FOREST PROTECTION 147 



d. In nature, SO 8 is largely mixed with SOt, the former 

 being less active than the latter. In the glass 

 case, usually, only SO, is developed. 



E. FACTORS OF DAMASE. 



Without a doubt, a slight admixture to the atmosphere of either 

 SO 2 or SO 3 has a certain influence on vegetation; such in- 

 fluence being irregularly proportioned to the amount of the 

 admixture. 



After Stoeckhardt, the one-millionth part of the air consisting of 

 SO 2 results, in the course of time, in discoloration (335 fumi- 

 gations discolor wet leaves in six weeks, dry leaves in eight 

 weeks). 



The degree of injury depends on 



a. The continuity of the fumigation which is governed 

 by the steadiness of the wind direction and which 

 decreases, step by step, with increasing distance 

 from the smelters. 



6. The sensitiveness of the plants which is governed 

 by species, quality of the soil, preceding injury 

 by fire, pasture or general neglect. 



c. The number of months per annum during which the 



leaves show physical activity. In the case of 

 hardwoods, this number is about 3^, extend- 

 ing from May 1 to August 15. 



d. Atmospheric conditions which may allow the gases 



to remain in bulk after emission from the smoke- 

 stack, thus concentrating the damage on such 

 parts of the country toward which the smoke 

 happens to drift in bulk. 



It has been proven by experiments as well as by the experience 

 of all observers in nature, that days of great atmospheric hu- 

 midity, days on which fog forms and days following nights 

 of heavy dew are particularly prolific in breeding acute dis- 

 coloration or damage. On the other hand, very bright weather 

 as well as heavy rains seem to minimize the damage by inten- 

 sive dilution and may prevent damage entirely. 

 The toxic influence of sulphur gases might be considered either 

 as an acute or as a chronic disease. Acute cases appear only 

 in the near proximity of smelters where clouds of smoke kept 

 in bulk under certain atmospheric constellations actually ex- 

 ercise a corroding influence on the leaves. 



On the other hand, where the diluted gases are inhaled by the plants 

 during a long number of days under the influence of a steady 

 wind, there chronic discoloration and chronic disease will enter 

 an appearance. 



