METABOLISM 261 
substances which are useless to them or which may even 
be deleterious. There are numerous products which 
come under this category, but from the mode of construc- 
tion of the body of the plant they are not cast off as they 
would be from the animal organism under similar conditions. 
Instead of being eliminated entirely they are only removed 
to such localities as ensure their being withdrawn from 
the spheres of vital activity. They are generally deposited 
in such regions as leaves which are about to be shed, or 
the bark of trees, which is a collection mainly of dead 
matter; or they may be stored away in special cells, or in 
cell-walls, or intercellular passages, or elsewhere. These 
bodies really correspond to excreta, and the processes of 
their formation and deposition are called processes of 
excretion. 
Most of the katabolic constructive processes are directly 
applied to the production of substances which are of great 
use to the plant. Emanating as these do directly from the 
protoplasm, their formation is generally termed secretion. 
Though they originate, however, directly in and from 
the living substance, the latter does not always present 
them in the form in which they are found in the adult 
plant-body, for various changes both of the mature of oxida- 
tion and reduction may take place in them after they have 
been secreted. 
The processes included under the general term katabolism 
are thus seen to be very varied. During the course of such 
changes many substances are frequently formed which 
seem to have no direct bearing on the vital processes, and 
whose meaning is still obscure. These are often spoken of 
as the bye-products of metabolism. 
We may now pass to consider in some detail some of 
the more prominent processes of secretion. 
For many reasons the formation of such enzymes as are 
used during digestion may be regarded as the most typical 
of these. A cell which is about to secrete is generally 
found to be filled with colourless hyaline protoplasm in 
