BACTERIA OF THE SOIL 71 



2. Fill tubes, plug, and sterilize in the Arnold sterilizer. 



3. Inoculate with different strains of B. radidcola. At first 

 growth will be slow, but between the fourth and tenth days 

 many bacteroids may be found. 



Exercise 102. The Formation of Tubercles upon Roots of Legumes 



If time permits, this experiment may be performed by the class, 

 different legumes being assigned to different students. 



1. Obtain new flowerpots, or sterilize old pots at 150° C. for 

 an hour. Fill the flowerpots with sand which has been baked at 

 a temperature of 150° C. for one to two hours. 



2. Plant seeds of various legumes in the sand. Inoculate 

 half the pots by adding a suspension of B. radidcola obtained 

 either from a pure culture or from soil in which the legume 

 has previously been grown. Water the sand as needed with the 

 followiug solution : 



3. Watch the plants for differences in growth. When the 

 differences are marked, pull out the plants and examine the roots 

 of inoculated and uninoculated plants for tubercles. 



Exercise 103. The Reduction of Sulphates by Bacteria 



When grown under anaerobic conditions, certain bacteria are 

 able to reduce sulphates. In water-logged swamps ferrous sul- 

 phate may be rediiced and give rise to deposits of bog iron. 



1. Prepare Van Delden's solution for the cultivation of 

 sulphate-reducing bacteria (see Exercise 46). 



2. Fill six small flasks nearly full of the solution. Plug and 

 sterilize the flasks iu the autoclave. 



3. Set aside two flasks for controls ; inoculate two with a few 

 grams of garden soil and two with a few cubic centimeters of 

 sewage or ditch water. 



4. Incubate for one to two weeks at room temperature. 



