Botany is a subdivision of Biology – the study of life. Specifically, botany is the scientific study of plant life – it is at the forefront of everything that affects human life and includes every aspect of plant life from origins, climate, habitat, migration, reproduction, chemical content of both the plant and volatile oil, to a plant’s historical names. Humans and animals have had a deep and abiding relationship with plants for millennia. They are essential for both human and animal survival, as a primary source of food, together with clothing, shelter, medicine and fuel. They provide oxygen in the earth’s atmosphere which is vital to life, remove excess carbon dioxide and help to reduce global warming.
Plants own needs are comparatively simple: carbon dioxide, water, sunlight, a few essential minerals, protection from predators and extreme weather. From these few ingredients, they can photosynthesise their own energy and create immensely complex structures, both physically and chemically.
Simplistically, plants create two types of metabolites to sustain life:
Primary metabolites are chemicals that are essential for life i.e, growth, flowering, fertilisation and reproduction. They are formed from the first product of photosynthesis, i.e. glucose. Fixed oils are primary metabolites, in that they are produced by the plant as an energy source for young seedlings that have germinated.
If primary metabolites are the basics for life; secondary metabolites are in the ‘luxury goods’ section. Secondary metabolites are much more complex chemicals – they are more costly for the plant to produce in terms of resources and energy. The full purpose of secondary metabolites has yet to be fully understood, but they are mainly to do with higher processes, such as defence against predators, bacterial/fungal infections, aid pollination and healing, etc. Some plants prefer to protect themselves by developing defensive structures such as spikes or thorns, but many more prefer to produce ‘chemical weapons’, e.g. β-farnesene is an alarm signal for aphids.
Essential oils are secondary metabolites, since they are not necessary for the primary/vital cellular functioning of the plant. However, they are responsible for giving the plant its aroma and flavour, both of which can have a profound effect upon people and animals.
Essential oils can be harvested from every part of plants, including the leaves, roots, stem, fruit, seeds, roots, rhizome, flowers, buds, etc. But, only about 15% of flowering plants contain essential oils and these are stored in various specialised secretory structures within the plants. The type of structure is generally family or species specific.
Different types of plant storage cells produce different types of oil, which helps to explain why oil produced from the same plant can be chemically different depending on which part it has been extracted from, e.g. bitter orange produces different oils from the peel, flowers (neroli) and leaves/twigs (petitgrain).
Secretory structures include the following:
Secretory cells - These are specialised single cell structures within plant tissue that differ from the cells adjacent to them, both in content and size.
Secretory cavities - can be found throughout the entire plant and are large intercellular spaces. They are formed when essential oils are deposited between the walls of cells, or if the walls of several cells collapse and dissolve. The cavities left are spherical in shape and mainly found in the Myrtaceae, Styraceae families (in the bark), and Rutaceae family (‘cavities’ in leaves and fruit, or sometimes called ‘sacs’ in the fruit).
Secretory ducts and glands - Ducts are elongated cavities within plant cells that branch to create a network from the roots, to the stem, leaves, flowers and fruits. The cavities are formed by special cells that also divide to create secretory epithelial cells which secrete oils. The oils fill the cavities and the cavities join up to form ducts. These ducts/glands are generally found in plants from the Apiaceae, Asteraceae, Burseraceae, Pinaceae, and Cupressaceae families. The Cupressaceae and Pinaceae families commonly have resin ducts, both in the needles and the wood.
Glandular Trychomes - The surface of many plants are pubescent, ie covered with fine hairs (called ‘trychomes’). Trychomes are modified epidermal hairs which, in some cases, cover the entire plant, and in other cases, just specific parts such as the leaves, stems or calyx of the flower – they give the plant a ‘furry’ feel. Not all hairs are designed to excrete essential oil, but in those that are, special secretory cells can be found that have developed from a single epidermal cell that has a large nucleus and dense cytoplasm. Families that most commonly have trychomes are: Lamiaceae, Verbenaceae, Asteraceae and Geraniaceae.
Epidermal Cells - Essential oils found within flowers, e.g. rose, jasmine, etc. generally diffuse through the cytoplasm and walls of epidermal cells, finding their way to the outside of the plant. The yield of oil from epidermal cells is usually very low.
The full role of essential oils in plants is still not completely understood but regarded to include:
- Repelling bacterial, viral or fungal invasion
- Defence against predatory animals or insects
- Wound healing/antiseptic
- A source of energy
- Attraction of suitable pollinators
- A form of hormone regulation
- Survival in difficult conditions
- Allelopathy (i.e., compounds that prevent the growth of nearby competing vegetation by inhibiting germination).
Many factors influence the production of essential oils within the plant. The French often use the word ‘terroir’ when summarising the effects of growing conditions on a plant and the oil it produces. These variations in the growing conditions of plants can greatly affect the chemistry, yield and quality of any essential oil produced. From an aromatherapy point of view, this is all part of the dynamic healing ability of essential oils. So, the plant’s habitat is fundamental in the development of the therapeutic properties of its respective oil(s) and this habitat includes the following considerations:
- The soil and its mineral content;
- The climate, e.g. tropical, temperate, desert, etc.;
- The type of plant species grown;
- The weather conditions of the particular growing season during which the plant developed;
- How much sun/water the plant received.
Taxonomy and Nomenclature
Attempts at naming and classifying plants go back thousands of years and taxonomy is the name given to the science of classification which aims to identify and understand how plants are related to each other. In view of the millions of plant species in the world, it is important to have some systematic, universal way of identifying them. For example, it is important to know the difference between St John’s Wort, Deadly Nightshade and Blackcurrant berries – all look very similar, but one is deadly poisonous. So, it has always been essential to know the names of edible as well as poisonous plants in order to communicate acquired experiences to other members of the family and the tribe.
The use of only ‘common’ names with which to identify plants can be extremely confusing and dangerous for a variety of reasons:
- a plant may have one or more common names originating from English folklore;
- plants may also have a regional name, e.g. sticky buds vs cleavers;
- or, may share the same name with a similar, but fundamentally different plant;
- some plants can be known by different names in different countries, making accurate identification virtually impossible.
Once taxonomy has grouped and classified a plant, botanical nomenclature attempts to provide scientifically meaningful names for individual plants. The language that forms the basis of botanical nomenclature is Latin.
Carl Linnaeus (1707-1778) a Swedish botanist, physician and zoologist created a binomial (two-name) system, using Latin and Greek words that were both simple and short.
The first word in the name is a noun that identifies the plant’s genus, or generic name, the second word identifies the plant’s species, or specific name, and is descriptive, identifying something distinctive about the plant. The species name can be highly descriptive, revealing much about the plant such as:
- Origin – atlantica, mexicana, zeylanicum
- Growing habitat – flexuousus
- Colour of its flowers or leaves – viridiflora or aurantifolia
- Purpose - vinifera (wine-bearing) and Vitis (vine), i.e. Grapeseed.
NB: It is sometimes helpful to think of the genus as a surname and the species as a Christian name, e.g. Jones, Sarah.
A large number of both genera and species names are followed by a capital ‘L’, e.g. Thymus vulgaris L. This indicates that it was ‘L’innaeus who first classified them. As new species are discovered, or old ones re-classified, the name of the botanist who most recently created the name is added in an abbreviated form after the end of the binomial plant name.
A lot can be learned about a plant from its botany, making the study of essential oils even more interesting and rewarding.