WEST VIRGINIA GEOLOGICAL SURVEY. 



69 



to exude a resistant substance when exposed to the air. Bruises 

 and broken limbs and roots are quickly healed over, especially 

 in young and vigorous trees, and the natural place of entrance 

 of many disease-producing organisms is thus closed. A resistant 

 Govky tissue is sometimes formed beneath the scars of tender 

 growing parts and is also found about the lenticels, or breath- 

 ing pores, in the bark. Leaves, the tenderest and most exposed 

 growing parts of the tree, present several protective features. 

 Leaves are structurally protected from excessive light and from 

 the escape of too large quantities of water from their upper 

 surfaces. A further protection is afforded them by the hard- 

 ened epidermis or skin-like covering, and many leaves are pro- 

 tected, also, by a dense covering of hairs or spines. The seeds of 

 trees are protected, in some cases, by spiny or bone-like cover- 

 ings, as in the nuts of chestnut, hickory and walnut. 



The Decay of Dead Wood. 



From the ordinary commercial standpoint trees become 

 useful only after they are cut down. When this is done the 

 protection they have had in life is, of course, removed. ^lore- 

 over, enemies of living wood now give place to a larger number 

 of insects, fungi and bacteria that attack dead wood and quick-' 

 ly bring about its ruin and decay. 



Conditions That Favor Decay. 



Saplings felled and left upon the ground decay witltip-j«L 

 few months. This is especially true when the bark is not ^rff-p, 

 moved and when they are left in damp, shady places. j\Iany 

 wood-destroying fungi and insects prefer to work in young 

 trees, and in the sap wood of older ones, thriving only where 

 there is a large percentage of moisture. Just the conditions 

 that favor the most rapid decay are afforded by leaving logs, 

 sills, ties and other timbers upon jthe moist ground or in other 

 places where thorough seasoning is not possible. 



