The Relation to Nitrogen 227 



may be made by the green plant. The air contains about 

 78 per cent of ' free ' nitrogen, but this vast source is 

 wholly inert naturally except, as later indicated, either 

 through the intermediary of certain microorganisms or 

 by means of electric discharges, whereby this free nitrogen 

 is combined. Nevertheless, since the original rocks seem 

 to contain no nitrogen, the air is the original source of all 

 that at present found in arable soils - - constituting often 

 from .1 to .3 per cent of the dry weight of the soil. 



119. The nitrogen content of plants.- -Nitrogen 

 enters into a variety of organic compounds among which 

 the proteins are of the greatest importance, for these in 

 turn are apparently the main constituents of protoplasm, 

 whether living or dead. Some other compounds, occur- 

 ring in plants, which contain nitrogen are various amino 

 and amido acids, certain alkaloids, and also nitrates. 

 The protein content is greatest in seeds, storage organs, 

 and meristematic tissues. In beans and other legumi- 

 nous plants it may amount to 25 per cent of the dry weight, 

 while in wheat straw it constitutes only about 3.5 per cent. 



120. Synthesis of nitrogenous bodies. - - Animals ob- 

 tain nitrogen, for the most part, as protein foods, furnished, 

 of course, by the bodies or products of other animals or 

 plants. On the contrary, the rule is that green plants and 

 many fungi and bacteria are able ultimately to construct 

 amido compounds, proteins, and other nitrogenous bodies 

 from certain of the raw materials; that is, from some 

 of the mineral nutrients and photosynthates (carbohy- 

 drates) . 



Proteins represent an immense group of compounds 

 (sections 147-148) with a relatively enormous molecule. 



