RESPIRATION 299 
Turning from the question of respiration to study other 
changes which subserve a similar purpose with regard to 
the local supply of energy, we may first examine such 
processes as are oxidative. In them all we cannot fail to 
mark the activity of the protoplasm in carrying them out. 
The living substance does not, however, act as a general 
oxidising agent, but different protoplasts possess specific 
powers. Certain micro-organisms can cause the oxidation 
of ammonia and the consequent formation of a nitrite ;. 
others can convert the nitrite into a nitrate, but neither 
can do the work of the other. Others have not such 
limited powers ; a certain bacterium can cause the oxidation 
of alcohol to acetic acid, and after the exhaustion of what 
alcohol may be present, can further oxidise the acetic acid 
to carbon dioxide and water. The exact way in which the 
protoplasm acts as a carrier of the oxygen without 
apparently undergoing decomposition is very obscure. It 
may perhaps combine with the oxygen and pass it on to 
these oxidisable substances, acting as a carrier only. 
It has recently been found that besides exerting a direct 
oxidative power, protoplasm can secrete an enzyme, or 
perhaps a variety of enzymes, each with a special peculiarity, 
through whose instrumentality the oxidation is effected. 
These enzymes have been termed oxidasés, and they are 
probably widespread in the vegetable kingdom. A dis- 
cussion of their peculiarities would be beyond the scope of 
this volume, but we may call attention to their general 
features. 
The first one discovered is known as laccase; it has a 
very wide distribution, occurring in the roots, stems, and 
leaves of various plants, and in a very large number of 
fungi. It appears to oxidise various constituents of plants, 
but particularly the colouring matters. Another, known 
as tyrosinase, occurs in other fungi, and oxidises chiefly 
tyrosim. Others oxidise various colouring matters, together 
with tannin. 
Many very complex disturbances set in when a normally 
