How fragrances function

The fresh scent of lavender, the delicate note of roses: a pleasant scent delights the senses. The effect of essential oils is particularly intense. Extracted from flowers, an essential oil contains up to 400 volatile chemical substances that stimulate our sense of smell. Via the olfactory cells in the nose, they send a signal to the brain, where the scent is evaluated. Our nervous system reacts by releasing messenger substances that act on our psyche and on our endocrine and immune systems.

Even without the scientific explanations we have today, people in ancient times observed that plants react to odours. In Egypt, it was known that the smell of slashed figs could trigger the fruits of a whole tree to ripen. In China, incense was burned to make hard pears edible. Today we understand why: ethylene, an odorous gas found in smoke but also produced by every plant, sends a signal that starts the ripening process. While humans only smell with the nose, plants do so with their entire organism. All plant cells, whether in the roots or in the leaves, have olfactory receptors.
 

“All smells produced by plants – for example those of rosemary, basil or liquorice – are equivalent to precise messages: they are the plants’ ‘words,’ their lexicon! Millions of different chemical compounds function like signs in a real plant language, about which we know very little,” explains plant neurologist Stefano Mancuso. He has compared the complex language of plants with Egyptian hieroglyphics, which were deciphered only after endless attempts. 
However, it is easier to observe how well the plant language works: trees whose branches and roots do not touch one another can communicate by odours passed through the air, for example to warn of predators. For example, the African acacia tree, when nibbled by giraffes, emits a gas that warns nearby trees of the danger. Within minutes the other trees build up toxins in their leaves, making them inedible for the giraffes.
 

Fragrances are also a language for humans, one that helps us to store our memories. Freshly cut grass, baking Christmas cookies or a certain perfume: smells can be a key to the past. If a certain smell reaches our nose, it all comes back in a flash, even after decades. Neuroscientists assume that this is due to the path olfactory information takes through our brain – directly from the nose to the hippocampus, our brain’s “memory control centre”. 

References
Daniel Chamovitz: What a Plant Knows: A Field Guide to the Senses. New York: Scientific American / Farrar, Strauss and Giroux: 2012

Stefano Mancuso, Alessandra Viola: Brilliant Green: The Surprising History and Science of Plant Intelligence, Washington, DC: Island Press, 2015

Kathrin Meyer und Judith Elisabeth Weiss (Hrsg. für das Deutsche Hygiene-Museum Dresden): „Von Pflanzen und Menschen“, Wallstein, 2019
Eva Heuberger, Iris Stappen, Regula Rudolf von der Rohr: „Richen und Fühlen. Wie Geruchssinn, Ängste und Depressionen zusammenspielen“, fischer & gann, 2017

www.pflanzenforschung.de/de/themen/lexikon/ethylen-ethen-10096
www.pflanzenforschung.de/index.php?cID=5681
www.ds.mpg.de/2495238/Gerueche_und_Erinnerungen