1 The genesis of essential oils
Aromatherapy is the use of essential oils, all of which are derived from plants. Anyone wishing to practise aromatherapy must gain as full an understanding of the plants concerned as possible, so that the oils can be used knowledgeably to their best effect. This chapter enables the practitioner to do this, looking beyond the oil in the glass bottle to the plant from which it was extracted, its growing environment and the family to which it belongs.
To be an effective aromatherapist it is crucial to a good outcome that aromatherapeutic-quality essential oils pertinent to the particular client be employed, and to be able to do this the therapist must be able to discriminate between therapeutic-quality oils and those produced for other industries, which is the overwhelming bulk of essential oils produced. To be able to select such oils is not possible unless the therapist has a basic knowledge of some aspects of botany, and in particular the nomenclature used.
In the early 18th century the identification of plants was in a chaotic state, for example John Tradescant brought spiderwort to England from North America and – including his own name after the fashion of the time – named it Phalangum Ephenerum Virginianum Johannis Tradescanti.
Then along came the Swedish naturalist Carl von Linné or Linnaeus (1707–1778) and changed everything. He devised the binomial system and applied it universally, making the precise nominal identification of plants possible (the spiderworts mentioned above are now known as Tradescantia andersoniana, a simple binomial title which is recognized the world over). Binomial means a two-name system; millions of people are differentiated by a family name and an individual personal name; in similar fashion plant names are made up of a generic name and an individual specific descriptive name. Binomials are written in italics and may be followed by the name (perhaps abbreviated) of one or more persons, e.g. Panax quinquefolius L: the L stands for Linnaeus, the author of this name for American ginseng. Sometimes there is a double citation (a second botanist) and this means that the plant has been reclassified, the original author being put first, in parentheses; although not essential, this does give an abbreviated bibliographical reference. Over the years the Linnaean system of classifying organisms in groups according to their similarities has been subject to much modification but is still at the core of the international taxonomic system used today.
What is taxonomy? It is a study devoted to producing a system of classification of organisms which best reflects the totality of their similarities and differences. The word taxonomy comes from two Greek words (taxis – arrangement and nomia – method). Major taxonomic groups of the plant kingdom include categories as follows, and several subgroups:
• 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.
• 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.
• 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.
When procuring and prescribing essential oils therapists must take care to identify precisely the plants from which they are derived, and this means giving not only the generic and specific names but also specifying, where necessary, the chemotype, variety, etc.
Note on pronunciation: aromatherapists are sometimes worried about how to pronounce the Latinized names, but there are no strict rules and almost anything goes! The same names are used throughout the world, but there is a wide variation in pronunciation from country to country, and indeed by individuals within a country.
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.
Photosynthesis is the process by which green plants use the electromagnetic energy of sunlight, absorbed by the chlorophyll in the plant, to drive a series of chemical reactions leading to the formation of carbohydrates. The plant takes up water and minerals from the soil through its roots and carbon dioxide from the air, mainly through its leaves. This whole process is called photosynthesis, and because it is essential to the life of the plant it is termed primary metabolism. All animals, including humans, depend on this photosynthesis because it is the method by which the basic food, sugar, is created.
During the complex reactions of the first, light reaction stage of photosynthesis, light energy is used to split water (H2O) into oxygen (O2), protons (hydrogen ions H+) and electrons; the oxidation of water gives rise to free oxygen, a waste product for the plant. In the second, dark reaction stage, no light is required and the protons and electrons are used to reduce carbon dioxide, which enters the plant through the stomata, to carbohydrates in the form of simple sugars, providing food for the plant’s growth. A complex series of chemical changes occurs, which can be represented by the equation
(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).
Before seeing how an essential oil comes into being, it is worth reflecting on what value essential oils have for plants. This has been debated for many years and there is as yet no definitive answer. However, conjecture on the subject has thrown up many possible reasons:
• 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 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 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.
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).
Essential oils are synthesized and stored in different sites; they may be found in the leaves, seeds, petals, roots, bark, etc. Sometimes different oils occur in more than one site in a plant; for example, two different oils are produced by the cinnamon tree (bark, leaf), and three different oils by the orange tree (leaf, blossom, peel). The type of secretory structure is a characteristic of a plant family and it is possible to place secretory structures into the following categories:
Chemotype is a term applied to plants of the same genus and species which have the same external appearance but differ, sometimes considerably, in their internal chemical composition. These chemotypes usually occur naturally in plants growing in the wild, and can result partly from cross-pollination. The place and manner of a plant’s growing will also promote internal changes: many essential-oil-bearing plants, e.g. rosemary and thyme, are prone to this kind of change owing to genetic and environmental factors. They become resistant to local pests and diseases and have adapted to make the best use of the soil and other surrounding conditions. Such plants are termed ‘landrace’, and strains which yield specified chemical constituents are sought and selected for propagation by cloning: that is to say, cuttings are taken and then cultivated to produce the specific oils required. Included in this category are the thymes and lavenders flourishing wild on the sunny dry hills of Provence, which are extensively cloned and then grown commercially.