DRUG PLANTS 



yii 



tory was not very prevalent until almost thirty years later. It 

 soon spread over Europe, Asia, and Africa, and today its culti- 

 vation and use are world wide (fig. 245). Tobacco contains a 

 narcotic substance known as nicotine, which makes its way into 

 the human system by way of the mucous membranes of the 

 respiratory tract. Nicotine is extracted from tobacco stems and 



—7 ^^--- — ^-;;;::^^^Si^^ 



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Fig. 245. — Distribution of acreage devoted to growing tobacco 



in 1930. 



widely used as a contact insecticide, since it is powerful enough, 

 even in dilute solutions, to kill insects upon contact. 



Many types of tobacco are grown, all of which differ in their 

 soil, moisture, and temperature requirements. Considerable care 

 is necessary to establish and mature a tobacco crop. After the 

 leaves, or entire plants are harvested, they are hung in sheds to 

 cure for three to six weeks. Then they are piled and allowed to 

 undergo another curing period of about six weeks after which 



