THE ACTIVITIES OF NITEOGEN-BACTEEIA IN SOIL 173 



activities of organisms which are killed by sterilisation. In 1888 

 Beyrinck succeeded in making a pure culture of one of these nitrogen- 

 fixing organisms. This one he called Clostridium Pastorianum (Fig. 

 105). He was able to cultivate it artificially, and the medium he 

 employed was composed of the following substances : 



Potassium hydrogen phosphate, - - 0*1% 



Magnesium sulphate, - - 0'02% 



Sodium chloride, 0-001-O002% 



Ferrous sulphate, 1 



/- ------ traces 



Manganese sulphate, f 



Dextrose, - - 2-4% 



Water, 100 c.c 



His method of "capture " was the same as is employed in making 

 pure cultures from any mixture. A solution con- * I 



taining the above substances was inoculated with j % .^^ 



a small portion of soil. Owing to the nature of 9 ~ 1 ^ 

 the medium the other soil bacteria were not able ^ I 



to effect multiplication. When growth had taken FlG 10 5._ciostridium 

 place, plate-cultures were made in the usual way, 

 inoculations being taken from one of the colonies that had developed 

 on the plates. 



Clostridium Pastorianum forms rods 1-2-1-3 //, thick and 1-5-2-0 ft 

 long. It forms endospores and excretes butyric acid. The name 

 " Clostridium " was given to it on account of the peculiar form which 

 the individuals assume when they form endospores. When not forming 

 spores the individuals cannot be distinguished from the members of the 

 genus Bacillus. Another characteristic feature is that the action of 

 iodine produces a peculiar violet-brown colouration of the individuals, 



FIG. 106. Clostridium Pastorianum. FIG. 107. Clostridium Pastorianum. 



In sporogenous condition. Germination of spore. 



this also being a characteristic of the other butyric acid bacteria. This 

 organism grows best when deprived of oxygen, though capable of 

 growth when oxygen is present. The ripe spores are 1 '6 /x long and 

 1'3 //, broad, and often lie in a roughly triangular covering (Fig. 106). 

 The germination is polar (Fig. 107). In one experiment, in which a 

 litre of nutrient solution containing no nitrogen was inoculated with 



