NITROGENOUS ORGANIC COMPOUNDS OF PLANTS 231 



tubers, the amides constitute a third or more of the nitroge- 

 nous matter. In fodders, the amount depends upon 

 the stage of growth at which the crop is cut. When 

 mature, from 10 to 15 per cent, of the nitrogenous matter 

 is in the form of amides, while in the early stages of 

 growth, there are two or three times as much. The 

 amides and amines form a part of crude protein (see 

 Section 307). In comparing the crude protein content 

 of food, the amount of amide nitrogen should be con- 

 sidered, because the amides are of less food value than 

 the proteids. 



319. Protein Production and Disintegration. The 



following cycle of changes takes place in the production 

 of proteids in plants : 



1 i ) Ammonia is taken from the soil. 



(2) An amide is produced from ammonia. 



(3) A proteid is finally formed from the amide. 

 When plants are used as food, the reverse order of 



changes takes place in the animal body : 



(1) The proteid of the food undergoes digestion, and 

 is made over into proteid tissue in the body. This pro- 

 teid tissue is finally broken up into amides. 



(2) The amide is expelled from the body as waste 

 matter. 



(3) In the soil, the amides are changed to ammonia, 

 and are then ready to begin anew this cycle of changes. 



Alkaloids 



320. General Composition. The alkaloids are nitroge- 

 nous organic compounds present in many animal and 



