Causes of Diseases in Crops 79 



The sources of smoke may be classified into 

 three divisions: (1) Smoke from large buildings or 

 from manufacturing plants. (2) Smoke from loco- 

 motives. (3) Smoke from chimneys of dwelling- 

 houses. Smoke is generally produced because of 

 improper furnace construction, improper draft, 

 overloaded boiler, insufficient air space, insufficient 

 air supply to boiler room, and finally carelessness of 

 operation. 



Smoke contains large quantities of carbon dioxide, 

 steam, and sulphur dioxide, besides its characteristic 

 soot. The latter consists of carbon, tar, and mineral 

 matter mixed with small quantities of sulphur, 

 arsenic, and nitrogen compounds which are of an 

 acid nature. Soot adheres to plants, especially to 

 foliage, giving these a burned, contorted appearance. 

 Another effect of soot and smoke is to close up the 

 stomata or respiratory openings of the leaf, which 

 results in asphyxiation. The effect of smoke on 

 plants is loss of leaflets in case of compound leaves, 

 and abnormal vegetation because of curling and 

 distortion. Lesions and spots may be formed on the 

 foliage as a result of the sulphur dioxide which is 

 present in smoke. The spots are at first small, but 

 soon enlarge and finally involve the whole leaf, which 

 dries and becomes gray. Smoke injury, although of 

 a mechanical nature, may also be considered from 

 a physiological point of view. The after effect of 

 smoke on plants resolves itself into a question of in- 

 sufficient food supply and assimilation. This is 

 indirectly brought about by diminished illumination, 



