RECLAIMING LOST NITROGEN. 159 



in this respect is greatly increased by the stimulation that 

 comes from the presence of tubercle bacteria in their roots. 

 These conclusions are certainly not demonstrated and are 

 quite generally discredited. There is practically no evidence 

 for a belief in such a general nitrogen-fixation power. 



Xor have we any reason for believing that the property of 

 living in symbiosis with bacteria, and thus fixing nitrogen, is 

 possessed by many plants outside of the family of legumes. 

 There are only a few other instances satisfactorily attested. 

 Hiltner has shown that the alder has a relation to soil bacteria 

 almost identical with that of legumes. Like the legumes it 

 develops tubercles on its roots, and these tubercles are pro- 

 duced by bacteria, similar to those developing in the roots of 

 legumes. The bacteria have been isolated and successfully 

 studied in water cultures. While similar to those found in the 

 tubercles of legumes they are apparently incapable of producing 

 tubercles in legumes, but they cause the development of these 

 bodies only upon the alder, with a corresponding fixation of 

 atmospheric nitrogen. Somewhat similar facts have been de- 

 termined for three other families of plants, Myricaceae, Elaea- 

 ginaceae and Scrophulariaceae, in each of which families tuber- 

 cles are occasionally produced ; but the property is a rare one 

 in other families, while it is practically universal among the leg- 

 umes. While then the possibility of fixing nitrogen is not de- 

 nied for other plants, it appears to be a property of little sig- 

 nificance except in the great family of legumes. To this 

 family, growing in a condition of symbiosis with the soil bac- 

 terium B. radicicola, we must look for the chief agency ex- 

 isting in the soil for fixing atmospheric nitrogen. 



SUMMARY. 



A brief general summary of the significance of soil bacteria 

 in agricultural processes may now be appropriately given. 



