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the following comes from National Association for Holistic Aromatherapy
What are Essential Oils?
Essential Oils Defined
The term “essential oil” is a contraction of the original “quintessential oil.” This stems from the Aristotelian idea that matter is composed of four elements, namely, fire, air, earth, and water. The fifth element, or quintessence, was then considered to be spirit or life force. Distillation and evaporation were thought to be processes of removing the spirit from the plant and this is also reflected in our language since the term “spirits” is used to describe distilled alcoholic beverages such as brandy, whiskey, and eau de vie. The last of these again shows reference to the concept of removing the life force from the plant. Nowadays, of course, we know that, far from being spirit, essential oils are physical in nature and composed of complex mixtures of chemicals.1
The International Organization for Standardization (ISO) in their Vocabulary of Natural Materials (ISO/D1S9235.2) defines an essential oil as a product made by distillation with either water or steam or by mechanical processing of citrus rinds or by dry distillation of natural materials. Following the distillation, the essential oil is physically separated from the water phase.
According to Dr. Brian Lawrence “for an essential oil to be a true essential oil, it must be isolated by physical means only. The physical methods used are distillation (steam, steam/water and water) or expression (also known as cold pressing, a unique feature for citrus peel oils). There is one other method of oil isolation specific to a very limited number of essential oil plants. This is a maceration/distillation. In the process, the plant material is macerated in warm water to release the enzyme-bound essential oil. Examples of oils produced by maceration are onion, garlic, wintergreen, bitter almond, etc.”.2
NAHA explores different methods of extracting essential oils here.
For now, lets explore the biological role of essential oils within aromatic and medicinal plants.
The Biological Role Of Essential Oils Within Plants
While essential oils are in the plant, they are constantly changing their chemical composition, helping the plant to adapt to the ever-changing internal and external environment. Recent scientific research has shown that plants produce essential oils for a variety of purposes including:
To attract pollinators and dispersal agents
Insects have been pollinating flowers for over 200 million years. Insects, like humans, are attracted to specific plants for one of three possible reasons: its aroma, its color, or its morphology or physical structure. Scent appears to be more ancient than flower color as an attractant to insects.3 Various insects, including bees, butterflies, and even beetles, are known to be attracted by the aroma of a plant.4
To play a role in allelopathy, a type of plant-to-plant competition
Allelopathy occurs when a plant releases chemicals to prevent competing vegetation from growing within its area or zone. An often-cited example is in southern California, home to the dominant shrubs Salvia leucophylla (sage bush) and Artemisia californica (a type of sage). Both species release allelopathic terpenoids, eucalyptol and camphor, into the surrounding area, which effectively prevents other plant species from growing around them. This is allelopathy. Chemicals that deter competing growth (terpenes, for example) are referred to as allelochemics.
To serve as defense compounds against insects and other animals
Plants, like other living things, need to protect themselves from various types of predators. Plants use terpenoid compounds to deter insects and other animals from approaching them. Shawe pointed out that “insects are very rarely found on peppermint plants and the presence of linalol in the peel of citrus fruits confers resistance to attack by the Caribbean fruit fly.”5 The Douglas fir tree releases a complex mixture of volatile oils, or terpenes, from their needles to defend against the spruce budworm. Even more fascinating is that the Douglas fir trees “will vary the composition and production of terpenes each year thus decreasing the ability of the budworm to develop widespread immunity to specific compounds.6
To protect the plant by their antifungal and antibacterial nature
Resins and complex combinations of terpenes are released by some plants and trees, such as evergreens, to act as antimicrobial, antifungal, and antibacterial agents against a wide range of organisms that may threaten the survival of the plants. Compounds such as sesquiterpene lactones found in plants such as feverfew, yarrow, and blessed thistle, have been found to play a strong antimicrobial role as well as a protective role from herbivores.
The Storage Of Essential Oil Within The Plant
Plants store essential oils either in external secretory structures, which are found on the surface of the plant, or internal secretory structures, which are found inside the plant material. Usually with plants having external secretory structure, you just have to lightly touch them and you will notice an aroma imparted to your skin. With plants having internal secretory structures, you may need to break the leaf or seed in order to get to the aroma/essential oil.
External Secretory Structures
External secretory structures in plants are called glandular trichomes. They can be found on the surface of the plant (such as herbaceous leaves) and are thought to be responsible for the production of chemicals that deter or attract pests or pollinators. Glandular trichomes are most commonly found in the Lamiaceae (syn. Labiatae) family. The oil storage capacity varies from species to species and also between trichomes. Biochemical experiments have shown that these volatile oils are synthesized by highly refined enzyme reactions taking place within the plant.
Common essential oils that have glandular trichomes: Basil, Lavender, Marjoram, Melissa, Oregano, Peppermint, Rosemary, and Spearmint
Internal Secretory Structures
Secretory cavities and ducts
Secretory cavities and ducts consist of large, intercellular spaces that are formed either by the separation of the walls of neighboring cells, or by the disintegration of cells.7 Cavities occur as spherical spaces and are most commonly found in the Myrtaceae and Rutaceae families. Ducts are more elongated spaces and are most commonly seen in the Asteraceae (syn. Compositae), Pinaceae, Apiaceae (syn. Umbelliferae), and Coniferae families.
Common essential oils with secretory cavities:
Citrus oils: Bergamot, Grapefruit, Lemon, Lime, Orange, and Tangerine; Eucalyptus species; Clove bud; and Resin trees: Benzoin, Frankincense, and Myrrh
Common essential oils with secretory ducts: Angelica, Caraway, Carrot seed, Dill, Fennel, Fir, Cedar, Pine, Spruce, Juniper, and Cypress
Essential oil cells
Essential oil cells are found within the plant tissue and are unique from other cells in content and size. They can often be found throughout the plant and are most commonly seen in the Lauraceae, Piperaceae, Gramineae, and Zingiberaceae families.
Common essential oils with cells: Bay Laurel, Black pepper, Cardamom, Cinnamon, Citronella, Ginger, Lemongrass, Nutmeg, Palmarosa, and Patchouli
Read more from National Association for Holistic Aromatherapy at the link below
1 Sell, Charles. (2010). Chapter 5: The Chemistry of Essential Oils. (Can Baser K H, and Buchbauer G. Editors) in the book Handbook of essential oils : science, technology, and applications, (pp. 121-150). Boca Raton, FL: CRC Press, Taylor & Francis Group.
2 Lawrence, B. (2000). Essential Oils: From Agriculture to Chemistry. NAHA’s World of Aromatherapy III Conference Proceedings, pp. 8–26.
3 Shawe, K. (1996). The Biological Role of Essential Oils, Aromatherapy Quarterly, 50, 23-27.
5 Shawe, K. (1996). The Biological Role of Essential Oils, Aromatherapy Quarterly, 50, 23-27.
6 Buhner, S. (2002). The Lost Language of Plants. White River Junction, Vermont: Chelsea Green Publishing.
7 Svoboda, K. (1996). The Biology of Fragrance. Aromatherapy Quarterly, 49, 25-28.