56 



GENERAL CONDITIONS FOR QROWTH 



OXYGEN SUPPLY. 



Oxygen is one of the constituents of protoplasm and is 

 therefore necessary for all organisms. This does not mean 

 that all organisms must obtain their supply from free oxygen, 

 however, as animals and plants generally do. This fact 

 is well illustrated by the differences among bacteria in this 

 respect. Some bacteria require free oxygen for their growth 

 and are therefore called aerobic bacteria or aerobes (sometimes 

 strict aerobes, though the adjective is unnecessary). Others 

 cannot grow in the presence of free oxygen and are therefore 

 named anaerobic bacteria ov anaerobes (strict is unnecessary). 

 There are still other kinds which may grow either in the 

 presence of free oxygen or in its absence, hence the term 

 facultative anaerobes (usually) is applied to them.i The dis- 

 tinction between facultative aerobe and facultative anaerobe 

 might be made. The former means those which grow best 

 in the absence of free oxygen, though capable of growing in 

 its presence, while the latter term means those which grow 

 best in the presence of free oxygen but are capable of grow- 

 ing in its absence. 



The amount of oxygen in the atmosphere in which an 

 organism grows may be conveniently expressed in terms 

 of the oxygen pressure, i. e., in millimeters of mercury. 



^ It is popularly supposed that in canning fruit, vegetables, meats, etc., 

 all the air must be removed, since the organisms which cause "spoiling" 

 cannot grow in a vacuum. The existence of anaerobic and facultative 

 anaerobic bacteria shows the fallacy of such beliefs. 



