340 ROOT-NODULE BACTERIA 



Nitrogen fixation. — In spite of numerous investigations on the assimilation of ni- 

 trogen in the absence of the host plant, this phase of the root-nodule problem has 

 proved most baffling. The nodule bacteria under proper conditions will combine small 

 amounts of atmospheric nitrogen, 2-3 mg. of nitrogen for each 100 cc. of culture. 

 Compared with the amount fixed in the plant, this is insignificant. A satisfactory 

 explanation for this difference in behavior in the absence of the host plant has not 

 been found. 



Strain variations. — Only recently has the importance of the variation between 

 strains within a cross-inoculation group been realized. The relatively small differ- 

 ences between strains in cultural characteristics, carbohydrate fermentation, gum 

 production, and serological properties have been shown to be correlated. The practi- 

 cal importance of this lies in the fact that certain strains in symbiosis with plants have 

 been more efficient in fixing nitrogen, and this activity of the strain has been shown to 

 be related to the behavior of the strain in laboratory tests. 



CULTURES OF BACTERIA FOR THE INOCULATION OF LEGUMES 



Since 1887' it has been known that the transfer of soil from a well-inoculated field 

 may exert a beneficial effect on the growth of leguminous plants in a new location. The 

 bacteria carried in the soil cause nodule development and thus furnish nitrogen to the 

 plants. More recently much easier and quicker methods have been found for intro- 

 ducing nodule bacteria. Pure cultures of the bacteria are grown in liquid or solid 

 media, and a water suspension of the bacteria is applied directly to the seed. In this 

 way the bacteria are brought in close contact with the young plant. To insure nitro- 

 gen fixation the use of bacteria to inoculate the seed has become a well-established 

 practice, especially when a leguminous plant is seeded for the first time. 



The property of leguminous plants to accumulate atmospheric nitrogen in large 

 quantities gives them an enviable role in economic agriculture. 



' Kellerman, K. F.: Centralbl.f. Bakleriol., Abt. II, 34, 42. 191 2. 



