28 



SHADE TREES. 



Fig. 12. Every tree well guarded against injury while building goes 

 on. Paris. Photo by Dr. W. A. Murrill. 



SALT. 



Salt in excess is fatal to tree roots. Be careful if it is 

 used to melt ice on the sidewalk, and don't allow brine from 

 an ice cream tub to be emptied near a tree. 



GAS. 



Illuminating gas from defective mains is a common cause 

 of injury to street trees. Other gases, especially sulfurous 

 acid, when discharged into the air in large quantity may 

 damage or kill them, but this happens only near chemical 

 works and under conditions not at all common. If a buried 

 gas pipe is not perfectly tight the escaping gas poisons the 

 soil about it and affects nearby trees more or less seriously in 

 proportion to the quantity of gas discharged into the soil pene- 

 trated by the roots. Thus, other things being equal, the 

 nearer a tree is to a leak, the greater ihe danger that it will 



