148 AGRICULTURAL BACTERIOLOGY. 



nitrogen fixation has been demonstrated, the plants have been 

 grown in soil wholly deprived of nitrogenous foods, and prac- 

 tically free from nitrogen. In other experiments, when the 

 legumes are grown in soils already well supplied with nitrog- 

 enous foods, the increase in fixed nitrogen is very much less, 

 or even inappreciable. In other words, the legumes appear to 

 prefer taking their nitrogenous material directly from the 

 nitrogenous foods of the soil when these are present in abun- 

 dance. But if the soil does not furnish the proper nitrogen, 

 then recourse is had to atmospheric nitrogen through the 

 agency of the tubercle organisms. From this, of course, it fol- 

 lows that in a soil already well supplied with nitrogenous 

 plant foods there is little or no further fixation of atmospheric 

 nitrogen by leguminous plants ; but when the soil becomes 

 poorly supplied with nitrogenous foods the atmospheric nitro- 

 gen is seized by the leguminous plants and the store of fixed 

 nitrogen consequently increased. In other words, this phe- 

 nomenon of fixation of nitrogen by leguminous plants in asso- 

 ciation with bacteria, occurs only in soils where such fixation 

 is needed to renew the fertility of the worn-out soil. 



DIFFERENT SPECIES OF THE TUBERCLE ORGANISM. 



The question whether proper bacteria are always in the soil 

 ready to produce tubercles is one which has caused a considerable 

 amount of dispute and much experimentation. It is practically 

 certain that nearly all soils contain bacteria capable of living in 

 symbiosis with leguminous plants. Nearly all soils, unless it 

 be the extremely sandy soils that support little or no vegeta- 

 tion, will support leguminous plants readily enough and de- 

 velop tubercles on their roots. One cannot examine the roots 

 of legumes anywhere without finding tubercles, a fact which 

 shows that the bacteria in question are very widely distributed 

 in nature. 



But as the subject has been studied more carefully it has 



