100 THE VEGETATIVE FUNCTIONS OF PLANTS 



that is active in converting sugar into carbon dioxide and 

 alcohol is a zymase, called alcoholase. 



101. How Enzymes Work. It has previously been 

 stated that enzymes have the ability to cause changes in 

 other substances without themselves being altered or 

 consumed thereby. The mystery of this fact has never 

 been fully explained, but the simile used by Huxley helps 

 us to form a crude mental picture of the process. "There 

 can be no doubt," says Huxley, "that the constituent 

 elements of fully 98 per cent, of the sugar which has 

 vanished during fermentation have simply undergone 

 rearrangement; like the soldiers of a brigade, who at the 

 word of command divide themselves into the independent 

 regiments to which they belong. The brigade is sugar, 

 the regiments are carbonic acid, succinic acid, alcohol, 

 and glycerine." We may add that the commanding 

 officer is the enzyme, secreted by the yeast. 



102. Many Kinds of Enzymes. Two kinds of enzymes 

 have just been mentioned that which converts starch 

 to sugar (diastase), and that which causes alcoholic 

 fermentation (alcoholase). In our own bodies we are 

 familiar with the ptyalin, or "animal diastase," of the 

 saliva, which can also convert starch to sugar, the pepsin 

 of the gastric juice, which can change the insoluble pro- 

 teins of meat into soluble proteins, the pancreatic juice, 

 and others. Among plants is found cytase, which can 

 liquefy the cellulose of cell-walls. It is by this means that 

 the delicate threads of fungi which grow on trees can pene- 

 trate the hard, solid wood. The enzyme, secreted by the 

 fungus, softens and liquefies the wood, and the delicate 

 fungal thread may then penetrate with ease. When 

 leaves fall in the autumn, the final stage in the process is 



