12 ELEMENTARY FORESTRY. 
are readily detected by their being smaller than adjoining rings 
and less distinctly defined. The age of trees could be told by 
the rings of the outer bark nearly as well as by those of the 
wood were it not for the fact that the outer layers of bark fall 
off as the tree grows older. 
In some experiments the bark of rapidly growing branches 
was peeled back in the spring for a few inches, the wood cov- 
ered with tin-foil and the bark replaced. At the end of the sea- 
son there was found a ring of wood outside of the tin-foil, thus 
showing where the annual growth of the tree was made. 
The Bark covers the whole exterior surface of the trunk, 
branches and roots and serves as a protection. It is made up 
of two parts, the outer or corky layer which is dead bark and 
the inner or live bark. These vary much in appearance and 
thickness on different kinds of trees. For instance, on the 
White Birch the corky layer is pure white, very thin and tough, 
while on our White Pine it is very dark brown and often an inch 
or more in thickness and quite brittle. 
The Sapwood is the portion of the wood next to the bark. 
It varies much in thickness in different species and in trees of 
the same species; the most rapidly grown trees contain the 
largest amount. It is the most active portion of the wood in 
the growing tree, and contains considerable plant food and 
more water than the heartwood. 
The Heartwood is the wood in the center of the trunk and 
is generally distinguished from the sapwood by its more com- 
pact structure and darker color, though in some cases it may 
be lighter colored than the sapwood. It is also harder and 
more valuable for fuel, shrinks less in drying, and is more dura- 
ble in contact with the soil than the sapwood. There is very 
little movement of the sap in the heartwood. 
The Roots furnish water and nourishment that the plant 
receives from the soil, but only the young roots have the power 
of taking up the soil water; the older roots are most useful in 
holding the tree in place. It is common to classify roots into 
surface roots and tap roots, depending on their shape and the 
depth they go in the ground. Some trees have nearly all surface 
roots, as the Birch and Spruce, others have nearly all tap roots, 
which often go to a great depth on dry land, as those of the Bur 
