GROWTH AND NUTRITION 69 



glycocoll (the simplest), glutaminic acid, lysine, arginine, histi- 

 dine, tryptophane, tyrosine, and cystine. In addition to these 

 ten or more other cleavage products have been identified. 



It soon became evident that proteins may yield the same 

 amino acids but yield them in quite different proportions. For 

 example, gliadin, present in wheat, contains about one per cent 

 lysine, and casein contains approximately 7.5 per cent. Lactal- 

 bumin contains about 9 per cent glutaminic acid, gliadin, about 

 44 per cent. It is also known that certain amino acids are en- 

 tirely absent from some proteins. Thus, glycocoll has never 

 been identified in the hydrolytic products of gliadin, zein, 

 casein, or lactalbumin. 



We have then taken it for granted that there are different 

 kinds of proteins, and it has been demonstrated that there are 

 indeed many different ones. Reference to our diagram should 

 make that clear. Evidently a change in the arrangement of the 

 individual amino acids would make theoretically possible an 

 almost limitless number of different geometrical patterns, and 

 so of different proteins. 



If the complexity is further increased by changing the pro- 

 portions in which the amino acids exist, then there is for all 

 practical purposes no limit to the number of these compounds. 

 Thus the tissues of one animal are in some degree unlike those 

 of every other animal. Furthermore, the various tissues of one 

 animal contain many different kinds of proteins. We also know 

 that the nutritional value of different proteins varies. In general 

 we say that animal proteins have a high biological value and 

 plant proteins have a low biological value. Numerous demon- 

 strations of this fact have been described, but we will content 

 ourselves with one. 



It is a commonplace among feeders of live stock that corn 

 alone is a very poor diet for growing animals. From a nutri- 

 tional standpoint this cereal, and others too, for that matter, 

 are inadequate as a complete diet in at least two respects ; one of 

 these is the protein factor. Our textbooks commonly state that 



