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 cut of a knife or the careless wielding of an axe, to the boring of an insect, or to the decay of a fungous disease. (See Fig. 117.) Whatever the cause, the remedy lies in cleaning out all decayed wood, removing the loose bark and covering the exposed wood with coal tar.

In cutting off the loose bark, the edges should be made smooth before the coal tar is applied.  Loose bark, put back against a tree, will never grow and will only tend to harbor insects and disease.  Bandages, too, are hurtful because, underneath the bandage, disease will develop more rapidly than where the wound is exposed to the sun and wind.  The application of tin or manure to wounds is often indulged in and is equally injurious to the tree.  The secret of all wound treatment is to keep the wound smooth, clean to the live tissue, and well covered with coal tar.

The chisel or gouge is the best tool to employ in this work.  A sharp hawk-billed knife will be useful in cutting off the loose bark.  Coal tar is the best material for covering wounds because it has both an antiseptic and a protective effect on the wood tissue.  Paint, which is very often used as a substitute for coal tar, is not as effective, because the paint is apt to peel in time, thus allowing moisture and disease to enter the crevice between the paint and the wood.

[Illustration:  FIG. 117.—­A Neglected Surface Wound.  Note the rough surface of the wound, the want of a coal tar covering and the fungous growth that followed.]

Cavities:  Deep wounds and cavities are generally the result of stubs
    that have been permitted to rot and fall out.  Surface wounds allowed
    to decay will deepen in course of time and produce cavities. 
    Cavities in trees are especially susceptible to the attack of
    disease because, in a cavity, there is bound to exist an
    accumulation of moisture.  With this, there is also considerable
    darkness and protection from wind and cold, and these are all ideal
    conditions for the development of disease.

    The successful application of a remedy, in all cavity treatment,
    hinges on this principal condition—­that all traces of disease
    shall be entirely eliminated before treatment is commenced.