DIGESTION 269 



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 Urea, 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 proteids, 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 studied from the point of view of 

 their utility to the plants which secrete them. 



The conversion of zymogens into enzymes is much 

 facilitated by a gentle warmth, particularly when a trace of 

 free acid is present. The red rays of light exercise a similar 

 influence in some cases. 



The fermentative activity of protoplasm was alluded to 

 at the opening of this chapter. The living substance of 

 many cells is capable of setting up various fermentative 

 decompositions, apparently identical with those that have 

 been described. Various cells can convert starch into 

 sugar, can peptonise proteids, and carry out other digestive 



