LEAF DISEASES AND INJURIES 25 



of food, and consequently the whole tree suffers. Diminished 

 growth, both in width of the annual ring and in length of the 

 twigs, is a general symptom resulting from the direct injury 

 to the leaves. Some writers believe that the gradual accumula- 

 tion of the poisonous products in smoke, lodging in the soil, 

 may have a detrimental effect on the trees. Some indication 

 as to whether or not smoke is causing damage in a given locality 

 may be determined by observing the presence or absence of 

 lichens and the common green Pleurococcus which normally 

 grow on the north sides of trees and posts. These plants are 

 very susceptible to smoke-injury and will not appear where 

 smoke is present. The common garden bean is one of the most 

 susceptible of annuals and the behavior of this plant maybe 

 taken as an indication of possible smoke-damage. 



Cause. 



The toxic property of smoke, and the accompanying invisible 

 gases which issue with it from chimneys, is due to several 

 substances, the most important of which is sulfur-dioxide. 

 All coal contains some sulfur and when burned this escapes 

 as a sulfur-dioxide gas. This gas is heavy and accumulates 

 in the lower strata of the air. On further oxidation and 

 contact with water, it dissolves and forms sulfuric acid which is 

 corrosive and toxic to the living cells of the leaves. The 

 cumulative effect of small amounts of sulfuric acid is to produce 

 the burning symptoms noticed in acute injury, and cause the 

 stunted and deformed foliage in more resistant plants. 



Soot and ashes falling on foliage also carry with them a certain 

 amount of sulfuric acid. Soot itself causes added injury by 

 accumulating as a tarry coating on the leaves and thus diminish- 

 ing their photosynthetic power. The particles of soot may 

 also clog the stomates of the leaf and interfere with the normal 

 exchange of gases necessary for starch formation. The amount 

 of soot falling where smoke is abundant is enormous. In the 



