226 PLANT PHYSIOLOGY 



light in the synthesis of proteins may be an indirect one. Light 

 is necessary for the synthesis of carbohydrates, which are con- 

 sumed in the formation of protein. At any rate, Ught, whether 

 directly or indirectly, plays an important part in the assimilation 

 of nitrates; and the primary source of energy for the synthesis 

 of nitrogenous compounds is the same chlorophyll-bearing 

 parenchyma of the leaf where the assimilation of carbon and 

 water elements takes place. 



When ammonium salts are used for the nutrition of plants, 

 the necessity of reducing the nitrates for the synthesis of proteins 

 is naturally eliminated. It seems, therefore that ammonia 

 ought to be more easily assimilated than the nitrates. But 

 owing to its toxicity, apparently, it is not carried through the 

 plant in an unaltered state, but is changed even in the roots 

 into an organic substance, such as asparagine, and in this form 

 is transferred to the leaves, where the final transformation takes 

 place. This rather compHcated mechanism seems to complicate 

 the nutrition of plants with ammonium salts, and thus under 

 ordinary conditions nitrates may be considered a better source 

 of nitrogen for plants. 



The formation of protein substances in the leaves from inor- 

 ganic compounds of nitrogen and products of the assimilation of 

 carbon is called the ''primary synthesis of protein substances." 

 This synthesis can be reahzed only in plants and is of no less 

 significance for the existence of the animal world than is the 

 synthesis of carbohydrate from carbon dioxide and water. 



The reduction of nitrates passes through the stage of the 

 formation of nitrites, salts of nitrous acid, oxygen being not 

 released in a free state but utilized for the oxidation of organic 

 compounds present in the cell. It is very probable that sugars 

 play the role of such oxygen acceptors. The reduction of 

 nitrates is especially favored by the formation in the process of 

 photosynthesis of active forms of carbohydrates. This process 

 is regulated by a special enzyme, belonging to the group of 

 oxidoreductases. 



The reaction does not stop with the formation of nitrites but 

 continues down to ammonium, which also does not accumulate 

 but either is transformed into asparagine or reacts directly 

 with hydroxy acids, which arise from carbohydrates in the 

 reduction of nitrites, and gives, with the latter, amino acids. 



