DAMAGE TO TREES BY ACID FUMES. 699 



The . acid, equivalent to 24^ grammes of sulphuric acid 



or 18 grammes of hydrochloric acid in 1,000,000 parts of 

 water, seemed to have no action. Two greenhouse plants were 



submitted to a daily spray of the ,,-7^ acid for a month, but 



z,uuo 



showed no corrosion. 



3. Damage done. 

 a. General Account. 



Among the direct kinds of damage done by acid fumes to 

 trees are : loss of increment, thinning out of woods and 

 formation of blanks, injury to fruit, especially in the case of 

 orchard-trees, loss of fodder by destruction of grass in a 

 forest. Damage is done indirectly by rendering the woods 

 liable to insect-attacks, to fire and other dangers. 



I). According to Species. 



Conifers suffer more than broadleaved species from smoke, 

 even although the needles under similar conditions absorb 

 less sulphur dioxide and are in themselves less sensitive 

 and hardier than other leaves. This is due to the longer 

 duration of the needles and their consequently increased 

 exposure to the bad influence of the gas, and to the greater 

 powers of recovery possessed by broadleaved species. 



Thus evergreen conifers are not only longer exposed each 

 year, but the evil accumulates from year to year as long as 

 the needles remain on the tree, whilst broadleaved trees 

 annually throw off their leaves. 



Schroder* found that 1,000 square centimeters of leaf- 

 surface, containing double that quantity above and below, 

 will, within 36 hours, absorb sulphuric dioxide as 



follows : 



c. cm. 



Silver-fir needles, young . . . . 1'8 



old ... . 1-4 



Alder leaves ....... 7*9 



* " Thar. Frstl. Jhrbch.," vol. xxii., 1872, p. 193. 



