FORESTRY 



197 



should be pruned before they are set out. If the soil is 

 quite sterile, as city soil commonly is, fertilizers should be 

 used. While the tree is young, the soil around it should be 

 kept broken and free of weeds. If branches are cut away, 

 wounds larger than two inches in diameter should be 

 covered with thick paint or tar, or by a plate of zinc. The 



View in a beech forest in Indiana. 



life of decaying trees is often saved by removing the decayed 

 parts and filling the cavities with asphalt or concrete. 

 Smoke or gas frequently causes the ill health of trees in 

 cities. Plants are sensitive to quantities of gas far too 

 small to be detected by the sense of smell ; a gas leak may 

 cause injury to plants many feet away on account of the 

 gas-poisons which work through the soil. 



Some insects cause great damage to trees, both by devour- 

 ing leaves and boring into the stems. One of the most 

 striking examples of this is the case of the gypsy moth. In 



