198 PART III. PHYSIOLOGY. [ 45 



which free oxygen does not enter, provided that suitable material 

 is accessible ; these processes are termed fermentations, and th6 

 plants organised ferments. Thus, Bacteria cause putrefaction and 

 other similar fermentations in the most various organic substances 

 with which they happen to come into contact. Similarly Yeast is 

 the cause of the alcoholic fermentation of sugar, which may be re- 

 presented by the equation 



C 6 H 12 6 = 2C 2 H 6 + 2C0 2 . 



The chief kinds of enzymes which have been found in plants 

 are: 



1. Those that act on carbohydrates, converting the more complex 

 and less soluble carbohydrates into others of simpler composition 

 and greater solubility. 



2. Those that act on fats, decomposing them into glycerin and 

 fatty acid. 



3. Those that act on glucosides, glucose being a constant 

 product. 



4. Those that act on the more complex and less soluble proteids, 

 converting them into others which are more soluble and probably 

 less complex, or decomposing them into non-proteid nitrogenous 

 substances (amides, etc.). 



The chemical action of some of these enzymes are illustrated by the 

 following equations : 



1. Conversion of starch into sugar (amylolytic enzyme, commonly termed 

 diastase) : 



Starch. Maltose. Dextrin. 



3 (C 6 H 10 5 ) + H 3 0= C 12 H 2 ,O n + C 6 H 10 5 



2. Conversion of cane-sugar into grape-sugar (invert enzyme) : 



Cane-sugar, Dextrose. Lsevulose. 

 C 18 H 22 U + H 2 0-C 6 H 12 6 + C 6 H 1S 6 

 8. Action of fat-enzyme : 



OleYn. Oleic acid. Glycerin. 



CjjH^Oe + SHsO- 3C 16 H 34 2 + C 3 H 8 O 3 



It will be noted that, in every case, the action of the enzyme involves 

 the taking up of one or more molecules of water. 



The action of the enzymes which act on proteids (protedytic enzymes) 

 cannot be represented by equations, inasmuch as no formulae for the 

 various proteids have at present been arrived at. It may be generally 

 stated that their effect is, like those of the other forms, to induce de- 

 composition with the assumption of water. The proteolytic enzymes, 

 acting some in an acid medium, others in an alkaline, convert the more 

 complex proteids, such as globulins, into simpler ones, such as peptone ; 



