232 MASS. EXPERIMENT STATION BULLETIN 170. 



hence one part of sulfur dioxide per million might be present for a few 

 moments at any particular point, while a few moments later only slight 

 traces would be found. 



The preparation of asphalt and tar on streets lined with shade trees 

 sometimes results in burning of the foliage; and this is also true of steam 

 rollers when employed for road work. 



Sewer gas has often been suspected of injuring shade trees. The con- 

 stituents of this gas are, however, extremely variable. Some of them are 

 toxic, and in sufficient quantities are capable of injuring vegetation. 

 As a matter of fact, however, injury to plants from sewer gas seldom 

 occurs; on the contrary, sewers and cesspools furnish one of the best 

 environments for root growth. Even when the poisonous gases of the 

 sewers reach rather high percentages they are seldom produced in large 

 enough quantities to do harm, and soon become diffused in the atmosphere. 



In summarizing the effects of smoke on vegetation the following factors 

 should be considered : ■ — 



Smoke is the product of combustion diffused in the air, and may be 

 either visible or invisible, affecting vegetation in the following ways: — 



By retarding growth and development of plants in consequence of the 

 presence in the atmosphere of noxious gases, acids, etc. 



By causing a direct burning of the foliage resulting from the gases 

 present. 



By causing asphyxiation through the deposition of soot on the foliage. 



By reducing the light intensity and thereby affecting photosynthesis 

 or carbon assimilation. 



By constituting an important factor in the formation of fogs, which 

 increase the susceptibility of plants to injury from gases. 



By combining with certain soil constituents to form an acid soil, thereby 

 affecting the roots of plants. 



Smoke affects plants both directly and indirectly, although the effects 

 are often slow in asserting themselves. 



The direct effects of smoke arise from the products of combustion, such 

 as soot and sulfurous gases, which affect the foliage and young shoots, 

 also the soil, and, consequently, the roots of plants. 



The indirect effects of smoke follow as a result of fogs, which are due 

 to the solid particles present in the smoke and which also interfere with 

 the normal light conditions, thereby affecting photosynthesis or carbon 

 assimilation. 



The factors involved in burning from gases may be classified as follows : — 



1. Inherent susceptibility to burning, which is determined by the 

 anatomical and physiological characteristics of the organism. 



2. Susceptibility of a periodic nature, which is associated with the 

 activity of some particular life cycle function. 



3. Susceptibility associated with meteorological conditions or agencies. 



