No. 6. DEPARTMENT OF AGRICULTURE. ^381 



Nitiogen assimilation, therefore, appears to be the work, not of a 

 single organism, but of two or more growing in symbiotic relationship 

 to one another. 



Symbiosis is a term which has been frequently misapplied, it be- 

 ing confounded with mixed infection, and concurrent growths of two 

 organisms. jS[utual advantage should be the test of symbiotic re- 

 lationship. Two organisms which grow together, grow symbiotically 

 when they live to mutual advantage. An aerobe, or macroaerophile, 

 in its greed for oxygen robs the medium of this element, and makes it 

 better fitted for the development of an anaerobe. Thus B. subtlUs 

 and B.'tetani may grow together in the same medium to their mutual 

 advantage, or in symbiotic relationship. Frequently we do not 

 understand in what this mutual advantage consists, although the 

 fact exists as in the symbiotic relationship of clover to the root 

 tubercle organisms. 



Beijernick some years ago pointed out that certain organisms, 

 while not strictly anaerobes, thrive best under a diminished supply 

 of oxygen, or in other words, under diminished pressure of atmos- 

 pheric oxygen. 

 These organisms he terms Microaerophiles. 



If some form of Microaerophile be included in a hanging drop, 

 the bacteria instead of grouping themselves at the periphery of the 

 drop, as in the case with the aerobes or macroaerophiles, will keep 

 within a circle some distance within the so called respiration line 

 where there is diminished oxygen pressure. 



Beijerinck showed that the Granulobacter are microaerophiles, 

 and, as such, grow best when associated with some macroaerophilitic 

 form like Azotobactcr, which by its urgent demand for oxygen is 

 capable of ridding the medium of that portion of oxygen which in its 

 fullness would be unfavorable to the development of the Granu- 

 lobacter. 



Nitrogen assimilation, therefore, not only becomes a question of 

 two organisms living in symbiosis, but of two organisms possessing 

 distinct physiological properties; of Macroaerophiles with micro- 

 aerophiles. 



Whether their exact relationships are always necessary is, how- 

 ever, a question which may be reasonably doubted. 



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