230 MASS. EXPERIMENT STATION BULLETIN 170. 



through a series of years until they finally die. There are many instances 

 in New England, particularly in large industrial centers, where the expec- 

 tation of life of elm trees is reduced from one-half to one-third the normal, 

 owing to the presence of noxious atmospheric gases, and no amount of 

 soil renovation or tree surgery can correct these conditions. 



It is questionable whether injury ever occurs to vegetation from smoke 

 derived from wood, although in one or two instances injury to crops has 

 been surmised. In each case the crops were located near brick kilns. 



Lichens are the most sensitive organisms to smoke, although the smoke 

 and gases derived from wood combustion appear not to affect them. 

 These lowly organized plants are invariably absent on trees in cities, and 

 in the thickly inhabited parts of towns where coal is burned, but may be 

 observed in suburban settlements where wood is more used as fuel. These 

 organisms are apparently affected even by the minutest trace of sulfurous 

 gases in the atmosphere. 



The greatest injury to vegetation occurs near smelters, where sulfur 

 dioxide and other gases contaminate the atmosphere. In some places 

 vegetation is affected 75 to 100 miles from such establishments. Where 

 sulfur is used for bleaching purposes, and the atmosphere becomes polluted, 

 vegetation is likely to suffer, and many manufacturing establishments 

 which make use of coal-tar products, naphtha, ammonia, carbolic acid, 

 creosote oil, etc., frequently fill the atmosphere with poisonous gases 

 which injure vegetation and animal life. However, the manufacture of 

 sulfuric acid by smeltering companies has done away with much of the 

 injury formerly occurring to vegetation in their vicinity. In the manu- 

 facture of sulfuric acid the furnace smoke, which is heavily laden with 

 sulfur dioxide, is used, and in modern equipments most of the sulfur con- 

 tents are removed. Sulfur dioxide is much heavier than air, and possesses 

 a pungent and characteristic odor. Persons familiar with the odor of 

 sulfur dioxide are comparatively rare who can detect 2 to 1,000,000 parts 

 when present in the atmosphere. Even 3 to 1,000,000 parts is detected by 

 only few, while 4 to 1,000,000 parts is discernible to those of average 

 sensitiveness. 



The limitation of injury from sulfur dioxide to the most sensitive plants, 

 or threshold of discoloration as we term it, is according to some experi- 

 menters 1 to 1,000,000 parts. This, however, is regarded as the theoretical 

 limit, since it would require many, hours to produce visible injury to the 

 most sensitive plant with this concentration, and, as a matter of fact, 

 burning or visible injury probably never occurs in nature with this dilu- 

 tion. Very sensitive plants will show discoloration when subjected to 

 sulfur dioxide from 3 to 4 parts to 1,000,000 if they are exposed to this 

 concentration for a number of hours. Or, in other words, to produce 

 burning a concentration would have to exist in the atmosphere for some 

 hours, even when present in sufficient quantity to be discernible to the 

 sense of smell. Burning in general from various gases presents different 

 appearances, and the same gas will produce entirely different pathological 



