ENZYMES AND THEIR ACTION 459 



plastids of the seedling construct the starch which has been 

 referred to, its formation indicating, as in other oases, a tem- 

 porary surplus of carbohydrate supplies. 



There are other enzymes with a more restricted distribution, 

 about whose value to the plant little or nothing is at present 

 known. The cells of a particular yeast plant, known as Torida 

 UrecE, decompose urea with the formation of ammonium carbo- 

 nate, and an enzyme having the same power can be extracted from 

 them. Many enzymes can be prepared from bacteria, which 

 set up various changes in proteids, partlj' peptonising and partly 

 putrefactive. 



The fermentative activity of the protoplasm was alluded to 

 at the opening of this chapter. One of the most familiar of its 

 manifestations is the production of alcohol from sugar, which is 

 set up by yeast and which goes on not only in the yeast-cell but 

 in the fluid around it. The same transformation takes place in 

 some ripe fruits, the protoplasm of their parenchymatous cells 

 conducting the fermentation. _ Similar changes lead to the 

 formation of acetic acid from alcohol by the fungus Mycoderina 

 aceti, and of other acids in the cells of the higher plants. The 

 dependence of these fermentations upon the vital activity 

 of the protoplasm is evident from the fact that no enzyme can 

 be extracted from any of these cells which can set up the par- 

 ticular 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 their purification, 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 solution of ^comInon salt, or with water containing a trace 

 of an antiseptic. After a period of ten or twelve hours the 

 extract should be strained and subsequently filtered, when the 

 enzyme may be precipitated by strong alcohol. It is very 

 evident that this will not yield it pure, for the solvents em- 

 ployed will dissolve many constituents of the tissue besides the 

 enzymes, particularly proteids and sugars. The former will be 

 thrown down with the enzyme by the alcohol. 



