DIGESTION 251 
their destruction. They are often injuriously affected by 
neutral salts, alkalies, or acids, though in this respect there 
exists considerable diversity throughout the group. 
The enzymes are manufactured by the protoplasm of 
the various cells in which they occur, being produced from 
its own substance, in a manner somewhat similar to that 
of the formation of the cell-wall. Usually their presence 
is accompanied by a marked granularity of the protoplasm, 
due to the formation in it of an antecedent substance, known 
as a zymogen, which is readily converted into the enzyme. 
This granularity does not, however, always occur, though 
we have reason to suppose that the secretion of the enzyme 
always takes place by successive stages. The zymogen 
has not, however, been definitely detected in all cases. 
We find various degrees of completeness of differentia- 
tion of the cells which produce these enzymes. In the 
simplest cases, such as the mesophyll of the leaves of most 
plants, or the great majority of seeds, or the tubers of the 
potato, the enzyme is found in all the cells which contain 
the reserve materials, so that a rapid transformation of 
the latter is readily possible. In the Horse-radish and 
many allied plants the cells which secrete the enzyme do 
not themselves contain any reserve materials, but are 
situated among those which do, so that the enzyme has to 
pass from the seat of its formation to other cells in order 
to discharge its function. This is a very slow and gradual 
process, and is probably carried out through the agency of 
the delicate filaments of protoplasm which extend through 
the cell-walls, for enzymes are not capable of dialysing 
through a membrane. 
The occurrence of guch cells, which are apparently set 
apart especially for the secretion of an enzyme, gives us, 
as it were, the starting points of the special structures 
known as glands, whose function is similar, but whose 
structure is more complex. In some of the plants belong- 
ing to the natural orders Capparidaceew and Tropcolaceee, 
the glandular cell divides several times to form a little mass 
