450 Plant Physiology 



be found to hold good for the other organic substances 

 mentioned, it is apparent that the accumulation of such 

 bodies in the soil in amounts which might be toxic (wholly 

 neglecting the possibility of their immediate destruction) 

 would require long periods of irrational cropping. 



275. Illuminating gas. It has long been known that 

 illuminating gas is injurious to vegetation. Even small 

 leaks in gas pipes are fatal to the roots of trees in the vicin- 

 ity. Vegetation in cities suffers greatly from this cause. 

 The danger is greatly increased by the fact that gas diffuses 

 through the soil to considerable distances, particularly 

 when the surface of the ground is frozen or compact, as 

 when streets or roadways supervene. Many decorative 

 plants are reported to fail as house plants when illuminat- 

 ing gas is burned. This may be due to gas-escape at the 

 time of lighting burners (since, as will be shown subse- 

 quently, the amount of gas needed to cause injury is ex- 

 tremely small) , or it may be due to incomplete combustion 

 of the gas. 



Crocker and Knight have shown that ethylene, although 

 present in very minute quantities, is apparently the chief 

 toxic constituent of the illuminating gas with which they 

 worked. They employed as indicators flowers and buds 

 of the Carnation, Boston Market and the pink Lawson. 

 After an exposure of three days the young buds of these 

 plants were dead, and bursting buds were prevented from 

 opening by a concentration of one part of gas in 40,000 

 parts of air ; while after an exposure of twelve hours 1 part 

 to 80,000 caused the flowers that were already opened to 

 close. In ethylene of 1 part in 1,000,000 buds in which 

 the petals were just showing failed to open after an ex- 



