CLOSTRIDIUM PASTORIANUM 173 



and like the latter substance stains a violet colour with 

 a solution of iodine (Ex. 40). 



Spores arise at the end of the mother-cells, but when 

 fully formed they occupy the middle of the swollen cell, 

 which becomes transformed into a kind of spore capsule. 

 The ripe spores are about 1.6 ^ long and 1.3 //, broad, 

 and readily germinate when placed in a sugar medium 

 under anaerobic conditions. Grown on potato or carrot 

 in a vacuum or in nitrogen, the bacillus produces raised 

 greyish-yellow colonies about I mm. across, and gives 

 rise to a cheesy odour. The individual bacilli are 

 longer and thicker than they are when cultivated in 

 liquid media, and sometimes develop vibrio or coccus-like 

 involution-forms incapable of spore-formation or nitrogen 

 fixation : long sporeless threads are formed in solutions 

 containing peptone, asparagin, or other nitrogenous com- 

 pounds, but these die out rapidly. It does not thrive well 

 on sugar agar medium, and refuses to grow on gelatine. 



C. pastorianum is widely distributed, but is not 

 present in all soils ; it is generally met with in associa- 

 tion with aerobic species. 



Winogradsky found in soils of South Russia and Paris 

 other forms of species of Clostridium with nitrogen-fixing 

 powers. One of these was thicker than C. pastorianum 

 (1.6 to 1.8 thick), with spores 1.9 long and 1.5 thick: 

 they all proved difficult to isolate, and were not further 

 studied. 



Freudenreich also obtained from Swiss soils an 

 organism which closely resembled Winogradsky's Clos- 

 tridium in all its characters, and had the power of 

 fermenting .mannite, which the latter is said not to 

 possess. 



Another nitrogen-fixing species is Clostridium ameri- 



