THE OXIDASES IN RELATION TO PLANT PATHOLOGY 301 



substance, to which the colour of the infusion is due. The 

 pungency, on the contrary, is dependent on the amount of 

 uiaoxidized tannin, while the flavour is caused mainly by 

 an essential oil. By carefully regulating the different pro- 

 cesses of withering, rolling, and oxidation, the qualities of 

 the tea may be altered within limits. All the operations 

 involved must be carried out with the utmost cleanliness 

 to avoid bacterial contamination as far as possible, as such 

 gives rise to sourness, rendering the tea unfit for con- 

 sumption. 



In the procuring of cocoa beans, too, fermentation pro- 

 cesses are involved which loosen the seeds in the fruit. In 

 these stages yeasts and acetic-acid-producing bacteria are 

 active. Oxidases are also at work both during the fer- 

 mentation and subsequent drying, as shown by Loew, the 

 change from the violet colour of the fresh bean to a deep 

 brown being due to their agency. 



TOBACCO TRADE. 



The curing of tobacco again involves a fermentation. 

 The leaves, after a preliminary withering, are " sweated " 

 in moderate-sized heaps, and fermented in very large heaps 

 containing many tons. It was at one time thought that 

 this was a bacterial process, but, owing to the work of Loew 

 and other American chemists, it has been shown to be 

 mainly one of respiration of starch, sugars, and tannin, 

 brought about by oxidases in conjunction with hydrolytic 

 enzymes. Boekhout and De Vries (1909) are, however, of 

 the opinion that some of these changes are due to oxida- 

 tion without the intervention of an enzyme. 



TIMBER TRADE. 



Another instance of the importance of oxidases in com- 

 merce is that of the blemish known as " sap stain " in 



