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It's only natural: The chemistry of essential oils

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“Imagine your feet are nailed to the ground and you want to eat. There’s food over there but you can’t reach it. Now, imagine you want to drink and there’s water but you can’t reach that either. Then, you want to canoodle with a young woman or a young man, but you can’t reach them. And suddenly there’s a predator coming towards you, but you can’t move…” Dr. Jo Elworthy - plant expert and director of interpretation at the the Eden Project - poses a terrifying question. In all of these situations the outcome for a human would be almost certain death - a particularly quick one in the case of the predator.

For plants, however, it’s a different story. They exist in the same spot, usually for their entire lifespan; getting the things they need from the environments around them, and protecting themselves and their offspring with devious disguises and chemical artifice. What’s more, they are crucial to the ecosystem - without plants humans would not exist. But what is it that makes them so powerful, and oh-so crucial? The answer is elementary - it’s in their chemical make-up.

Over the millions of years they’ve been on the planet, plants have evolved to cope with the harshest of environments. Whether it’s by developing huge root systems to search out water in the driest of deserts, or mimicking animal pheromones to encourage insects to pollinate them - plants are masters of adaptation - and the essential oils they produce play a significant role.

Jo explains: “Plants wouldn’t have evolved to produce essential oils unless they were serving a purpose - and by that I mean a direct purpose for the plant. Everything else is additional. Why else would a plant make a chemical that complicated, if it didn’t have a function?”

One way plants keep themselves alive is by producing secondary metabolites, that’s organic compounds that have a wide array of effects - they can be toxic, fragrant, corrosive, hallucinogenic or foul-tasting. Whatever their effects though, they can generally be categorised as repellant, protective or attractive. And, because of these varied effects, humans have found many uses for them, in fact caffeine, cocaine, morphine, rubber and essential oils are all secondary metabolites.

Some secondary metabolites are volatile - which means that they evaporate easily. Volatile organic compounds (VOCs) have many functions, one of which is to help plants retain the moisture they need to survive. During long, hot summers - like those in the Mediterranean where temperatures can reach way over 30℃ - plants need every drop of water they get, and the VOCs they release help them to retain it.

These VOCs are often fragrant, which is one of the reasons humans love them so much, and they are usually strong tasting too. Think of a garden full of thyme, rosemary, and oregano - all are great smelling herbs we use to flavour food and drink. But these strong tastes aren’t made to flavour our spag bol, they serve a purpose - to deter animals and insects from eating their foliage. A mouthful of rosemary and garlic focaccia might taste good to us, but for some insects the aromatic plant tastes potent and is best avoided.

But it’s not all about deterrence, plants need to communicate and interact with other plants and organisms in order to reproduce. Jo explains: “Roses and other flowers are a bit different. They are for the other thing. Sex. Petals are modified leaves and they produce oils to attract insects so that they can take pollen from one flower to another.”

So, in much the same way that humans use the scent of roses to attract and allure, roses themselves create a romantic vibe in order to guarantee a date where they can exchange pollen and make baby plants - okay seeds. Jo emphasises that every inch of a plant exists for a reason, while a rose’s petals help it to reproduce, its leaves acts as solar panels with waxy surfaces to help keep the plant from drying out.

More than just producing chemicals that we use to flavour our foods and scent our bodies, plants are vital to life on Earth. Jo explains: “If we go back to basics, everything that keeps us alive is either grown as a plant or mined - so a rock or natural material.

“Plants trap the energy of the sun. Using their leaves, they turn sunshine into chemical energy by they combining it with carbon dioxide and water to make sugar. They then use the sugar like we do, as a supply of energy and as a building block to make all the other things they need. They mix carbon, hydrogen, oxygen, and nitrogen - essentially what the air is made of - with minerals from the soil, then they combine and recombine all of these things into really complex stuff.

“We eat plants because we need them to release the energy they have created, we need them to build our bodies. We also use them as building materials, fuels, and as medicines. What’s more, we have done that forever. Humans have used plants like this for as long as they’ve existed.”

But it’s not just humans that plants are vital to. The whole planet relies on them. We’ve all heard the phrase ‘lungs of the Earth’ when referring to rainforests, but the grip plants have on the entire ecosystem goes much further than the carbon dioxide they absorb and the oxygen they emit.

The very chemicals and VOCs rainforest trees release into the atmosphere have a direct effect on climate control. The VOCs create nuclei that water droplets can form on, leading to the generation of clouds and, eventually, rainfall. Within the rainforest this is the equivalent of turning on the tap - and means that plants and organisms are able to continue to function.

On a wider scale, scientists are now exploring how they can mimic this natural process to help tackle our own, man-made climate crisis. Clouds both reflect sunlight, which cools the Earth, and trap heat in the same way as greenhouse gases, thus warming the Earth - in other words they are crucial to tackling the damaging effects of climate change.

But again, Jo is keen to point out that the trees in the rainforest do not regulate climate on purpose, it is a happy coincidence that has led to an evolutionary advantage.

“Volatile organic compounds from rainforest trees help clouds form and help it to rain. And, because it rains more, the trees thrive and produce more VOCs. They don’t do it on purpose to form a cloud because they aren’t sentient, but the cloud is produced regardless and it produces an evolutionary advantage.”

The regulation of climate in just one of the many roles plants play in the ecosystem, and Jo warns against messing too much with the natural order of things. She said: “We are part of the global ecosystem, which is plants, microbes, other life, air and water, and soil and rock - and it all interacts in a phenomenal recycling system.

“For the last 200,000 years since we have been on the planet we have interacted with this environment, but we are beginning to not interact with it but to affect it and cut things down. There is no action without reaction. We don’t know what will happen when we mess with this stuff.

“It is about respecting nature, having awe and wonder at how it keeps us alive and how it feeds us, gives us fuel, gives us building materials, creates our air, gives us all these services where it regulates the climate, and gives us all the senses that we have. We are part of the ecosystem, if we adversely affect the environment it will adversely affect us.”

Jo’s message is clear, plants are cool - but they are also crucial. It’s easy to forget that the everyday objects that we use, eat and drink are all sourced from nature, including essential oils. What’s more, they serve a purpose within the ecosystem and humans have no right to monopolise that. In a world where flavours and scents are commonplace, it’s important to bear in mind the responsibility humans hold in keeping our planet thriving.