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 
