FERMENTATION 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. Ifa 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 
ig 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. 
