68 THE DESTRUCTR'E AGENTS OF FORESTS. 



supply; insufficient and improper plant food; severe winter 

 freezing and late spring frosts; wind storms; lightning; snow, 

 sleet and hail ; an excess or insufficiency of light ; and crowding 

 by other trees. Closely related to these are the unfavorable 

 artificial conditions found in the regions of coke ovens, pulp 

 millSj copper smelters and blast furnaces, all of which produce 

 large quantities of smoke or sulphur gases. Second, there are 

 the dangers from living organisms, such as the wounding of 

 trees by deer^ bears, and other forest inhabiting mam Trials ; the 

 knawing of roots and carrying of disease germs by small bur- 

 orwing quadrupeds; the ravages of innumerable insects; and 

 the destructive work of fungi and bacteria and of parasitic 

 flowering plants. The wonder is that trees can live at all, when 

 we consider all these things, and especially when we reflect that 

 they have no power to remove themselves from unhealthful sit- 

 uations and out of reach of infectious diseases, but must submit 

 passively to whatever com.es their way. A brief consideration 

 of the structural protection which Nature has given trees will 

 help us to understand how so many of them are able to sur^dve 

 in the face of great odds. 



Protective Structure of Trees. 



Every liidng tree, after it has reached a certain age, is 

 made up of two distinct parts, namely^ the dry, dark-colored 

 heartwood and the living and growing sapwood, bark and leaves. 

 Every part has its own peculiar protection. The heartwood. 

 which extends through the trunk and larger limbs and roots, 

 is effectually dead and now acts only as a support to the living 

 parts which surround it. The heartwood of a few trees, such 

 as the osage orange, the red cedar and others, is supposed to 

 contain some chemical properties that are distasteful to insects 

 and that resist the entrance of fungi; but in nearly all cases it 

 has no power in itself to resist the elements of decay. There- 

 fore, it is completely enclosed by living wood which is enabled, 

 normally, to ward off many of its enem.ies. The liidng woody 

 parts are partially protected by the thick bark of the trunk and 

 limbs and by the exudation of giuns and resins which occurs in 

 many species. The wood of trees that are not resinous is said 



