228 PLANT PHYSIOLOGY 



substances, there is a continuous secondary synthesis of proteins 

 from the products of their decomposition, the amino acids. This 

 synthesis has already been considered in the discussion of the 

 processes occurring during germination (Art. 19). This secon- 

 dary synthesis of protein substances in plants likewise depends 

 to a great degree on adequate provision of oxygen and ceases 

 entirely in an oxygen-free medium. It has been established by 

 the investigations of Zalessky (1901) and others that the dissec- 

 tion of massive organs, such as b^ulbs and potato tubers, increases 

 considerably both their respiration and the synthesis of protein 



substances. 



This connection between respiration and protein synthesis has 

 been explained up to the present time by the supposition that 

 respiration delivered the free energy necessary for synthesis. 

 However, both hydrolysis of proteins and their inverse synthesis 

 from amino acids are accompanied by very insignificant changes 

 of energy supply. The participation of oxygen in the synthesis 

 of protein substances is much more comprehensible from the 

 viewpoint of the activation of proteolytic enzymes during reduc- 

 tion and their inhibition during oxidation. 



52. Assimilation of Molecular Nitrogen of the Atmosphere 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 utiUze substances in a highly dilute state. The 

 average concentration of carbon dioxide of the atmosphere is but 

 0.03 per cent by volume, while the average amount of nitrogen 

 contained in soils is between 0.1 and 0.5 per cent by weight. 

 But because of a high mobility of the atmosphere, plants have at 

 their disposal practically all of the enormous amount of CO2 

 surrounding the earth. Besides, the total amount of carbon 

 dioxide in the air is continuously augmented not only through 

 the respiration of animals, plants, and microorganisms, but also 

 by the processes of combustion and by volcanic eruption. Con- 

 ditions appear to be much less favorable in regard to the presence 

 of nitrogenous substances. The amount contained in the soil 

 is hmited and is increased only through the disintegration of 

 such plants and animals as have existed within the same regions 

 of the earth's surface. Moreover, a constant transformation of 

 fixed nitrogen into free molecular nitrogen takes place in nature. 

 This happens, for instance, when nitrogenous organic matter 



