362 



UNITY AND DIVERSITY IN BIOCHEMISTRY 



and more available for bacterial decomposition. The soluble phosphates are 

 concentrated by the roots and transported by the xylem in the plant to the 

 active cells. 



III. NITROGEN 



In general, plants remove nitrates from the soil. Moreover the other 

 forms of nitrogen in the soil, such as amino nitrogen and ammonia, are 

 rapidly converted to nitrates by the action of numerous heterotrophic 

 bacteria and autotrophic micro-organisms which derive their energy from 

 the conversion of ammonia into nitrite (nitrous bacteria) or nitrite into 

 nitrate (nitric bacteria). The absorbed nitrate can be accumulated by the 

 plant, or reduced to nitrite and then to ammonia which the plant uses, 

 notably for the formation of amino acid amino groups. 



N 







End 



Fig. 97 — (Guilliermond and Mangenot). Structure of a root nodule 

 in a legume {Vicia faba). 



I. A section along the long axis of the nodule. — R: a transverse section through a root; 

 N : a longitudinal section of the nodule; c.b. : branched bacterial filament having infected 

 the root. 



II. A transverse section of the nodule. End : endoderm of the root (continuing into the 

 nodule). 



In addition, a limited number of organisms are able to utilize atmo- 

 spheric nitrogen by reducing it to ammonia or amino groups. 



The ocean is poor in nitrogen-fixing organisms. It receives its nitrogen 

 from inflowing rivers. The Mississippi alone pours 360,000 tons of nitrogen 

 per annum into the sea in the form of nitrates. The nitrogen of the land 

 is therefore being continuously removed in this way. It is restored by 

 putrefaction, by the conversion of the nitrogen of the air into nitrates by 

 electric discharges during storms, and by the action of nitrogen fixing 

 organisms, free bacteria or bacteria living in symbiosis with plants. 



In fact, the fixation of nitrogen is a very common and wide-spread 

 phenomenon in the biosphere. Still little is known about its mechanism, 

 but it does not appear to be everywhere the same. 



