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Essential oil, also defined as essence, volatile oil, etheric oil or aetheroleum, is a complex mixture of volatile constituents biosynthesised by living organisms. Essential oils can be liberated from their matrix by water, steam and dry distillation, or expression in the case of citrus fruits [1-5]. Their occurrence and function in nature is still a question and the subject of ongoing research. However, there is evidence that organisms produce essential oils for defence, signalling or as part of their secondary metabolism.As a consequence essential oils comprise an important bioresource for renewable natural products [1-25].

Thymus is a perennial herb of the Labiatae family (Fig. 1A) and can grow to a height of some 30 cm. There are more than 100 varieties of Thymus distributed throughout Europe. It has dark green leaves and light mauve-pink flowers in early summer. Thyme is a good rock-garden plant, as it spreads easily. It originates on the coast of the Mediterranean Sea and in the region of Asia Minor. Now it is cultivated in numerous European countries, Russia, Canada, America, and some of the North African countries. Thyme is one of the great culinary and medicinal herbs, and is a classic constituent of a bouquet garni (Fleming 1990), but only a few species are used in the kitchen, e.g., common thyme (T. vulgaris) and lemon thyme (T. citriodorus.) The essential oil (Marhuenda et al. 1988; Blazquez and Zafra-Polo 1989) of thyme has also been reported to be effective in food preservatives, antioxidants (Farag et al. 1990), antimicrobials (Morris et al. 1979), breath-freshen- ing dentifrices (Ishikawa and Shibuya 1989), medical disinfectants, etc. In general, the dried leaves contain 0.7–2.5% dry wt. of the essential oil. The major components are thymol (15–62%), carvacrol (2–44%), p-cymene, and γ-terpinene. The typical chemical structure is shown in Fig. 2.

Thymus vulgaris L. or common thyme belongs to the family Labiatae = Lamiaceae. Jalas (1972) in Flora Europaea, gives 66 species of Thymus, divided into two subgenera, Coridothymus, with only one species, and Thymus with 65 species. Thymus vulgaris belongs to the sixth section of Thymus. Classification of this species is difficult and therefore methods of comparative enzyme study may be useful for identification (Elena-Rossello 1979). The variation in T. vulgaris is caused not only by genetic constitution, but also depends on ecological and environmental conditions. All of these factors influence the yield of volatile oil produced by thyme plants and the composition of the oil, which is very important.

Aiye, Folium Artemisiae argyi, is the dry leaf of Artemisia argyi Levl. et Vant. (Asteraceae) collected in summer before the plant blooms. It is listed officially in the Chinese Pharmacopoeia and recommended for use as an analgesic and hemostatic.

Yinchen, Herba Artemisiae scopariae is the dry young sprout of Artemisia scoparia Waldst. et Kit. or A. capillaris Thunb. (Asteraceae) collected in spring when the sprouts have reached a height of 6–10 cm. It is listed officially in the Chinese Pharmacopoeia and used mainly as a choleretic, antiinflammatory, and diuretic agent in the treatment of epidemic hepatitis.

Nanheshi, Fructus Carotae, is the dry ripe fruits of Daucus carota L. (Apiaceae) collected in the fall when the fruits have become ripe. It is officially listed in the Chinese Pharmacopoeia and used in traditional Chinese medicine mainly as an anthelmintic.

Concurrent with the development of commercial gas chromatography in the late 1950’s and early 1960’s, botanists and chemists began to realize the value of a “new” suite of characters for the analysis and classification of plants — the terpenoids. Not only are the terpenoids under strong genetic control (Irving and Adams 1973), but the use of electronic digital integrators give quantitative characters amenable to multi-variate and geographic analyses (Adams 1970a; Adams 1972a). Given this “new” suite of quantitative chemical characters, the field of terpenoid chemosystematics rapidly expanded during the next two decades. The purpose of this chapter is to orient the novice to some of the basic procedures of analyses of terpenoids (particularly steam volatile components) and to show the utility of these compounds for the analyses of patterns of variation within and among forest tree taxa.

Essential oils are mixtures of many substances, predominantly terpenes (ea. 90%) and phenylpropane derivatives. Other components include simple phenols, sulphurcontaining compounds (mustard oils), methyl anthranilate and coumarins.