FROM THE GROUND UP: Time to smell the ... flowers

The sweet-spicy fragrance of iris blossoms is one of the treats of spring.
The sweet-spicy fragrance of iris blossoms is one of the treats of spring. PHOTO BY PAM BAXTER

This is such a delightful time of year, with plants bursting into flower and leaf, making the landscape come alive again. But would spring be quite as wonderful if it didn’t smell so good? How many of us plant flowers for their fragrance as much as for their visual appeal? I’m thinking hyacinths, magnolias, lilies, lilacs, irises, roses, lily-of-the-valley, heliotrope, viburnum, honey locust, honeysuckle, jasmine.

A few days ago, after burying my face in yet another iris bloom to inhale its spicy sweetness, I started to wonder, “What is fragrance? When we smell a flower, what is it that we’re actually smelling?” In all the botany I studied in college and all the exploration of the plant world that I’ve done since then, I’ve never come across an explanation. Nowhere in the 748 pages of my college botany text was the subject of fragrance even mentioned.

I wanted to explore the mystery of how something so intangible could affect my senses. Touch, sight, and taste seem straightforward: there is an object that one comes in contact with. Even with sound there is something tangible there; we can feel the vibration. But smells hover unseen in the air, and with flowers the fragrance is tantalizingly ephemeral.

I found the best explanation in an online article, “Making Scents: The aromatic world of flowers,” by Ben Guarino, (http://scienceline.org/2013/01/making-scents-the-aromatic-world-of-flowers/). Formerly a freelance science journalist, Guarino now writes for the Washington Post.

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Not surprisingly, fragrance comes down to chemistry. Says Guarino, “Each scent compound is an organic molecule known as a volatile, a chemical that vaporizes into a gas when released by a plant.” He describes these molecules as miniscule, so tiny that that they can be sensed by the olfactory organs of creatures as small as the insects that are enticed by aromas to pollinate the plants producing the fragrance.

The scent molecules have “spines” built of carbon atoms. “What makes floral scents distinctive are the length of the carbon chains and the other types of atoms glued to these backbones,” notes Guarino. For instance, on the stinky side of things, it’s sulfur atoms attached to the carbon chain that give carrion flowers their putrid (nose-curling) stench.

What I find ven more fascinating is the complexity, the choreography, of a flower’s production of fragrance. To start with, Guarino notes that “the part of the plant that releases fragrance depends upon the species.” Some plants produce scent in specialized glands (e.g., orchids), while some produce scent “throughout the outer layer of petals and other parts of the flower.”

Some flowers contain a reward for pollinators: sweet, nourishing nectar, or even nest-building materials. Some flowers have “learned” to mimic the scent of those nectar-rewarding flowers, thus luring insects to pollinate them, while giving back nothing in return. Smart plants, since it costs flowers resources and energy to produce nectar.

Smart plants! Some flowers (orchids, again), have developed fragrances that mimic that of female bees to attract male bees that try and mate with the flowers, pollinating them in the process.

Smart plants! They can sense when they’ve been pollinated. “Plants must generate enough smell to induce insects to fertilize their flowers, but not so much that they waste energy and carbon. For many species, scent emission is not constant; snapdragons decrease scent production 36 hours after pollination,” explains Guarino.

How many compounds might there be in a single fragrance? Says Guarino, “Until the past few decades . . . the biochemical process of scent production remained a mystery. In 1953, chemists knew of 20 chemicals in a rose’s fragrant bouquet; by 2006, they had discovered roughly 400.”

One thing I’d still like to know is how many scent molecules it takes for us to be able to detect their presence; for us, breathing in the fragrant spring air, to say, “Mmmm . . . .. that smells good!”

Pam Baxter is an avid organic vegetable gardener who lives in Kimberton. Direct e-mail to pamelacbaxter@gmail.com, or send mail to P.O. Box 80, Kimberton, PA 19442. Join the conversation at “Chester County Roots,” a Facebook page for gardeners in the Delaware Valley. Go to Facebook, search for Chester County Roots, and “like” the page. To receive notice of updates, click or hover on “Liked” to set your preferences.