318 STRUCTURE AND LIFE HISTORIES 



after season, but if there is an alternation or rotation of 

 crops of different kinds. It was also recognized that the 

 best results are obtained when one of the crops in rota- 

 tion is a legume (a member of the family Leguminosae) 

 such as clover, peas, beans, vetch, lentils, and others. 1 

 A clear explanation of the value of rotation was not 

 possible until, in 1889, Hellriegel discovered that legumes 

 are able to utilize the free nitrogen of the air, and that 

 this is made possible by the bacteria that produce the 

 characteristic tubercles on their roots. The researches 

 of Hellriegel and others, at about this time, proved that 

 the fixation of nitrogen is due to the bacterium that 

 causes the tubercles, Pseudomonas radicicola (Fig. 61). If 

 clover or other legumes are grown in a sterile soil, free 

 from the presence of all bacteria, the tubercles do not 

 form (Fig. 228), and the fixation of free nitrogen ceases. 

 Thus it is seen that bacteria are essential to one of the 

 oldest and most fundamental practices of agriculture. 



289. Nitrifying Bacteria in the Soil. In addition to 

 the symbionts causing leguminous tubercles, there exist, 

 in all soil, at least two other forms of nitrifying bacteria, 

 which grow independently of other plants. The first, by 

 the addition of oxygen, transforms the ammonia in the 

 soil into nitrites, while the second, by the addition of 

 more oxygen, transform nitrites into nitrates.' 2 ' It is only 

 in the latter form that nitrogen, so indispensible to 

 nutrition, can be utilized at all by plants. 



299. The Nitrogen Cycle. From the above facts it 

 is seen that there is, in nature, a nitrogen cycle quite as 



1 The subject of rotation of crops is more fully discussed in para- 

 graph 90 (pp. 91-93)- 



z Cf. Chapter VII, pp. 82-83. 



