256 VEGETABLE PHYSIOLOGY 
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 
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 quite recently the 
alcoholic fermentation of sugar was attributed to such an 
action in the yeast-cell, and in the cells of certain 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 Myco- 
derma or Bacterium aceti. Similar protoplasmic action is 
responsible for the production of various acids in the cells 
of the higher plants. The dependence of these fermenta- 
tions 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 
