1 The genesis of essential oils
Botany for aromatherapists
Taxonomy
In aromatherapy it is sufficient for identification purposes to know:
• The family that the plant belongs to (all family names end in -aceae).
• The genus: generic names are based on structural characteristics and are always written in italics with an initial capital letter and can be used alone.
• The species: these are adjectival, describing the genus, and are never written with a capital letter, even when it is after a person, e.g. smithii: the whole word is in lower-case italics and cannot be used by itself.
However, there are further divisions below this level, such as:
• Subspecies: often denotes a geographic variation of a species.
• Variety: indicates a rank between subspecies and forma. They are named by adding ‘var.’ in Roman font and the italicized variety name, e.g. Citrus aurantium var. amara. The label ‘var.’ is used to indicate a major subdivision of a species, or a variant of horticultural origin or importance (although these are now labelled cultivar). Many names of horticultural origin reflect the historical use of the variety rank.
• Forma: denotes trivial differences.
• Cultivar: indicates a cultivated variety, and a rank known only in horticultural cultivation. These names are non-Latinized and in living languages (usually the name of, or chosen by, the originator, in the following case Monsieur Maillette). They are not italicized, and appear within quotation marks, e.g. Lavandula angustifolia ‘Maillette’.
• Chemotype: indicates visually identical plants but having different, perhaps significantly so, chemical components, resulting in different therapeutic properties. Chemotypes occur naturally in plants grown in the wild, some species throwing up many chemical variations; they can be propagated by cuttings for cultivation and they are named by the abbreviation ‘ct.’ followed by the chemical constituent, e.g. Thymus vulgaris ct. thujanol-4, T. vulgaris ct. geraniol, T. vulgaris ct. carvacrol, etc. Chemotypes are plants that look the same from the outside, but have different chemical constituents inside. (By contrast, phenotypes are plants that look different on the outside but are chemically similar inside.)
• Hybrid: indicates natural or artificially produced crosses between species. The name contains ‘x’ (in Roman font) which means the plant is a hybrid produced by sexual crossing, e.g. Mentha x piperita, which is a cross between Mentha aquatica and Mentha spicata.
The genesis of essential oils
Plants are capable of transforming the electromagnetic rays from the sun into energetic substances including a major group of compounds, the terpenes. According to Harborne (1988) more than 1000 monoterpenes and possibly 3000 sesquiterpenes have so far been identified. The phenylpropenes constitute another much smaller but significant group: they always consist of a 3-carbon side chain having a double bond attached to an aromatic ring. In essential oils most of the components belong either to the terpene group, based on the mevalonic acid pathway, or to the phenylpropene group, formed through the shikimic acid pathway.
Synthesis of volatile oils
(in this example the formation of glucose).
The elements in sugar (carbon, hydrogen and oxygen) are the same as those in essential oils, but differently grouped, and hundreds of chemicals are produced by the decomposition/glycolysis of sugars with aid of enzymes: enzymes are highly specific and assist in only one particular reaction (as they do in humans). Mevalonic acid goes through phosphorylization, decarboxylation and dehydration to become five-carbon isoprene units, which are the basic building blocks for the terpenes found in essential oils (Fig. 1.1). The phenols are arrived at via a different route – the shikimic acid pathway.
Chemicals produced by plants that do not have an obvious value to the producer plant are known as secondary metabolites; the array of secondary metabolites, which of course includes volatile oils, is enormous (Waterman 1993 p. 31). Secondary metabolism products include alkaloids, bitters, glycosides, gums, mucilages, saponins, steroids, tannins and essential oils, which are not necessary for the vital functions of the plant (see Fig. 1.1), and of these secondary metabolites the essential oils have the greatest commercial significance, being used in many industries (Verlet 1993). Volatile oil secondary metabolites vary widely in chemical structure and their purpose and function in the plant is little understood.
With genetic techniques, it is now possible to intervene in these pathways and change both the quality and the quantity of essential oils – a prospect which brings new dimensions into the natural balance (Svoboda 2003).
Why does a plant contain essential oil?
• To prevent attack by herbivores: both mono- and sesquiterpenes are involved in various ways, such as acting as insect hormones to interfere with the development of the feeding insects, or having a straightforward repellent action. Essential oils and other secondary metabolites can render plant tissue bitter and unpalatable.
• To prevent attack from insects: it has been shown that the number of oil glands in a plant increases when it is under attack by insects (Carlton 1990, Carlton, Gray & Waterman 1992).
• To prevent attack by bacteria, fungi and other microorganisms: there is ample proof available from studies done in vitro on the antifungal and bactericidal properties of herb volatile oils (see section on aromatograms in Ch. 4).
• To aid pollination by attracting bees and other insects such as moths and bats (Harborne 1988).
• To help in the healing of wounds inflicted on the plant itself.
• To act as an energy reserve.
• To help survival in difficult growth conditions: for instance by the production of allelopathic compounds, such as 1,8-cineole and camphor, which are freely given off from the plant and find their way to the soil, where they prevent other plants from growing (Deans & Waterman 1993).
• To prevent dehydration and afford some degree of protection in hot dry climates by surrounding the plant with a haze of volatile oil, thus helping to prevent water loss from its foliage. Leaves with a dense covering of glandular hairs can help trap the water molecules that evaporate through the stomata. One of the oldest plants in the world, the leaves of which can be as much as 10% oil by weight, is the eucalyptus. Living root stock of this plant has been found dating back thousands of years to the Ice Age (Dr Mike Crisp, Australian National Botanic Gardens, unpublished information 1986). The free oil vapour emanating from other ancient plants, e.g. pine trees, can be smelt easily when walking in pine forests on a sunny day.
Secretory structures
Essential oils and their mixtures with resins and gums are commonly found in special secretory structures. Secretory structures in plants are divided into two main types: those occurring on the plant surfaces, which usually secrete substances directly to the outside of the plant (exogenous secretion), and those which occur within the plant body and secrete substances into specialized intercellular spaces (endogenous secretion) (Svoboda 2003).
• Cavities, sacs, oil reservoirs (schizolysigenous)
Essential oils obtained from flowers such as roses are usually not secreted by glandular hairs, but by the actual epidermal cells of the petals. The amount of essential oil in flowers (Rose, Acacia, Jasminum sp.) is very low, usually between 0.02 and 0.08% (v/w).
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