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THE SPECIFIC PHYSIOLOGICAL JOTECTS OP CA2B0E DIOXIDE. 



Tlie frequent presence in the soil air of con- 

 siderable percentages of. carbon dioxide makes it neces- 

 sary to inquire into the possibility of specific effects 

 of this gas on plant roots in addition to the effect of 

 its mere presence in decreasing the percentage of oxy- 

 gen. We know that gaseous nitrogen is physiologically 

 inert and has no specific effect on plants. .his does 

 not appear to be true of carbon dioxide. The effects of 

 the carbon dioxide in the atmosphere are of little 

 interest to the present inquiry. The process of photo- 

 synthesis is dependent on the presence of carbon dioxide 

 and probably on its partial pressure in the air, and the 

 rate of photosynthesis is, iSf course, important to the 

 growth of a green plant as a whole, but nothing of this 

 kind occurs in soil. The effect of increases or decreases 

 in atmospheric carbon dioxide may be passed, therefore, 

 with very brief notice. Slight increases in the percent- 

 age of carbon dioxide in the air have usually been found 



