IOWA ACADEMY OF SCIENCE 
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or valley, the S0 2 will do more harm even if the city was located on 
a plain or elevation. To overcome this feature, it is proposed to erect 
tall smoke stacks, which will dilute the S0 2 to a concentration sufficiently 
low, so that it will not he effective in killing the, leaves of trees and 
shrubs. Of course, on foggy days the S0 2 will concentrate itself in the 
valley, but the number of foggy days are not numerous in a year, and 
so for that reason, this solution is reached as being the most practical 
under present conditions. 
In the report of the smoke inspector of the city of Chicago, consider- 
able emphasis is placed upon the smoke as emitted from locomotives. 
He states that co-operation of the railroad and smoke inspector is 
one of the best means of lessening the amount. Breekenridge admits 
in his report that he is not able to suggest methods whereby Illinois 
coal can be burned without smoke, but that careful firing is the most 
effective method of smoke reduction. However in a thickly populated 
areal, the only solution is electrification. Recently at Pittsburgh* the 
railroads have agreed to equip all locomotives entering Pittsburgh with 
locomotive stokers. 
Another partial assistance in the solution of the problem is to plant 
only such forms as are able to withstand at least traces of S0 2 . The 
survey of the restricted area at Chicago has shown that there is a 
difference in the resistance power of different species. The cotton- 
wood, the willow and the white poplar have been shown to be the least 
susceptible to injury from smokes and gases. By a reference to the 
species listed for the various zones, an idea can easily be obtained with 
reference to the forms- best suited to meet the conditions at hand. 
SUMMARY AND CONCLUSION. 
As a direct result of the above study, the following conclusions are 
reached : 
1. Gases and smoke have a detrimental effect upon vegetation. 
2. The presence of vegetation in the vicinity of a manufacturing 
concern depends to a large degree upon its proximity to the plant in 
question. The territory reaching out from an industrial center is 
marked out into various belts. Each belt has a certain characteristic 
flora that marks it out from the others. 
3. An industrial city shows conditions that are similar to those 
of a single large manufacturing concern. 
* Industrial World, Feb. 3, 1913. 
