9 



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an environment other than the optimum, lose at least some 

 of their distinguishing characteristics. Many of these organ- 

 isms, when transferred to nutrient agar slants, grow vigorously 

 without pigment production. 



The azotobacter develop on the Ashby medium in great 

 profusion, however, the differentiation of the nitrogen-fixing 

 bacteria is not sufficiently established to render an enumera- 

 tion of them possible. 



THE FIXATION OF FREE NITROGEN BY BACTERIA 



The relation of bacteria to nitrogen is perhaps the most 

 important problem which presents itself to the agriculturist; 

 the reason being that while the nitrogen forms a very large 

 proportion of the constituents necessary to the building up of 

 plant tissue, it is present in the soil in a very limited quantity, 

 and consequently constant cropping would tend toward ex- 

 hausting the supply. 



The fixation of free nitrogen by bacteria is consummated in 

 two widely different ways, commonly designated as the sym- 

 biotic and non-symbiotic relation. Symbiosis involves a favor- 

 able influence of one species upon another. Many observers 

 contend that this symbiotic relation is detrimental to the host. 

 The symbiotic relation existing between the leguminoceae and 

 certain bacteria enables the former to absorb free nitrogen 

 from the air and elaborate it into nitrogenous compounds. 

 This metamorphosis takes place within the leguminous nodules, 

 the earliest description of which was given by Malpighi in 

 1687, and this observer referred to them as galls, i. e., diseased 

 excresences, an opinion also shared by later writers. 



Treviranus, in 1853, was the first to regard these nodules 

 as normal growths, and thirteen years later they were studied 

 by Woronin, who made the subsequently important observa- 

 tion, that the formation contains entirely closed cells filled with 

 bacteria. 



Beyerinck, in 1888, indubitably established the fungoid 

 nature of these bacteria by isolating them from the nodules, 

 and cultivating them further in artificial media. Some ex- 

 hibited certain slight but undeniable differences which were 

 not so extensive as to make their discoverer feel justified in 



