The Secrets of Scents

How many times a day do you smell something? I’m not talking about that first sip of coffee in the morning or the time you smelled the bouquet of flowers you were given. I’m talking about those scents that you barely notice, those subtle fragrances that surround us and to which we generally pay no attention.

Our sense of smell is probably the one we use the least. We look, we listen, we touch, we taste… but smell? We do it almost inadvertently. We notice a smell when it is very present, like when we enter a cheese shop, or when we mow the lawn, but that’s it.

I Must Admit That It Causes Me a Certain Amount of Frustration!

As a teenager, I read Perfume: The Story of a Murderer by Patrick Süskind and it changed the way I used my nose. I use my sense of smell A LOT, and I’ve noticed that, like a muscle, the more you work it, the more it develops. It helps me identify plants or mushrooms by their odor, tell when a vegetable is nearing the end of its life, or even balance the flavors in a recipe… Is something missing? Is it thyme? (I take a good whiff of thyme.) No. Paprika? (Sniff sniff.) No. Coffee? (Sniff) Yes!

And that’s how I ended up with instant coffee in my spice rack… In chili, in tomato sauce, I’m telling you, it adds something! But I never would have thought of cooking with this flavor if my nose hadn’t first told me that it worked.

But I digress: let’s return to the scents of nature and their raison d’être.

In your garden, each plant gives off a unique olfactory signature: peppermint smells different from spearmint, and wild roses have a completely different scent from cultivated roses. In the fall, decomposing leaves have a particular, almost melancholy smell. These scents are not there by chance: they are the result of complex survival strategies, human intervention and, sometimes, chance!

Fragrances as a Form of Communication

Plants don’t just smell nice for our pleasure. Scents are messages, tools for survival. Flowers release specific scents to attract their pollinators: some will be more attractive to bees, others to butterflies or hummingbirds, depending on their olfactory preferences. What would be the point of having a scent that attracts bumblebees, for example, if the flower is tiny and there is not enough nectar for them? It is in the interest of this small flower to have evolved to attract ants or small flies, which can distribute its pollen or, possibly, its seeds (in the case of ants!).

But don’t think that all plants are trying to seduce with their sweet scents! Some use olfactory messages as a means of defense. Rosemary, for example, gives off a strong, resinous smell that repels many insects. Its essential oils contain compounds such as camphor and cineole, which act as natural repellents. When a herbivorous insect approaches, the intense smell of rosemary warns it that it may find an unappetizing meal.

Lavender, for Example

Lavender, a close cousin of rosemary, is said to be an effective repellent against ticks: a good reason to put on some perfume before going down on your knees in the garden. In Europe, some birds, the blue tit being the best known, even use certain aromatic plants, including lavender, to protect their nests from parasites!

For other plants, it is when a plant is attacked that volatile compounds escape and attract the predators of its aggressors. For example, when caterpillars attack a corn plant, (E)-?-farnesene and linalool escape from the “wound”. Parasitoid wasps that lay their eggs in the caterpillars are attracted by this smell, which promises them plump caterpillars that will become incubators for their eggs. The corn plant therefore has the satisfaction (as much as a corn plant can be satisfied!) of seeing these caterpillars die and the birth of the new little soldiers that will protect it. I know: nature is violent!

For the curious: linalool is present in many plants and is partly responsible for the fresh, floral scent. The (E)-thing, on the other hand, is more complicated to describe. For the curious: linalool is present in many plants and is partly responsible for the fresh, floral scent. The (E)-thing, on the other hand, is more complicated for the human sense of smell to describe!

(Am I the only one who can picture a violent scene of caterpillars being murdered with a sweet floral background, beautiful spring sunshine and a little classical music?)

Artificial Odor Selection

Humans have not been mere spectators of these olfactory strategies developed by nature. Artificial selection has played a crucial role in the evolution of the scents of our cultivated plants. Take basil: there are now more than 150 varieties, each with its own aromatic signature. Humans are able to select, hybridize and appropriate nature: if they can create new varieties of citrus fruits or new flower colors, why not new smells? After all, herbs and spices play a major role in our diet (and even in pharmaceuticals!).

Lemon basil, Thai basil, purple basil… each variety has been selected not only for its taste, but also for its fragrance. Some varieties have been developed for their culinary properties, others for their medicinal or ornamental qualities. Thai basil smells completely different from large green basil: spicier, more intense, like cinnamon, with almost minty notes.

This diversity is no accident. Gardeners and botanists have worked for generations to isolate and multiply the plants with the most interesting aromas, meticulously choosing the varieties that best corresponded to our gustatory and olfactory desires. And here we are, laidback gardeners, choosing a packet of seeds from among five or six varieties, as if nothing had happened!

Accidental Odors

Not all odors have an immediate ecological function. Sometimes molecules simply have an odor! Let me give you a very simple example: flatulence. When our body digests, it produces gaseous residues that are not absorbed and are not necessary for the functioning of our body. These “waste” molecules are simply evacuated and the resulting odor has no specific role, except perhaps to ward off colleagues…! This phenomenon of molecules that smell without having an olfactory role also exists in the plant world.

Some Examples

The smell of wet earth, for example: it is the result of a molecule produced by a bacterium, geosmin, which meets the essential oils of a plant, and then gets wet! The resulting smell, mixed with other compounds, can then give the smell of rain, spring or autumn.

My favorite smell is that of decomposing leaves. They give off a characteristic smell with a hint of vanilla due to the sugars in the leaves. Mixed with geosmin, after a good rain, you get the smell of fall!

Smells are very complex, and I’m simplifying it all for you, but you have to admit that it makes you want to go out for a good whiff. This smell, geosmin, is thought to be a by-product of the decomposition process and the life cycle of the bacterium, a bit like how flatulence is a by-product of digestion. It is a scent from nature that has no purpose, accidental, but oh so pleasant!

Another example of an accidental odor that is less “mixed” is the “green” odor. You know what I’m talking about, don’t you? Whether it’s a lettuce, basil or poplar leaf, there is a characteristic scent of the green parts of plants. This smell comes mainly from hexanal, a compound released when a leaf is cut or crumpled. It is not a defense signal, it is only the molecule that escapes from its cell, where it fulfilled a role other than smelling good! Every green plant shares this olfactory signature: it has no choice, it is a basic compound of plant cells!

Developing Our Sense of Smell: A Sensory Journey

Taking the time to smell, really smell, is to open a window on an invisible world. Our sense of smell is often relegated to simple utilitarian functions: detecting curdled milk, appreciating a dish, recognizing a loved one.

But smelling is much more than that. It is decoding complex chemical messages, understanding the survival strategies of plants, perceiving stories that only our nostrils can tell. Each smell is a story: that of a defense, a seduction, a communication or simply a biological process. And each one is worth smelling, recognizing and appreciating.

The next time you take a stroll in a garden, take the time to breathe. Really breathe. The flowers, the fruit, the leaves, the roots, the soil… You will be surprised at all the subtleties you will discover!