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 i)revents, 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 intlict 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 

 J any wound, if proper!}- assisted, can be healed by the pro- 

 cess of callusing, described on pages 91-98. 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, 



