DIGESTION 255 



suspected, it is only within recent years that it has been 

 demonstrated. Like the decomposition which is brought 

 about by myrosin, the splitting up of the sugar is apparently 

 not a process of hydrolysis. It may be expressed by the 

 following equation : 



C o H i 2 G = 2C0 2 + 2CH 3 CH 2 OH. 



In the reaction the sugar is decomposed, alcohol is formed 

 and carbon dioxide given off. 



This enzyme, which has been called zymase, has been 

 proved to exist not only in yeast, but in certain fruits, being 

 formed there when the fruits are kept in an atmosphere 

 which contains no oxygen. 



The physiological explanation of this observation will 

 be discussed more fully in a subsequent chapter. 



There are other enzymes with a more restricted distri- 

 bution, about whose value to the plant little or nothing is 

 known at present. The cells of a particular microscopic 

 organism, known as Micrococcus urece, decompose urea 

 with the formation of ammonium carbonate, and an 

 enzyme, urease, having the same power, can be extracted 

 from them. Many enzymes can be prepared from bacteria, 

 which set up various changes in proteins, some resulting in 

 the formation of peptone, and others producing toxic sub- 

 stances. Many bacteria excrete a variety of diastase. 



Another class of enzymes has recently been discovered 

 which do not apparently take any part in digestion, but 

 which may be briefly alluded to here. They set up a 

 process of oxidation in the substances they attack, and have 

 consequently been named oxidases. They are apparently 

 very widely distributed, and perform very various functions, 

 being often concerned in bringing about the presence of 

 particular colouring matters. They occur very prominently 

 in Fungi, but are by no means confined to them. They 

 have not at present been very fully studied from the point 

 of view of their utility to the plants which secrete them. 



The conversion of zymogens into enzymes is much 



