PHYSIOLOGY 477 



from many other combinations. They are never assimilated in their 

 uncombined state. 



Assimilation of Nitrogen 



Nitrogen constitutes a relatively small part of the weight of the 

 plant, sometimes less than 1 per cent, and never more than 7 per 

 cent to 8 per cent, but is, nevertheless, an indispensable constituent 

 of the protoplasm. Like the other elements, it always enters the 

 plant in combination, the only exception to this being the ability of 

 certain Bacteria to utilize the free nitrogen of the atmosphere. Green 

 plants, with few exceptions, obtain their nitrogen in the form of 

 soluble nitrates from the soil. 



Various nitrogen Bacteria are of great importance in supplying the 

 higher plants with the nitrogen compounds necessary for their growth. 

 The Bacteria which occur in the tubercles found upon the roots of 

 Leguminosae and a few other plants (Elceagnus sp.) can utilize the 

 atmospheric nitrogen, and provide the host with a much larger 

 amount of nitrogen than is found in the soil where it is growing. 

 This power of fixing nitrogen makes leguminous plants (Clover, 

 Beans, Peas, etc.) of such great value in restoring the fertility of 

 worn-out soils, and also accounts for the high value of these as food- 

 plants. The best known of the tubercle-forming Bacteria is Bacillus 

 radicicola, while Clostridium Pasteurianum is a nitrogen-fixing form 

 which lives free in the soil. 



Nitrite and Nitrate Bacteria 



The production of the nitrates, which are the principal source of 

 nitrogen for the higher plants, is also largely the work of Bacteria. 

 These are of two kinds, those which oxidize ammonia to form 

 nitrites, and those which oxidize the nitrites to nitrates, available 

 for the use of the higher plants. Their importance in the economy 

 of nature is sufficiently evident. 



Processes in Nitrogen-assimilation 



As in the case of carbon-assimilation, the steps by which the 

 higher organic compounds are built up are still very imperfectly 

 understood. It seems probable, however, that they are chemo- 

 synthetic, rather than photosynthetic in their nature. While the 

 carbon compounds are relatively simple in structure, the nitrogen 

 compounds are extraordinarily complex. (A full discussion of this 

 very difficult subject is given in Pfeffer (26 a), pp. 388^10). 



