SYNTHESIS, STORAGE AND BREAKDOWN 127 



occurs extensively during germination. Information regarding 

 these protein-splitting enzymes is as yet rather incomplete : for 

 the present purpose it will suffice to refer to them collectively as the 

 Proteases. 



For the synthesis of proteins in the plant a supply of nitrogen 

 is required. We must therefore consider the sources from which 

 nitrogen is secured for this purpose. The typical plant is dependent 

 for its nitrogen supplies on combined forms of the element, absorbed 

 from the soil, and is incapable of utilising the free nitrogen of the 

 atmosphere. Water culture experiments of the type described on 

 p. hi lead to the conclusion that plants are incapable of making 

 appreciable growth if no compound of nitrogen is supplied to them. 

 Further, experiments show that for the majority of plants nitrates, 

 ammonium salts, and to a lesser extent nitrites, are the most suit- 

 able sources of nitrogen ; and there is no doubt that it is in these 

 forms, especially the first two, that the plant obtains its nitrogen 

 from the soil. Soluble organic forms of nitrogen also occur in soils, 

 and may be used to some extent. The supply in the soil of soluble 

 nitrogen-compounds is maintained as the result of a constant cir- 

 culation of nitrogen between the plant world, the soil, and the atmo- 

 sphere. There is also a somewhat similar circulation of carbon. 

 After the death of a plant (or animal) the materials of which it is 

 composed become eventually added to the soil, and under the influence 

 of bacteria and fungi they pass through processes of decay. The 

 nitrogen present in the proteins is restored to the form of nitrate as 

 the result of a series of transformations described in detail on p. 452, 

 though some absorption by plants may occur before the nitrate 

 stage is reached. An important part is played by those soil bacteria 

 (Azotobacter, Clostridium and Bacillus radicicola) that possess the 

 special faculty, denied to the great majority of organisms, of utilising 

 or "fixing" atmospheric nitrogen. The second of these is discussed 

 on the page quoted, while the third, which lives symbiotically with 

 certain higher plants, is considered in Chapter XII. The activities 

 of these organisms result in an enrichment of the soil as regards 

 nitrogen, though this is counterbalanced by other factors which lead 

 to a loss of combined nitrogen. 



The nitrates or other nitrogen-compounds are absorbed by the 

 root hairs, and are probably transported through the plant by the 

 transpiration stream along with other salts, of which sulphates and 

 phosphates are also needed for protein synthesis. While the synthesis 

 of sugars is dependent on light and on the presence of chlorophyll, 



