PLANT FOOD AND PLANT GROWTH 37 



furnished to most growing crops, by a process (nitrification) that 

 is more fully explained in the following pages. 



Protein. Protein is the general name for organic nitrogen com- 

 pounds, including the proteids, or final products, and the amids, 

 or intermediate products. The amids and proteids of the protein 

 group might be compared with the sugars and starches (and fibers) 

 of the carbohydrate group in which the sugars are the intermediate 

 form and the starches (and fibers) the more permanent form. 

 Protein always contains nitrogen in addition to oxygen, carbon, 

 and hydrogen. 



The chemical reactions involved in the formation of proteids 

 are not yet well understood, although many of the intermediate 

 products (amids) are well known, and some can be made arti- 

 ficially. The amids are especially abundant in young or immature 

 plants, and they are also liberated as intermediate decomposition 

 products. Thus, carbamid, O=C = (NH 2 ) 2 , which is also called 

 urea, is a common nitrogen compound in urine, the medium by 

 which most of the nitrogen waste is thrown off from the animal 

 body. This compound might be considered as formic aldehyde, or 

 monose (O=C = H 2 ), in which the two hydrogen atoms are re- 

 placed by two amido groups, and the amido group ( NH 2 ) may 

 be considered as ammonia in which only two monovalent hydrogen 

 atoms are joined to the trivalent nitrogen atom, thus leaving one 

 free hand by which this group may be attached to other groups or 

 atoms in the building of molecules. The hydroxyl group ( OH) 

 and water (OH 2 ),in relation to oxygen, correspond to the amido 

 group ( NH 2 ) and ammonia (NH 3 ), in relation to nitrogen, and 

 also to the methyl group ( CH 3 ) and methane (CH 4 ), in relation to 

 carbon. The amido group ( NH 2 ) acts as a monovalent radicle, 

 and by replacing hydrogen atoms in various compounds forms new 

 compounds called amids, or amido compounds, and these by con- 

 densation or combination with other groups may form the final 

 nitrogenous organic compounds called proteids, which constitute 

 chiefly the flesh (not fat) and vital organs of animals, and the pro- 

 tein of mature plants. 



It has been suggested that amido formic aldehyde, H 2 NCHO, or 

 amido acetic aldehyde, CH 2 (NH 2 )CHO, or aspartic aldehyde (see 

 aspartic acid and asparagin in the following list) may furnish the 



