DIGESTION 26! 
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 urece, 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. 
Enzymes of another class do not apparently take any 
part in digestion, but 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 
tothem. Others of a similar character act exactly oppositely 
and have been called reductases. None of the members 
of either class have 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 
facilitated by a gentle warmth, particularly when a trace of 
free acid is present. The red rays of light exercise a similar 
influence in some cases. 
The fermentative activity of protoplasm was alluded to 
at the opening of this chapter. The living substance of 
many cells is capable of setting up various fermentative 
decompositions, apparently identical with those that have 
been described. Various cells can convert starch into 
