There are certain zones of ambiguity in the botanical classification. Species are relatively clearly defined by their morphological characteristics (appearance) and genetic similarities, but in the species itself there are differences that are not so great that a group of plants can be singled out as a separate species, but are classified in a lower classification: subspecies, variety and form .
These divisions are sometimes vaguely used, and it is known that over time a subspecies acquires the status of a new species, but also vice versa, that a particular species becomes a subspecies of another plant.
How do these differences come about? Evolutionarily, they are most often created by geographical separation. An isolated community of a plant species is in a different environment from another community of the same species, and through gene mutations, different characteristics develop that enable easier survival in the habitat. This process can take a long time, through thousands of years. Sometimes plant communities of two subspecies or varieties are not in isolated areas (although they may be), but may have a contact or even overlapping zone.
What exactly defines a plant community to become a subspecies, variety, or form within a species?
Todd F. Stuessy in his book Plant Taxonomy: The Systematic Evaluation of Comparative Data lists some of the criteria shown in the table.
|Category||Morphological differences||Geographical distribution||Genetic diversity||Possibility of hybridization||Hybrid fertility|
|subspecies||a few clear differences||separate, sometimes with contact zones||pronounced (different genes)||possible with contact zones||markedly reduced|
|variety||one to several clear differences||separate, with overlapping zones||different genes or different gene control||possible in the overlapping area||reduced|
|form||one difference||sporadically||gene control (often one)||always||complete|
Such criteria are often ambiguous. Genetic studies of gene diversity today are one of the useful methods used in defining the exact affiliation of a particular plant community.
Genus and species
It is completely logical that different plant species have different genetic traits, and thus different metabolic pathways in the synthesis of compounds. Therefore, the botanical characteristics of the plant material from which the essential oil is derived should always be well defined. Today, “mint”, “eucalyptus” or “lavender” oil is sold. It is a completely inaccurate name. There are over 700 different species of eucalyptus, of which over twenty species are commercially available as essential oils. They are, logically, of different chemical composition. There are over 40 species in the genus Lavender, and about a dozen different essential oils can be obtained commercially.
Here are two very vivid examples of the importance of the botanical definition of an essential oil.
About ten years ago, a small company that sold essential oils received lavender essential oil on offer at a good price. The owner of the company asked a colleague if it was wise to take such an opportunity. A colleague answered in the affirmative, but the owner asked another question: “it won’t bother you that this lavender is marked as stetchas?”. The colleague at first did not understand what the owner wanted to say, and then she saw the declaration. It was a species of Lavandula stoechas (it is read: “lavandula stehas”), Spanish lavender. Fortunately, she said “stetchas”, because Spanish lavender oil is extremely rich in ketone compounds (fenchone, camphor) and there are great restrictions in its use due to potential toxicity, unlike the essential oil of real lavender, which is dominated by esters and alcohols and certainly is for use.
Four years ago, I received a retail sample of Croatian peppermint essential oil. The name on the bottle was nicely marked in Croatian, even Latin (Mentha x piperita, Lamiaceae). But when I opened the bottle, the smell didn’t even closely match the smell of peppermint. It had the typical odour of a piperitone oxide compound (and more likely piperitenone oxide). It was clear to me then that the manufacturer was most likely collecting horse mint (Mentha longifolia) which contains these compounds in larger quantities. For him, it was all “mint”. The manufacturer is a very diligent and responsible man, but he still needed to seek advice to determine the exact type. The essential oil ended up at retail at the seaside, and the substitution of the species itself is very risky in this case. Peppermint is a typical oil that is also taken orally, while horse mint oil should never be taken orally. Even if the customer will not use this oil orally, the oil will not help with local application for ailments such as migraines or sore joints, where peppermint oil is widely used.
To make things even a little more complicated (or simpler), several different types of the same genus can give essential oils of almost identical composition.
In the example of tea tree, the European Pharmacopoeia clearly specifies in the monograph of tea tree (Ph. Eur. 01/2008:1837 corr. 7.0) that several types of essential oils can be called tea tree essential oil- Melaleucae aetheroleum:
- Melaleuca alternifolia (Maiden & Betch) Cheel
- Melaleuca linariifolia Smith
- Melaleuca dissitiflora F. Mueller
Technically, this means that essential oils of these plant species can be called tea tree oil, but the label in Latin should specify the species.
The case is similar with eucalyptus. The standard of the European Pharmacopoeia 07/2010:0390 allows essential oils obtained from three species of this genus, provided that they are extremely rich in 1,8-cineole to be called Eucalypti aetheroleum (eucalyptus essential oil):
- Eucalyptus globulus Labill.
- Eucalyptus polybractea R.T. Baker
- Eucalytpus smithii Р.Т. Baker
with compliance with the rules of stating the exact name in Latin. But here too we should be very precise: Eucalyptus polybractea has two chemotypes, the cineole chemotype and the krypton chemotype. Eucalyptus polybractea chemotype krypton does not contain 1,8-cineole as the dominant compound and its essential oil cannot be called Eucalypti aetheroleum.
The plant world is taxonomically very complicated. Sometimes it is not enough to define only the exact species, but also the subspecies (lat. subspecies, abbreviation ssp.). We must also think about this for the species that we consider “homogeneous”, i.e. we do not consider it important to identify the subspecies in order to be sure of the desired action of the essential oil. Let’s see the example of winter savory (Satureja monatana). Winter savory oil is known as a phenolic oil with a strong antibacterial effect. Obviously, we expect such a composition and such action from this species. However, Slavkovska et. al. reported that the composition of the essential oil of winter savory depends on the subspecies. S. montana ssp.pisidica does contain a significant share of phenol carvacrol in essential oil (26.2-70.5%), while S. montana ssp.montana contains less phenol, and predominantly contains the alcohols linalool, borneol, trans-sabine hydrate, and p-cimen-8-ol. Therefore, before considering picking wild plants for the production of essential oil, it is always necessary to conduct a field test for botanical and chemical classification, in order to be sure of the composition of the essential oil.
For some plants knowledge not only of the subspecies but also of the variety is required (lat. varietas, abbreviation var.). The most famous example is fennel. There are two commercially available fennels and their essential oils:
- Foeniculum vulgare ssp. vulgar var. vulgare Miller, Apiaceae – bitter fennel
- Foeniculum vulgare ssp. vulgar var. dulce Miller, Apiaceae – sweet fennel
As fennel essential oil is also used orally, it is important to use sweet fennel essential oil, which contains smaller amounts of ketones and is safer to use.
Another well-known example is hyssop. The essential oil of common hyssop (Hyssopus officinalis) is dominated by neurotoxic pinocamphon and isopinocamphon and is rarely used in aromatherapy today. Hyssop decumbens essential oil (Hyssopus officinalis var. decumbens) contains non-toxic oxides and they are used with greater safety.
Much less often the form of a plant appears as an important criterion for selecting plant material for distillation. But the most famous example is ylang ylang. There are two main forms of ylang ylang:
- Cananga odorata (Lam.) Hook f. and Thomson form genuine, real ylang ylang
- Cananga odorata (Lam.) Hook f. et Thomson form macrophylla, cananga
Ylang ylang essential oil has a sweeter, floral scent, while cananga oil has a deeper and less sweet scent. This is due to the different chemical composition. Cananga contains a smaller amount of esters that give a floral-sweet character of the fragrance and a larger amount of β-caryophyllene (gives a deeper character of the scent and cananga has 30-40% of it), while real ylang ylang contains a larger amount of esters and a smaller amount of β-caryophyllene (up to 19%).
Very often cananga oil ends up in a retail store called ylang ylang, or ylang ylang essential oil is forged with cananga.