INTRODUCTION 



In nature all proteins are the products of 

 life and each kind of living molecule elaborates 

 and contains its own specific protein. Some 

 forms of life are capable of constructing their 

 proteins out of inorganic matter, while others 

 can utilize only that which has been built up 

 by other cells into protein material. Plants 

 take the ammonia, nitrates and nitrites of the 

 air, soil and water, and by synthetical proc- 

 esses convert these into the proteins found in 

 their tissues. In this process there are two 

 stages. In the first the inorganic nitrogen is 

 synthesized into amino acids and in the sec- 

 ond these are combined to form proteins. The 

 higher animals cannot synthesize inorganic 

 nitrogen into amino acids. This is done for 

 them by plants and to some extent by bacteria 

 in conjunction with plants. By the symbiotic 

 action of certain bacteria and plants even the 

 free nitrogen of the air is drawn upon in the 

 construction of vegetable proteins. So far as 

 protein metabolism is concerned the vegetable 

 world is the synthetical or constructive labora- 



