DIGESTION 255 
suspected, it is only within recent years that it has been 
demonstrated. Like the decomposition which is brought 
about by myrosin, the splitting up of the sugar is apparently 
not a process of hydrolysis. It may be expressed by the 
following equation : 
C,H,,0, = 2CO, + 2CH,CH,OH. 
In the reaction the sugar is decomposed, alcohol is formed 
and carbon dioxide given off. 
This enzyme, which has been called zymase, has been 
proved to exist not only in yeast, but in certain fruits, being 
formed there when the fruits are kept in an atmosphere 
which contains no oxygen. 
The physiological explanation of this observation will 
be discussed more fully in a subsequent chapter. 
There are other enzymes with a more restricted distri- 
bution, about whose value to the plant little or nothing is 
known at present. The cells of a particular microscopic 
organism, known as Micrococcus wree, decompose urea 
with the formation of ammonium carbonate, and an 
enzyme, urease, having the same power, can be extracted 
from them. Many enzymes can be prepared from bacteria, 
which set up various changes in proteins, some resulting in 
the formation of peptone, and others producing toxic sub- 
stances. Many bacteria excrete a variety of diastase. 
Another class of enzymes has recently been discovered 
which do not apparently take any part in digestion, but 
which may be briefly alluded to here. They set up a 
process of oxidation in the substances they attack, and have 
consequently been named oxidases. They are apparently 
very widely distributed, and perform very various functions, 
being often concerned in bringing about the presence of 
particular colouring matters. They occur very prominently 
in Fungi, but are by no means confined to them. They 
have not at present been very fully studied from the point 
of view of their utility to the plants which secrete them. 
The conversion of zymogens into enzymes is much 
