or remain healthy very long. The reason for this becomes very
apparent when one looks into the nature of the living or active
tissue of a tree and notes how this tissue becomes affected by such
injuries.

[Illustration: FIG. 116.--A Surface Wound Properly Freed from Decayed
Wood and Covered with Coal Tar.]

This living or active tissue is known as the "cambium layer," and is
a thin tissue situated immediately under the bark. It must
completely envelop the stem, root and branches of the trees. The
outer bark is a protective covering to this living layer, while the
entire interior wood tissue chiefly serves as a skeleton or support
for the tree. The cambium layer is the real, active part of the
tree. It is the part which transmits the sap from the base of the
tree to its crown; it is the part which causes the tree to grow by
the formation of new cells, piled up in the form of rings around the
heart of the tree; and it is also the part which prevents the
entrance of insects and disease to the inner wood. From this it is
quite evident that any injury to the bark, and consequently to this
cambium layer alongside of it, will not only cut off a portion of
the sap supply and hinder the growth of the tree to an extent
proportional to the size of the wound, but will also expose the
inner wood to the action of decay. The wound may, at first, appear
insignificant, but, if neglected, it will soon commence to decay
and thus to carry disease and insects into the tree. The tree then
becomes hollow and dangerous and its life is doomed.

Injury to the cambium layer, resulting in surface wounds, may be due
to the improper cutting of a branch, to the bite of a horse, to the