10 THE PRODUCTION OF VOLATILE OILS AND PERFUMERY PLANTS. 



hydrolized during the curing process by plant enzymes or ferments 

 to the compound coniferyl alcohol, which in turn is oxidized by 

 oxydase to vanillin. In this case a characteristic odor is perceptible, 

 yet no volatile oil can be separated from the plant. A fuller dis- 

 cussion of this class of substances will follow. 



With only a few exceptions it may be stated that volatile oils ex- 

 ist in the tissues of a plant as minute globules, sometimes inclosed in 

 cells but in some instances in enlarged cavities so conspicuous as to be 

 seen without the aid of a lens or a microscope. By a careful ex- 

 amination of the leaf of a peppermint plant, especially at the time of 

 blossoming, tiny glistening particles of oil are clearly discernible. 

 The close scrutiny of the peel of a lemon or an orange discloses to 

 view small, circular oil glands under the epidermis, imparting to it 

 much of the characteristic roughened appearance. Such seeds as 

 cloves, fennel, and anise contain oil passages directly below the epi- 

 dermis surrounding the endosperm or embryo of the seeds. 



The volatile oils in plants do not represent simple substances but 

 are complex mixtures of numerous aromatic compounds which possess 

 a definite chemical composition. However complex the composition 

 of an oil may be, usually one constituent seems to impart the char- 

 acteristic odor and stands out conspicuously. Generally this con- 

 stituent attracts attention as the odor bearer of the plant or oil. 



The substances which supply the aroma to plants or to essential 

 oils may be resolved by chemical classification into several groups of 

 organic compounds, namely, hydrocarbons, acids, alcohols, esters, 

 aldehydes, ketones, oxids, phenols, and sulphur compounds. 



Volatile oils with but few exceptions contain constituents which 

 belong to two or more of the above-mentioned groups of organic 

 compounds. Although each of the groups may contribute to the 

 complex odor of a plant or of a volatile oil, usually compounds exist 

 in the oil which seem to the observer to be especially agreeable and 

 fragrant. The bearers of these pleasant odors which are so apparent 

 even in complex mixtures are for the most part either ester-like or 

 alcoholic in character. It is not unusual, howcA^er, that aldehydes, 

 ketones, or phenols play the role of odor bearers in a few oils or |ii 

 plants, as, for example, the principal odorous constituent of lemon 

 oil, which is the aldehyde citral, while the pronounced odor of pen- 

 nyroyal oil is chiefly duo to the ketone pulegone. The strongly aro- 

 matic odor of thyme is attributed to the phenol called thymol, wdiile 

 sulphur compounds are largely responsible for the aroma of the 

 mustard oils. 



Thus it may be perceived that while esters and alcohols impart 

 agreeableness to the majority of oils, there are exceptions, as already 

 stated. Such oils as peppermint, lavender, wormwood, rose, geran- 

 ium, ylang-ylang, orange flower, and numerous others owe their 



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