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? 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). 
1 Cf. Chapter VII, pp. 82-83. 
