68 A MANUAL OF BACTERIOLOGY 



2. Plate out Azotobacter from the flasks prepared in Exer- 

 cise 97. Examine and transfer from colonies to tubes of 

 Ashby's agar. 



3. The most successful method of cultivating Azotobacter is 

 upon large surfaces. Put 50 cc. of melted agar into an Erlen- 

 meyer flask of 1 liter capacity. After sterilization allow the. 

 agar to harden, or pour sterile Ashby's agar into Petri dishes 

 with a diameter of 20 cm. or more. Inoculate with a suspension 

 of Azotobacter in sterile distilled water. Tilt the culture once 

 or twice daily. The result is usually a thick film of growth 

 upon the agar. In this way enough material may be collected 

 for analytical work, but it should be removed within a week 

 from the time of moculation. 



Exercise 99. Symbiotic Bacteria which Fix Atmospheric Nitrogen. 

 Bacillus radicicola 



Beijerinck. Botanische Zeitung 46 : 723. 1888. 



Hellriegel. Zeitschr. Riibenzuokerind. deuts. Reichs. 1886. 



Koch. In Lafar's Technische Mykologie, Bd. III. Jena, 1905. 



LoHNis. Handbuch landw. Bakteriologie. Berlin, 1911. 



Marshall. Microbiology. Philadelphia, 1911. 



Peirce. Proo. Calif. Acad. Sci. (3d ser.) 2 : 295-328. 1902. 



Smith. Bacteria in Relation to Plant Diseases, Vol. II. Washington, 1911. 



The student in regular class work can get no more than an 

 introduction to the bacterial side of this problem, because most 

 experiments in this line require more time than is usually avail- 

 able in laboratory classes. Yet it is possible to gain an intelli- 

 gent idea of the subject by making a few simple experiments. 

 A casual examination of the literature shows that upon some 

 important questions uncertainty still prevails. For more ex- 

 tended work the student is directed to the researches cited 

 above and to others. 



1. Secure the roots of a leguminous plant which shows numer- 

 ous tubercles. Sketch a root, showing the shape and arrangement 

 of tubercles. Do they seem to have any regular arrangement ? 



2. Secure young roots of peas or vetch plants. Holding a 

 root with a young tubercle in pith, cut razor sections embracing 

 the tubercle and root tissue at right angles to the long axis of the 



