After the first year, trees grow by increasing the thickness of
the older buds. Increase in height and density of crown cover is
due to the development of the younger twigs. New growth on the
tree is spread evenly between the wood and bark over the entire
body of the plant. This process of wood production resembles a
factory enterprise in which three layers of material are engaged.
In the first two of these delicate tissues the wood is actually
made. The inner side of the middle layer produces new wood while
the outer side grows bark. The third layer is responsible for the
production of the tough, outer bark. Year after year new layers
of wood are formed around the first layers. This first layer
finally develops into heartwood, which, so far as growth is
concerned, is dead material. Its cells are blocked up and prevent
the flow of sap. It aids in supporting the tree. The living
sapwood surrounds the heartwood. Each year one ring of this
sapwood develops. This process of growth may continue until the
annual layers amount to 50 or 100, or more, according to the life
of the tree.

One can tell the age of a tree by counting the number of annual
rings. Sometimes, because of the interruption of normal growth,
two false rings may be produced instead of a single true ring.
However, such blemishes are easy for the trained eye to
recognize. Heartwood does not occur in all varieties of trees. In
some cases, where both heartwood and sapwood appear, it is
difficult to distinguish between them as their colors are so
nearly alike. Because it takes up so much moisture and plant
food, sapwood rots much more quickly than heartwood. The sapwood
really acts as a pipe line to carry water from the roots to the
top of the tree. In some of our largest trees the moisture is