SHADE TREES. 229 



various building materials, and is more or less injurious to plants and 

 animals. The amount of sulfurous gases in the atmosphere, however, is 

 often quite insignificant, and very exact methods of chemical analysis are 

 required for quantitative determinations. The most exact and refined 

 methods of analysis in use are hardly reliable for amounts less than 1 to 

 5,000,000 parts of sulfur dioxide. Particles of cement dust, such as may 

 be found near cement manufactories, injure vegetation, as does the soot 

 arising from incomplete combustion of coal. Moreover, dust particles, 

 which may equal 50,000,000 to a cubic inch, form the nucleus of fogs, 

 which in turn imprison various obnoxious gases, thus rendering the dust 

 particles indirectly detrimental to vegetation. 



Besides the injury to vegetation resulting from gases associated with 

 smoke, smoke affects vegetation by causing a deposit of soot on the leaves, 

 thus obstructing the light. The soot also clogs the breathing pores or 

 stomata of the leaves, causing asphyxiation. The acids resulting from 

 coal combustion which accompany smoke also affect the soil by producing 

 soil acidity. At Leeds, Eng., a manufacturing city, it is estimated that 

 the daily deposit of soot is about one-half ton, and in the vicinity of other 

 English cities, where much soft coal is burned, the soil has become so 

 impregnated with smoke acids as to be of much less value for agricultural 

 purposes. Soft coal contains more sulfur than hard coal, and combustion 

 is less complete, resulting in more smoke and solid particles, which are 

 conducive to fogs. Fogs hold the sulfurous gases down, and in cities 

 where considerable soft coal is burned such gases affect vegetation more 

 severely. 



Soft coal is burned on steam railroads, but the escaping smoke and 

 gases are readily dispersed in the atmosphere. Moreover, the exposure to 

 gases of the vegetation along railroads is of such brief duration that in- 

 jury to plants is seldom noticeable. Injury to trees is frequently dis- 

 cernible in the vicinity of railroad engine houses, or roundhouses as they 

 are called. Soot is often found deposited on the trunk and foliage of 

 trees in such situations, and the contained gases affect the size and color 

 of the leaves. 



Trees in general are affected by atmospheric gases, but some are much 

 more immune than others. The black locust, Ailanthus and peach are 

 especially so, while most conifers and some of the oaks are quite susceptible 

 to injury. Many herbaceous and annual plants, such as morning glory, 

 cosmos, ragweed, etc., are very susceptible to injury from gases. Short- 

 lived trees of rapid growth, such as poplars, willows, box elders, cotton- 

 woods and soft maples, will survive and resist smoke and gases more 

 readily than the oak, elm, hard maple, chestnut and linden. Our native 

 elm appears to be affected most seriously by atmospheric gases, although 

 the nature of the symptoms resulting from constant exposure to atmos- 

 pheric gases is such that few ever guess their true significance. The 

 pathological effects following exposure to gases indicate troubles of a 

 chi-onic rather than an acute nature, and the trees gradually lose vigor 



