56 TEXTBOOK OF PLANT PHYSIOLOGY 



that is, 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, namely, 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 

 only occur when plants are exposed to light. Lately it has been 

 shown that with a sufficient amount of carbohydrates present, the 

 synthesis of proteins may also take place in darkness. This has 

 given rise to another supposition, namely, that nitrates are not 

 reduced by radiant energy, but through the action of the chemical 

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

 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. Which of the two ideas is the 

 correct one has not as yet been established. At any rate, light, 

 whether directly or indirectly, plays an important part in the 

 assimilation of nitrates and the seat of 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 ammonium 

 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, asparagine, and in this form is transferred to the 

 leaves where the final transformation takes place. This rather 

 complicated mechanism seems to encumber the nutrition of plants 

 with ammonium salts, and thus under ordinary conditions nitrates 

 may be considered as a better source of nitrogen for plants. The 

 process is discussed in greater detail in Chap. VIII, where the 

 general cycle of nitrogen transformations is considered. 



19. Assimilation of Atmospheric Nitrogen by Soil Bacteria. — 

 When the sources of carbon and of nitrogen are considered, it will 

 be seen that in both instances green plants are obliged to utilize 

 substances in a highly dilute state. The average concentration 

 of carbon dioxide of the atmosphere is but 0.03 per cent, by volume, 



