PLANT PHYSIOLOGY 251 



understood, but they probably either contribute material for 

 the formation of protein or act physiologically in connection 

 with some of the metabolic changes which we have been dis- 

 cussing. The general discussion of the questions connected with 

 the plant's soil food constituents belongs properly to the 

 study of soils and fertilizers in inorganic agricultural chemistry. 

 As the nitrogen of the soil food of plants is, however, directly 

 connected with the organic or energy food materials of plants, 

 a further consideration of it is not out of place in our present 

 study. 



Fixation of Atmospheric Nitrogen. — The ultimate source of 

 all nitrogen is the atmosphere, where it is present in large amount, 

 about 80 per cent. 



In just what way this atmospheric nitrogen first entered the 

 cycle connected with living organisms is not certain, nor is it 

 desirable for us to discuss the question at length. Three possible 

 ways have been suggested by which nitrogen from the air could 

 have entered the soil. 



(i) By the decomposition of metallic nitrogen compounds, 

 e.g. the nitride of boron (BN). Such compounds were doubt- 

 less formed when the surface of the earth cooled. On later 

 decomposition with water, ammonia would be formed, and this 

 could then have been oxidized to nitrate compounds by bacteria 

 which at first used the ammonia directly as nitrogen food. Such 

 a process, though it may have taken place originally, is probably 

 not going on at present. 



(2) By Electrical Discharges in the Air. — During the elec- 

 trical discharges of thunderstorms there is a continual con- 

 version of appreciable amounts of the free nitrogen of the air 

 into oxides of nitrogen, which with water yield nitrous or nitric 

 acid. 



N + O + electric discharge —> NO 



Nitric oxide 

 2 NO + O2 t^2 NO2 



3 NO2 + H2O :^ 2 HNO3 + NO 



Nitric acid 



