88 



PLANT PHYSIOLOGY 



The composition of the air is about four-fifths or 80% nitrogen. 

 In addition there is also present in the air considerable quantities 

 of ammonia,— about 2.4 mg. in every 100 cubic meters of air. 

 But in spite of the abundance of nitrogen and nitrogenous com- 

 pounds in the air above the green plants, they are quite incapable 

 of drawing from this vast storehouse. The only important sources 

 of nitrogen for the green plant are the nitrogenous compounds in 



H,0 



Fig. 5. — Boussingault's experiment. Growing a plant in soil free from 

 nitrogen and air free from ammonia. The pan at the bottom contains sulfuric 

 acid to form a seal. 



the form of soluble salts in the soil; and such nitrogenous com- 

 pounds must be added to the soil supply of soluble materials in 

 some manner if the plant is to profit. This was first demonstrated 

 by Boussingault (1860), who grew various kinds of plants in a 

 sterile soil free from nitrogen. The culture pot was placed in a 

 shallow dish supported above the bottom of a larger vessel, in 

 which the bell jar covering the plant was placed. This larger 

 vessel contained sulphuric acid to prevent the entrance of am- 

 monia, and two tubes which led into the jar from the outside pro- 



