FEKMENTATION 309 



We must not suppose that anaerobic respiration is capable 

 permanently of taking the place of the normal aerobic pro- 

 cess. Though the stoppage of oxygen can be to a certain 

 extent compensated for, the vital mechanism gradually 

 becomes exhausted, and life ceases if the cessation of the 

 supply is prolonged. In the higher plants anaerobic is at 

 the best only capable of supplementing aerobic respiration, 

 and that for but a limited period. The commencing 

 asphyxiation serves as a stimulus to the protoplasm, which 

 responds by setting up the anaerobic changes, but, like all 

 stimulations, the ultimate effect is exhaustion and a failure 

 to continue the response. 



There are other plants, however, which do not require 

 oxygen for their vital processes, and accordingly do not 

 absorb it ; indeed many of them are incapable of carrying 

 on their life in the presence of oxygen. They are of a very 

 humble type, and occur only among the Bacteria and 

 Fungi. An instance may be found in the organisms which 

 induce the formation of butyric acid from sugar or lactic 

 acid. If a few of these are sown in a suitable liquid, and 

 this is then enclosed in a hermetically sealed flask from 

 which free oxygen has been removed, they multiply with 

 extreme rapidity, until indeed either their food supply is 

 exhausted, or the waste products of their metabolism 

 accumulate to an inhibitory extent. If a little free oxygen 

 is admitted their activity ceases and death ensues, or they 

 pass into a resting condition, which lasts as long as oxygen 

 is present. We must not, however, necessarily conclude 

 that their metabolism is of a totally different kind from 

 that of others, but rather that they set up the decomposi- 

 tion and reconstruction of their protoplasm in a different 

 way from those plants which need a supply of oxygen to 

 determine them. 



