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 My ' co- 

 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 



