes 

 the outer corky bark and nothing else. Like the true cambium, this 

 cork cambium may encase the whole tree, or it may form little sep- 

 arate films in the bark. In either case it dies from time to time, and 

 is re-formed nearer the wood. 



Trees, like many other plants, bear flowers and reproduce by means 

 of seed. When the tree enters on its long winter rest, it has its next 

 year's buds already formed. With the coming of spring, these buds 



