36 Disease and Death of Trees — Generalities 
A tough skin or epidermis protects the leaves and young 
shoots, and a corky bark the living portion of the older 
parts of the tree, and prevents, or at least impedes, access 
of insects and fungi to the tender living tissues. But there 
are innumerable opportunities of breaking through this 
protective cover and exposing the delicate tissues below. 
Minute punctures are made by insects; hailstones tear off 
portions of leaves, produce contusions on the young twigs 
and lacerate the bark; windstorms tear off foliage and 
twigs and break branches; frost and sun-scald split and 
kill portions of the bark, and lightning may rend the bole 
to the very heart. In addition, careless man and gnawing 
animals inflict wounds, small or large. 
The result is, first of all, an interference with the normal 
functions of the tree: certain areas of conducting tissue or, 
in the leaves, of assimilating tissue are destroyed, and by so 
much the equilibrium of the whole system is disturbed in 
its nutrition; and secondly, a way is opened for fungi to attack 
the living or supporting tissue. 
The recuperative power of trees is so great that almost 
any wound, if properly assisted, can be healed by the pro- 
cess of callusing, described on pages g1-98. All that the tree 
doctor can do, or needs to do, in most cases, is to see that 
this healing process is performed promptly and without 
interference. The sooner the injury is looked after the 
better, so that the secondary evils of fungus infection may 
be prevented. 
Fungus Diseases. Any portion of a tree which has 
been mechanically injured, so as to expose the living 
tissues of the wood, is subject to the attacks of fungi, 
although sometimes attacks may occur without such injury. 
There are fungi which live on trees as saprophytes, using 
the tree merely as a location; others which feed on the tree, 
