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CHAPTER XII 



ol; the green to yellow and of the yellow to brown start and 

 proceed generally in the same order, commencing at the mar- 

 gin and about midway between the lateral veins. The yellow 

 colour is due to a chemical change of the chlorophyll, while 

 the brown colour is due to the oxidation of several different 

 compounds contained in the cell sap. The brow^i colour shows 

 also in the veins of the leaf, but not the yellow^ since there is 

 too little chlorophyll in the veins. If the leaves are killed too 

 quickly by desiccation, instead of by slow starvation, when the 

 yellow colour is not formed, then the brown colour develops 

 directly upon the still green leaf. The chlorophyll can be 

 extracted by alcohol, which shows that the green is concealed 

 by the brown. Furthermore, when a fresh leaf is dried at a 

 moderate temperature in an air-bath, it will no longer turn 

 yellow when exposed for a considerable time to a moist atmo- 

 sphere, showing that the change from green to yellow in the 

 curing barn is still a process of life, although a pathologic one. 



" Oxidase acts on the chromogenic substances found in the 

 cell sap when the cells die."^ 



Curing, then, consists in subjecting the tobacco to a pro- 

 cess of gradual starvation under proper conditions. The prin- 

 cipal external factors involved being heat and moisture. The 

 method of curing is determined by the soil and climatic con- 

 ditions under which the crop has been grown, and the purpose 

 for which it is intended. The several tobacco-growing 

 countries, therefore, employ methods of curing to meet their 

 market requirements and individual economic conditions. The 

 tabulation given below^ indicates the several methods of curing 

 and the types of tobacco cured by each method. 



