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- 
