OUR FORESTS O 



A tree grows in thickness or girth through the addition each year 

 of a coat of new wood cells (fig. 2). The layer of wood thus de- 

 veloped is known as an annual ring, which, after it is once formed, 

 does not change in size or place during the life of the tree. The 

 annual rings as a rule maj; 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 



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 a 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 place 

 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. 



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 

 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 cam- 

 bium, this cork cambium may encase the whole tree, or it may form 



