GENERAL MORPHOLOGY AND PHYSIOLOGY. 139 



Circumstances affecting their growth. 



Nature of the Soil. Though we know the 

 elements necessary, we are, nevertheless, as yet 

 unable to provide a pabulum suitable for all kinds 

 of bacteria. Thus we are quite unable to cul- 

 tivate some species artificially. Others will only 

 grow upon blood-serum. Many grow upon nutri- 

 ent gelatine ; but some species only if it be acid 

 or alkaline respectively. Whether in the latter 

 case this is due purely to th reaction or to the 

 presence of the particular ingredients is an un- 

 settled point. Though the comma bacillus of Koch, 

 like the majority of organisms, grows best on an 

 alkaline medium, yet the surface of a potato is 

 acid, and on this it is well known to flourish at 

 the temperature of the blood. 



Effect of temperature.- In their behaviour to- 

 wards temperature bacteria vary considerably, but 

 still for the majority we may distinguish a maximu'/n, 

 optimum, and minimum temperature. 



Many grow best at the temperature of the blood, 

 and hence the value of nutrient agar-agar, which 

 is not liquefied at 37 C. The tubercle bacillus 

 will only grow at a temperature varying between 

 30 and 41 C. On the other hand, many forms 

 grow between the limits of 5 and 45 C. At 

 these temperatures their functional activity is 

 paralysed, but they are not destroyed, for by re- 

 moval to favourable conditions they spring again 

 to life. Bacteria seem to have a special power 



