BACTERIA IN THE SOIL 165 



of the fixation of free nitrogen in the growth of Leguminosce 

 seem to be: 



" I. That under the conditions of symbiosis the plant is en- 

 abled to fix the free nitrogen of the atmosphere by its leaves ; 



" 2. That the nodule organisms become distributed within the 

 soil and there fix free nitrogen, the resulting nitrogenous com- 

 pounds becoming available as a source of nitrogen to the roots 

 of the higher plant ; 



" 3. That free nitrogen is fixed in the course of the develop- 

 ment of the organisms within the nodules, and that the resulting 

 nitrogenous compounds are absorbed and utilized by the host." 

 " Certainly," he adds, " the balance of evidence at present at 

 command is much in favour of the third mode of explanation." 



If this is finally proved to be the case, it will furnish another 

 excellent example of the power existing in bacteria of as- 

 similating an elementary substance. 



Most authorities would agree that all absorption of free 

 nitrogen, if by means of bacteria, must be through the roots. 

 As a matter of fact, legumes, especially when young, use 

 nitrogen, like all other plants, derived from the soil. It 

 has been pointed out that, unless the soil is somewhat poor 

 in nitrogen, there appears to be but little assimilation of 

 free nitrogen and but a poor development of root nodules.^ 

 The free nitrogen made use of by the micro-organism is in 

 the air contained in the interstices of the soil. For in all 

 soils, but especially in well-drained and light soils, there is 

 a large quantity of air. Although it is not known how the 

 micro-organisms in legumes utilise free nitrogen and convert 

 it into organic compounds in the tissues of the rootlet or 

 plant, it is known that such nitrogen compounds migrate 

 into the stem and leaves, and so make the roots really poorer 



^ This has been denied recently in the official report by the chemist of the 

 Experimental Farm to the Minister of Agriculture at Ottawa {Report^ 1896, 

 p. 200). 



