262 VEGETABLE PHYSIOLOGY 
sugar, can peptonise proteins, and carry out other digestive 
processes, without the intervention of an enzyme. Though 
this property can easily be proved in the case of cells of 
the higher plants, it is especially prominent in many of the 
more lowly organisms such as the Bacteria. The processes 
of putrefaction generally depend on this property in the 
organisms which bring it about. Till the discovery of 
zymase the alcoholic fermentation of sugar was attributed 
to such an action in the yeast-cell, and in the cells of cer- 
tain ripe fruits under particular conditions, the chief of 
which was the deprivation of oxygen. Such an action 
leads to the formation of acetic acid from alcohol by the 
microbe Mycoderma or Bacteriwm aceti. Similar proto- 
plasmic action is responsible for the production of various 
acids in the cells of the higher plants. The dependence 
of these fermentations on the vital activity of the 
protoplasm is evident from the fact that no enzyme can 
-be extracted from the cells which can set up the particular 
changes in question. 
It is not difficult to prepare the enzymes from the tissues 
in which they work, but it would be extremely rash to 
say that they are in anything like a pure condition when 
obtained. Nor is it easy to say much about the purifica- 
tion, as they are not known except in close connection with 
the substances on which they act, or with the products of 
the decompositions they initiate. There is therefore no 
known test of their purity. 
They can be extracted by treating the tissue, which 
should be very finely divided or ground in a mortar, with 
glycerine, or with a solution of common salt, or with water 
containing a trace of an antiseptic. After a period of ten 
or twelve hours the extract should be strained and subse 
quently filtered, when the enzyme may be precipitated from 
the filtrate by adding strong alcohol. It is very evident 
that this process will not yield it pure, for the solvents 
employed will dissolve many constituents of the tissue 
besides the enzymes, particularly proteins and sugars. 
