THE CARE OF STREET-TREES 125 



the limb close to the trunk, by the time the wound heals 

 the decay may be carried deeply into the tree. 



A Dressing Must be Applied. — It is necessary, therefore, 

 to apply a dressing to the surface of a scar when a limb is 

 removed that will, as far as possible, prevent the decay of 

 the old wood until it is overcapped with new callus. Thick 

 paint makes a good dressing; but the best way of preserving 

 the condition of the exposed stub is by an application of 

 thick coal-tar.' This fills the pores of the wood; and, when 

 it sets, becomes as hard as enamel. It checks the evapora- 

 tion of the sap and prevents the entrance of water. The 

 coal-tar also acts as an antiseptic, and prevents the forma- 

 tion by moisture and dust of a fertile spot for the entrance 

 of fungus spores and insects. 



The function of dressings is not to hasten the growth of 

 the callus ; but simply to prevent the decay of the stub. In 

 the case of scars, two or three inches in diameter, on hard- 

 wood trees, one application of coal-tar will be sufficient to 

 keep the exposed wood intact until the healing process is 

 completed. Larger scars may need further applications of 

 tar, one in about every two years, until the surface is over- 

 grown. So long as the exposed wood is kept intact, there 

 is no danger of injury resulting from pruning, no matter 

 how long it takes the wound to heal. 



Training to Artificial Forms. — In the city of Paris, street- 

 trees are sometimes trained to artificial geometrical forms. 



' Coal-tar is a waste product of gas works obtained in the process of dis- 

 tilling bituminous coal in retorts. The crude tar contains carbolic acid and 

 other impurities that may corrode plant tissues if present in large quantities. 

 The coal-tar found on the market is usually a refined product from which the 

 injurious ingredients have been removed. The coal-tar sold for roof-coating 

 is an efficient and safe dressing for wounds. One must avoid the use of tars 

 having injurious elements in their composition. 



