ESSENTIAL OILS ' 399 



The universal occurrence of the lecithins and similar phospholipids in 

 plant cells suggest that they are in some way essential to the maintenance of 

 living protoplasm. According to Jordan and Chibnall (1933) some phos- 

 pholipids serve as reserve foods. A number of roles have been ascribed to 

 the lecithins, but there is very little experimental justification for any of 

 the assumptions which have been made regarding their physiological signifi- 

 cance. Lecithins are excellent emulsifiers and for this reason have often been 

 assumed to have an important influence on protoplasmic organization and 

 permeability phenomena. Phospholipids seem to have important influences 

 upon the action of oxidase systems in plants and hence are supposed to have 

 efiects on the process of respiration. Lecithins are also supposed to have 

 some relation to the formation of oils in plant cells, as described in Chap. 

 XXV. 



Essential Oils. — The usual first step in any analysis of plant tissues 

 for lipids is to dissolve them out of the dried and ground tissues with ether. 

 The resulting "ether extract" contains a number of other types of com- 

 pounds in addition to the lipids. Among these are the essential oils and 

 resins. These compounds are chemically quite different from the fats, but 

 because of their occurrence in ether extracts it has become customary to dis- 

 cuss them in connection with the fats. Most of these compounds are pungent 

 aromatic substances. They are responsible for many of the distinctive odors 

 and flavors of plants. 



The essential oils are not oils at all in the chemical sense of the word. 

 They fall into two groups which are quite distinct chemically. The first of 

 these consists of the terpcnes which are hydrocarbons of the general formula 

 CioHi6> of which a large number of isomers exist. A large proportion of 

 the oils of turpentine, lemon, pennyroyal, bergamot, etc., consists of terpenes. 

 Essential oils of the second group all contain oxygen, and some contain sul- 

 fur. They are complex alcohols, aldehydes, and ketones, many of them 

 being phenol derivatives. Menthol, camphor oil, thymol, and vanillin (the 

 flavoring principle of the vanilla bean) are examples. Oil of mustard and 

 oil of garlic are sulfur-containing essential oils. The latter is responsible for 

 the distinctive odor and flavor of onions, garlic, and radishes. 



The resins are apparently oxidation products of the terpenes, but little 

 is known of their exact chemical constitution. In addition to the hard resins, 

 Canada balsam and crude turpentine are generally classed in this group of 

 substances. 



The role of the essential oils and resins in plants, if any, is unknown. 

 They are probably by-products of the metabolic activities of the plant. 



