ESSENTIAL OIL AND RESINS 149 



in the different plants. Similarly, mustard oil is not present in 

 mustard seeds as such, but as a glucoside which, when hydrolyzed 

 by the enzyme myrosin which is always present in other cells of 

 the same seeds, yields C 3 H 5 NCS, KHS0 4 , and C 6 Hi 2 O 6 . 



THE RESINS 



The resins were formerly supposed to be the mother sub- 

 stances from which the terpenes are derived. It is now known, 

 however, that they are the oxidation products of the terpenes. 

 Their exact structure is still a matter of some uncertainty, as their 

 peculiar " resinous " character makes them very difficult to study 

 by the usual methods of chemical investigations. 



Resins are divided into two classes: (a) the balsams, and (6) 

 the solid or hard resins. Canada balsam and crude turpentine 

 are familiar examples of the first class. They consist of resinous 

 substances, dissolved in or mixed with fluid terpenes. Ordinary 

 resin, or colophony, consists chiefly of a monobasic acid having the 

 empirical formula C2oHsoO2, known as sylvinic acid, whose exact 

 structure is not known. Its sodium salt is used as the basis for 

 cheap soaps. 



The hard resins are amorphous substances of vitreous charac- 

 ter, which consist of very complex aromatic acids, alcohols, or 

 esters, combined with other complicated structures, known as 

 rescues, whose definite chemical nature is not yet known. Among 

 the hard resins are many substances which are extensively used in 

 the manufacture of varnishes, such as copal, amber, dammar, 

 sandarach, etc. 



There are also resinous substances, such as asafcetida, myrrh, 

 gamboge, etc., which are mixtures of gums (see Chapter VI) and 

 true resins. Some of these have considerable commercial value 

 for medicinal or technical uses, 



PHYSIOLOGICAL USES AND BIOLOGICAL SIGNIFICANCE OF 

 ESSENTIAL OILS 



No theory has yet been advanced concerning the possibility of 

 the use of essential oils and resins by plants in their normal meta- 

 bolic processes. The very great diversity in their chemical nature 

 makes it impossible that they should all be considered as having 



