ood is arranged in rings or layers and that the
outer part of the stem is covered with bark. We will notice also that
the wood near the centre of the tree is darker than the outer part.
This inner part is called the heart wood of the tree. The lighter
wood is called the sap wood. It is through the outer or sap wood that
the water taken in by the root is passed up to the leaves where the
food which it carries is digested and then sent back to the plant. The
returning digested food is sent back largely through the bark. Between
the bark and the wood is a very thin layer which is called cambium.
This is the active growing tissue of the stem. In the spring it is
very soft and slippery and causes the bark to peel off easily. This
cambium builds a new ring of wood outside of the old wood and a new
ring of bark on the inside of the bark. In this way the tree grows in
diameter.

Now if the bark is injured, or any part of the stem, all parts below
the wound are cut off from the return supply of digested food and
their growth is checked. When such a wound does occur, or if a wound
is made by cutting off a branch, the cambium sets to work to repair
the damage by pushing out a new growth which tends to cover the wound.
We can help this by covering the wound and keeping the air from it to
prevent its drying and to keep disease from attacking it before it is
healed.

HOW THE WORK OF THE STEM MAY BE INTERFERED WITH

If there are any peach trees near by, examine the trunks close to the
ground, even pulling away the soil for a few inches. You will very
likely find a mass of gummy substance oozing from the tree. Pull this
away and in it and in the wood under it will be found one or more
yellowish white worms. These are tree borers. They will be found in
almost all peach trees. They interfere with the work of the stem and
in many cases kill the trees. These worms may be kept somewhat in
check by keeping papers wrapped about the lower part of the tree. But
the surest way to keep them in check is to dig them out, spring and
fall, with a knife and wire.

Borers attack the other fruit trees and also ornamental trees and
shrubs.

Rabbits sometimes gnaw the bark from trees during severe winters.

Careless workmen sometimes injure the bark of trees by allowing plows
and mowing machines or other tools which they are using among them to
come in contact with the trees and injure the bark.

Young trees purchased from the nursery generally