3.  Look out for a limb that shows fungous growths.  Every fungus sends fibers into the main body of the limb which draw out its sap.  The interior of the branch then loses its strength and becomes like a powder.  Outside appearances sometimes do not show the interior condition, but one should regard a fungus as a danger sign.

4.  When a limb is full of holes or knots, it generally indicates that borers have been working all kinds of galleries through it, making it unsafe.  The silver maple and sycamore maple are especially subject to borers which, in many cases, work on the under side of the branch so that the man in the tree looking down cannot see its dangerous condition.

5.  A dead limb with the bark falling off indicates that it died at least three months before and is, therefore, less safe than one with its bark tightly adhering to it.

6.  Branches are more apt to snap on a frosty day when they are covered with an icy coating than on a warm summer day.

7.  Always use the pole-saw and pole-shears on the tips of long branches, and use the pole-hook in removing dead branches of the ailanthus and other brittle trees where it would be too dangerous to reach them otherwise.

8.  Be sure of the strength of a branch before tying an extension ladder to it.

STUDY IV.  TREE REPAIR

Where trees have been properly cared for from their early start, wounds and cavities and their subsequent elaborate treatment have no place.  But where trees have been neglected or improperly cared for, wounds and cavities are bound to occur and early treatment becomes a necessity.

There are two kinds of wounds on trees:  (1) surface wounds, which do not extend beyond the inner bark, and (2) deep wounds or cavities, which may range from a small hole in a crotch to the hollow of an entire trunk.

Surface wounds:  Surface wounds (Fig. 116) are due to bruised bark, and a
    tree thus injured can no longer produce the proper amount of foliage
    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,