CHAPTER XI 

 ESSENTIAL OILS AND RESINS 



INCLUDED in this group are all those substances to which the 

 characteristic odors of plants are due, along with others similar 

 in structure and possessing characteristic resinous properties. 

 They have no such uniformity in composition as is exhibited by 

 the oils which are included among the fats and waxes ; but belong 

 to several widely different chemical groups. Furthermore, there 

 is no sharp dividing line between the essential oils and certain 

 esters of organic acids on the one hand and the fats on the other. 

 For example, if an aromatic fluid essence is a light fluid, non-viscid, 

 and easily volatile, it is usually classed with the organic esters; 

 denser liquid substances, of oily or waxy consistency, and with 

 comparatively slight odor and taste are usually fats, while oils of 

 similar physical properties but possessing strong characteristic 

 odors are classed as essential oils, regardless of their chemical 

 composition. 



Included in this general class are compounds having a great 

 variety of chemical structures; e.g., hydrocarbons, alcohols, 

 phenols, organic sulfides and sulfocyanides, etc. Many of these 

 compounds are crystalline solids at ordinary temperatures, but 

 melt to oily fluids at higher temperatures. The characteristic 

 property which assigns any given plant extract to this group is 

 that it has a strikingly characteristic odor or taste, often accom- 

 panied by some definite physiological effect, or medicinal property. 



These compounds may be either secretions or excretions of 

 plants, sometimes normally present in the healthy tissue, and 

 sometimes produced as the result of injury or disease. 



The essential oils and the resins often occur associated together 

 in the plant; or, the resins may develop from the oily juice 

 of the plant after exposure to the air. 



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