FERMENTATION 803 
gradually becomes exhausted, and life ceases if the cessa- 
tion 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. 
