170 TREES AS GOOD CITIZENS 



and reducing fertility. Trees near factories and railroad 

 roundhouses are particularly liable to damage from soft 

 coal fumes. The evil effects of soft coal smoke are not 

 severe in the neighborhood of small manufacturing con- 

 cerns of the ordinary type, where the chimneys are carried 

 well above the foliage. Also in humid regions the dust is 

 washed from the foliage at rather frequent intervals. 

 Nevertheless, coniferous evergreens and other especially 

 susceptible trees do not thrive where soft coal is much 

 used. The use of sulphur for bleaching purposes in an 

 industrial plant releases gases harmful to vegetation. 

 Injury is also sometimes caused by the fumes or other 

 careless discharge of industrial wastes where naphtha, 

 ammonia, carbolic acid, creosote oil and coal-tar or its 

 products, or petroleum products are used. 



When damaged by atmospheric gases, young leaves 

 first show discoloration and then slowly droop and die. 

 The twigs show reduced rates of growth, and gradually 

 this reduction becomes apparent throughout the entire 

 tree. These symptoms are followed by the death of the 

 twigs and sooner or later by the death of branches and 

 trunk as well. 



Trees vary in their ability to resist injury from gases. 

 It might be supposed that the slow-growing trees of sturdy 

 nature would be less susceptible to this form of damage 

 than those of rapid growth and short life. The reverse, 

 however, has been found true. Among the trees which 

 suffer most are the Oaks, the Elms, the hard Maples and 

 the Lindens. At the other end of the scale, with the great- 

 est powers of resistance, are such trees as the Poplars, the 

 Box Elder, the Silver Maple and the Ailanthus, which will 

 survive gas attacks where other trees would succumb. In 

 England, the Elder has been found to be the tree which 

 most successfully resists injury of this nature. It is obvi- 



