240 



Plant Physiology 



The bacteria producing the tubercles seem to have 

 acquired at least racial specialization, so that no one form 



of the organism will 

 infect all leguminous 

 plants. The introduc- 

 tion of a particular 

 legume into a region in 

 which that plant (or a 

 closely related species) 

 has not been grown 

 may necessitate, for 

 best results, " inocula- 

 tion " of the soil or of 

 the seed employed. 



Formerly the organ- 

 ism was introduced 

 by importing soil 

 from a locality in which the legumes had been grown. 

 This method has many disadvantages, and at the present 

 time a very thorough test is being made of the practicabil- 

 ity of employing pure cultures of the organism desired. 

 Good results have been secured with pure cultures in many 

 cases, but in some particulars the method is still in the 

 experimental stage. 



134. Certain saprophytic soil bacteria. Evidence 

 that saprophytic soil bacteria are able to fix free nitrogen 

 was afforded when Berthelot found in 1885 that bare soil 

 with its normal population of micro-organisms may con- 

 siderably increase in nitrogen content over and above 

 that added through rainfall. At the same time, no increase 

 occurred upon eliminating micro-organisms by steaming. 



FIG. 62. Bacterioids from legume tu- 

 bercles : Melilottis alba (1), Medicago 

 saliva (2, 3, and 5), and Vicia vittosa (4). 

 [After Harrison and Barlow.] 



