METABOLISM 265 
which indeed constitute its food in the strict sense of the 
term. What is true of such cells as are actively growing 
and multiplying, which are found, as we have seen, in the 
special growing points or layers, is equally true of all cells 
so long as they are living. In all cases, though growth 
and division may not be evident, we have to do with 
processes of repair of the inevitable wasting of the living 
substance during the operations of its life. The same 
kind of change is evident in all cells, though the immediate 
results of such changes differ according to the part any 
particular cell takes in promoting the well-being of the 
whole organism. 
If we turn from these anabolic processes we find we 
have proceeding, side by side with them, a decomposition 
of the protoplasm, involving a separation from its complex 
molecule of various substances which are of less com- 
plexity than the living material itself. These often, in 
the first instance, include such carbohydrates and nityro- 
genous substances as it made use of in building itself up. 
These can again be used in reconstruction of the proto- 
plasm, or can be further broken down into simpler substances 
still or can be retained unaltered. So long as the proto- 
plasm is living, it is continually undergomg constant 
reconstruction and decomposition. 
Besides initiating those chemical changes in which it 
takes this prominent part, it is also the seat of a large 
number of others into which its own molecule does not 
immediately enter. Processes of both oxidation and reduc- 
tion are continually going on in its substance, in which ave 
involved the various materials which are found there, either 
in solution in the water which saturates it, or in amorphous 
form; substances which have been transported from other 
cells, or have been formed in the processes of the self- 
decomposition of the protoplasm. 
Two classes of enzyme have been discovered which 
may, and probably do, assist in these changes. They are 
the oxidases, to which allusion has been already made, 
