OUR FORESTS 5 
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 1t is once formed, 
cloes not change in size or place during the life of the tree. 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 1s 
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 part is spring wood. Darker 
part is summer wood. 
FicurE 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 
