impurities, that crops of radishes and lettuce grown in different sections of Leeds show the possibility of correlating the known atmospheric impurities with the yield of the crop. They also found that as trees automatically keep a record of yearly growth, the presence of any inhibiting factor will make itself known by the narrowing of the annual rings.
In 1913, a study of the  Mr. Bakke reached the following conclusions:of city smoke on vegetation was made in Des Moines by A. L. Bakke, of the Iowa State College.
1. That gases and smokes have a deleterious action upon vegetation.
2. That the vegetation about a manufacturing concern may be mapped off, in the form of concentric belt demarcation, each belt being represented by a certain form or forms of plant life, since certain plants are more susceptible to smoke injury than others.
3. That an industrial city like Des Moines, in its plant elimination process, is governed by the same set of conditions as are in operation for a single manufacturing plant.
Mr. J. F. Clevenger, as a result of his studies in connection with the Smoke Investigation of the Mellon Institute, declared that the fact that smoke injures vegetation is evidenced not only by the general external appearance of many of the constituent plants, but also by their internal appearance, as shown by the size of annual rings and by lesions in the leaves. Mr. Clevenger's studies were confirmed by controlled field experiments which he made. In these experiments growing plants enclosed in cases were subjected to small quantities of soot distributed uniformly over them. The leaves of the plants so treated displayed a tendency to drooping and many of the leaves began to die at the tips; a checking of growth of the plants was also apparent.
The effect of smoke on health has been a much-mooted question. For a long time it was held, and still is by some, that a smoky atmosphere is not injurious and at times even beneficial to public health. This supposition gained favor from an observation, largely erroneous, that coal miners are not prone to contract tuberculosis.
One of the most comprehensive studies of the direct effect of smoke upon the respiratory organs was made by Dr. Louis Ascher, of Königsberg. Dr. Ascher's statistical and experimental studies led him to the conclusion that the mortality of acute lung diseases is certainly increasing, especially among children and old people. The cause of this increase, he declared, is the impurification of the air by smoke, as the increase is greatest in industrial centers and not in agricultural districts. He further pointed out that within industrial districts a difference in
- Bakke, A. L., "The Effect of City Smoke on Vegetation," Bulletin 145, Iowa State College of Agriculture.
- Clevenger, J. F., Bulletin No. 7, "The Effect of the Soot in Smoke on Vegetation," 1913. (Published by Mellon Institute.)