LEAF DISEASES AND INJURIES 23 



Smoke- and Gas-Injury 



Caused by the products of incomplete combustion of coal 



Conspicuous injury to vegetation is noticeable around indus- 

 trial centers where smoke and invisible poisonous gases are 

 discharged into the air from chimneys. The manufacturing 

 plants, large and small, office buildings, apartments, private 

 residences, railroad locomotives, smelters, furnaces, kilns, and 

 the like, are sources of smoke and gases, which in nearly every 

 city or industrial center cause the death of certain species of 

 plants, and chronic injury to other species for miles around. 

 The topography of the country surrounding the source of smoke 

 and gas determines the distribution and extent of the injury. 

 Prevailing winds and natural air currents, caused by hills and 

 valleys, may result in but little damage at the source of the smoke 

 and cause severe damage at a distance where the smoke settles. 

 The zones of acute injury are readily traceable by the remains 

 of trees and other plants killed by the fumes. If the produc- 

 tion of poisonous fumes has been continued over a long period, 

 the last vestige of plant life may have disappeared. This is 

 especially true where fogs are common, which cause the fumes 

 to be held in concentrated form over the immediate locality. 

 When the source of smoke is in a valley, more injury may 

 result just beyond the brow of the surrounding hills than imme- 

 diately adjoining the source. The determination of the amount 

 of damage caused by chronic injury some distance away from the 

 source of smoke is sometimes difficult. Much of the trouble 

 in encouraging normal tree growth in the cities is traceable to 

 chronic injury by smoke and gases. In the open country, away 

 from the other pathologic factors which are met with in the 

 cities, the effects of acute injury have been found to extend for a 

 distance of ten to fifteen miles from the source of smoke and 

 gases, and the chronic injury to a much greater distance, some- 

 times probably fifty or one hundred miles. 



