ASSIMILATION OF NITROGEN BY PLANTS 225 



the protoplasm. The proteins are the products of nitrogen 

 assimilation. Their molecules, however, are too complex to be 

 considered as primary products of nitrogen assimilation. Pro- 

 tein formation is preceded by the synthesis of some simpler 

 compounds of the type of amino acids. Unfortunately, however, 

 the chemistry of nitrogen assimilation is not quite so clear as 

 is that of carbon assimilation. 



The utilization of nitrates is easy to trace in plants by means 

 of a series of color reactions. For instance, a solution of diphenyl- 

 amine in concentrated sulphuric acid produces a bright blue 

 color in the presence of the slightest traces of salts of nitric acid. 

 From this reaction, it can be shown that nitrates are absorbed 

 by the root hairs, and that they penetrate unchanged into the 

 vessels of the root system and then are transferred with water 

 in the same unaltered state through the vascular bundles of the 

 stem into the leaves. When the plant is placed in a dark room, 

 an accumulation of nitrates occurs in the leaves, primarily in the 

 chlorenchyma cells. This concentration is very small, however, 

 because the ascent of water is also considerably slower in dark- 

 ness. When the plaiit is exposed to light, nitrates disappear 

 from the cells of the leaf, and a simultaneous increase in protein 

 content may be observed. 



The process of nitrate assimilation thus is closely connected 

 with that of photosynthesis. It is of importance to note that in 

 nitric acid, nitrogen is combined with oxygen; whereas in the 

 protein molecule, nitrogen is in the form of the amino, -^XH2 

 group, i.e., in combination with hydrogen. Naturally, then, the 

 assimilation of the nitrogen from nitrates must be followed by 

 their reduction to an amino group. The plant being exposed 

 to light, this reduction proceeds parallel to and simultaneously 

 with the process essential to photosynthesis, viz., the reduction 

 of carbon dioxide. 



It was at one time supposed that the reduction of nitrates 

 was at the direct expense of light energy, and that consequently 

 it could occur only when plants are exposed to light. Lately 

 it has been shown that with a sufficient amount of carbohydrates 

 present, the synthesis of proteins may take place also in darkness. 

 This has given rise to another supposition, viz., that nitrates are 

 reduced not by radiant energy but through the action of chemical 

 energy that is stored in the carbohydrates. Hence the role of 



