EDWIN BROUN FRED 339 



bean groups possess numerous peritrichous flagella ; while those of the cowpea and 

 soy-bean groups possess a single flagellum attached at a corner of the cell (mono- 

 trichous) . 



Acidity relations. — The two general groups of nodule bacteria established by 

 flagellation may be subdivided according to their behavior toward acidity. The acid- 

 ity or critical pH for bacteria differs with the several cross-inoculation groups. In gen- 

 eral, the limiting acidity is that known to be injurious to the higher plant. Below is 

 given the critical pH for the bacteria of several of the more common leguminous 

 plants/ 



Critical or Limiting pH 



Alfalfa and sweet clover 4 



Peas and vetches 4 



Clovers and beans 4 



Soy beans 4 



Lupine 3 



Carbohydrate fermentation. — Differences between the cross-inoculation groups as 

 well as the strains within a given group are easily recognized by differences in the fer- 

 mentation of carbohydrates.^ Since the fermentation of carbohydrates is weak, a 

 medium of low-buffer capacity and a long period of incubation must be used if changes 

 in pH value are to be noted. 



In general, the members of the peritrichous group are much more vigorous fer- 

 menters than are those of the monotrichous group. Each of the cross-inoculation 

 groups may be recognized by the use of appropriate carbon compounds, and in several 

 of the cross-inoculation groups subgroups are established. For example, strains of the 

 alfalfa organism produce much acid from mannitol, while others do not. 



Serological reactions. — Because of the high specificity of the agglutination test, 

 it has been found of value in separating strains within a single cross-inoculation group. 

 The sera usually show a high titre, agglutinating homologous antigen in dilutions of 

 1-20,000, 1-30,000, or even higher. Repeated tests indicate that these differences in 

 serological properties remain constant after long cultivation under varying condi- 

 tions.^ 



Gum production. — One of the most noticeable characters of the nodule bacteria is 

 their pronounced gum formation, especially when grown on plant extracts plus sugar. 

 If pea or clover bacteria are grown on the extract of young pea plants, the formation 

 of gum may be so profuse that the entire culture becomes viscous, like a weak gelatin. 

 The addition of acetone or alcohol to these well-developed cultures precipitates this 

 gum. Great variation in the amount of gum production is observed between the cul- 

 tures of the different cross-inoculation groups and between strains within each group. 



Chemical tests show this gum to be a nitrogen-free carbon compound. It does 

 not reduce Fehling's solution in the natural state but is readily hydrolyzed by acid 

 or heat with the production of a reducing sugar. 



' Fred, E. B., and Davenport, Audrey: /. Agr. Research, 14, 317. 1918. 



= Baldwin, I. L., and Fred, E. B.: Soil Sc, 24, 219. 1927. 



3 Stevens, J. W.: ibid., 20, 45. 1925; Wright, W. H.: ibid., 20, 95. 1925. 



