138 SHADE-TREES IN TOWNS AND CITIES 



DUST, SMOKE, AND INJURIOUS GASES IN THE AIR 



Dust and smoke are liable to choke up the breathing 

 pores of the leaves, and their natural functions are severely 

 hindered. Some trees suffer more than others from this 

 nuisance. The leaves of the sugar maple are especially 

 susceptible ; their stomata or breathing pores become clogged 

 up by dust, and they acquire a hard, metallic state. 



In cities where large quantities of bituminous coal are 

 used and in the vicinity of manufacturing establishments, 

 such as fertilizer mills, paper-pulp mills, copper-smelting 

 and blast furnaces, particularly where sulfur gases are pro- 

 duced, the effects upon all kinds of foliage are very evident. 

 It has been shown that sulfuric-acid gas is the most injuri- 

 ous component of the fumes that prove injurious to foliage, 

 and European investigations have proved that the presence 

 of sulfuric-acid gas in the air, in the ratio of 1 to 50,000, is 

 enough to lead to the destruction of the leaves of deciduous 

 trees. The effects of sulfurous fumes are shown by the 

 turning of the leaves reddish-brown in spots or along the 

 edges, and eventually of their drying up entirely. 



All the evidence goes to show that little can be done 

 toward mitigating the trouble caused by poisonous gases in 

 the air. In cities suffering from the smoke nuisance it is 

 very difficult to grow many of the ordinary street-trees. 

 The European and the American planes will stand adverse 

 conditions better than any of the other good street-trees. 



OILING OF ROADS 



The oiling of roadways during the last few years has led 

 to a great deal of speculation regarding the effect of the 

 dust from such roads on the vegetation bordering them. No 



