130 PEINCIPLES OF SOIL MICROBIOLOGY 



Vicia sicula has been grown a year before was used for plating out on 

 these media. The capacity of the colonies developing on the plate to 

 inoculate plants obtained from disinfected seed grown in sterile soil was 

 then tested, and it was found that nearly half of the colonies were those 

 of the organism in question. 



Vogel and Zipfel 133 demonstrated by agglutination tests, using highly 

 potent immune serum, that the nodule bacteria can be readily isolated 

 from the soil; this method is even more reliable than the direct inocula- 

 tion test, since, with the latter method, negative inoculation results are 

 often obtained. 



Colony appearance. The colonies appearing on the plate are either 

 surface or deep colonies. The first are drop-like, watery, mucilaginous 

 in appearance, gray-white to pearly white in color, glistening, and semi- 

 translucent to opaque. The edges are smooth and even ; they frequently 

 attain a size of 1 cm. or more in diameter. The deep colonies are small, 

 lens or spindle shaped, with smooth and even edges, opaque, granular in 

 structure, and cream colored to chalky white. They slowly increase in 

 size, eventually appearing on the surface, when growth becomes rapid. 

 When first isolated, colonies may not appear before 6 to 14 days. Some 

 races grow much faster than others, as in the case of Pisum, Vicia, 

 Lupinus, Trifolium, Melilotus, and Medicago. To the slow growers 

 belong the Vigna (cowpea), Glycine (Soja, soybean), and others (No. 

 46, PI. VIII). 



Morphology and life cycle of organism. The organism varies greatly 

 in size and shape in the nodule. Many small, oval forms, described by 

 Beijerinck as swarmers, and normal rods are found together with a few 

 large club-shaped or branching forms (bacteroids) in the young nodules. 

 In the old, decomposing nodule, the branching forms are extremely 

 vacuolated, showing small, oval, deep staining bodies within. 134 These 

 bodies may be the motile swarmers or the branching form dividing into 

 bacilli. 



In pure cultures, the organism forms minute short rods, motile when 

 young by means of flagella. 135 The bacteroids may be produced also 



133 Vogel, J., and Zipfel, H. Beitrage zur Frage der Verwandtschaftsverhalt- 

 nisse der Leguminosenknollchenbakterien und deren Artbestimmung mittels 

 serologischen Untersuchungsmethoden. Centrbl. Bakt. II, 54: 13-34. 1921. 



134 de Rossi, G. Uber die Mikroorganismen welche die Wurzelknollchen der 

 Leguminosen erzeugen. Centrbl. Bakt. II, 18: 289-314, 481^89. 1907. 



136 Barthel, Chr. Die Giesseln des Bacterium radicicola (Beij.). Ztschr. Gar- 

 ungsphys. 6: 13. 1917. 



