© Janet Davis

When the lilacs bloom, they fill the garden with that enigmatic perfume so many of us associate with May, possibly triggering thoughts of spring-times long ago.

My earliest memories of growing up on the west coast all seem to swirl in their own clouds of fragrance. The pungent sweetness of Scotch broom reminds me of lazy summers and petty larceny, for it was on a hot hillside under these weedy, yellow-flowered shrubs that I hid with my younger brother, waiting for errant shots from the nearby lacrosse court to land. “Finders keepers” we’d shout over our shoulders as we high-bounced our purloined balls home. And I remember vividly the fresh, earthy smell as we crawled on all fours through a waist-high meadow of spring grasses, wild roses and buttercups, dodging the spit bugs and chewing on the sweet purple clover.

More than four decades later, I can still close my eyes and inhale those memories. So it comes as no surprise to discover that, unlike the other four senses, our sense of smell is linked directly with the limbic region of our brains, the area that governs emotion, sex drive and hunger—as opposed to the neocortex, or thinking brain.

Did you know that the average pair of lungs takes in about 500 cubic feet of atmospheric air each day, but of that, only 2% makes its way across the olfactory sensing area in our upper noses? In other words, we really only get a good sniff about 10 cubic feet of air every 24 hours. But when those scent molecules waft past, they activate specific olfactory receptors in certain cells in the nose’s lining. From there, a chain reaction occurs that passes the signal to the higher parts of the limbic region of the brain. The receptors allow us to recognize and remember approximately 10,000 different odors. Much of the newest information on smell was uncovered by American researchers Linda Buck and Richard Axel, who won the 2004 Nobel Prize for their work with odorant receptors and the organization of the olfactory system.

Scent in the Garden

In a diversely planted garden, the air is filled with the perfume of many kinds of flowers. As we lean to smell our favorite fragrant plants, we think, “Ahh, roses. Mmm, lilies. Ooooh, lilacs.” But what’s really going on between our noses and our brains? And what, for that matter is fragrance?

Those perfumed blossoms, in fact, are all business, emitting scent not for our pleasure but as genetically programmed siren call to their insect pollinators.

Most floral scents come from highly volatile essential plant oils usually contained within the epidermis of the petals, but sometimes found in the leaves or roots. Organic chemists analyzing these oils with a gas chromatograph would isolate chemical compounds like aldehydes, ketones, esters, phenol-ethers and so on. But for the bee or butterfly, the aromatic oils aloft on a breeze act like a flashing “Come hither” sign.

When we sniff a carnation or pink, our olfactory receptors inhale molecules of its essential oil, which has become liberated as a gas. Each receptor contains 5-million nerve cells, with filaments that sweep the air as it passes through. The carnation’s essential oil contains a phenol called eugenol that is also found in cloves, which is why these flowers remind us of this well-known spice.

Perfume makers have long exploited essential floral oils, extracting them through a variety of arduous processes. Modern perfumers use complex computer technology to analyze, then synthesize, otherwise elusive oils, often replicating scents like lily-of-the-valley or night jasmine that are too delicate or brief-flowering to be commercially viable.

Something to think about as the scent of lilacs wafts on a May breeze.

Adapted from a column that appeared originally in the Toronto Sun.

Back to Botany, Ecology & Insects