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FRAGRANCE IS A FLOWER'S WAY OF ADVERTISING
I gave my wife a dozen roses for our wedding anniversary. The arrangement included half a dozen decorative stems of baby's breath. After a few days, my wife decided the baby's breath had to go because its powerful fragrance triggered her allergies.
It's interesting how flower fragrance can have such a powerful effect, good or bad, on people. Fragrance is one reason flowers are such an integral part of weddings and funerals.
From a biological point of view, of course, flower fragrance has nothing to do with people and everything to do with insects and pollination.
While some plants are wind-pollinated (oak, birch, pine, some grasses), most rely on insects to spread the pollen that is required for reproduction. Fragrance is one way flowers advertise (color is another).
"The underlying biological purpose of flower fragrance is to advertise to insects that food (nectar or pollen) is available, or to fool the insects into a hungry or amorous condition," explains Charlie Rohwer, a horticultural graduate student at the University of Minnesota. "The insect visits the flower, looking for food or a mate. If all goes well for the flower, the insect will take some pollen with it before it leaves, then serendipitously deposit that pollen on the stigma of another flower of the same species, thereby cross-pollinating the plants."
Rohwer says fragrance may serve to make it easier for insects to find the visual cues of the flowers, and fragrance probably evolved before visual flower signals. From a distance, fragrance is more effective than visual signals at attracting a pollinator, especially to a small or hidden flower. Up close, however, the visual signals would help to accurately find the pollen or nectar.
Flowers tend to be the most fragrant when they have sufficient nutrition, according to Rohwer. In addition, moderate to warm temperatures and high light tend to increase fragrance, but fragrance also reduces the life of the individual flowers. If the flower uses energy to make fragrance, it can't use that energy to keep itself looking nice. In breeding programs for cut flowers, like roses, vase life is an important characteristic to select for. Therefore, as plants with longer flower life are selected, fragrance is typically disregarded. The same holds true for potted and garden plants. For example, cyclamen used to have a fragrant flower. However, breeding programs have ignored fragrance and it has been virtually bred out.
Flower fragrances are often strongest at certain times of the day or night. Snapdragons, for instance, are more fragrant during the day when bee pollinators are likely to be active. Nicotiana is more fragrant at night when its moth pollinators are out and about. "It would be a waste of energy for these plants to advertise to insects that don't even pollinate them," Rohwer explains.
The location of fragrance on a flower varies from species to species. Some plants have fragrant pollen. The oily coat of the pollen may contain its own profile of fragrant compounds that is distinct from the fragrance profile of the rest of the flower. Different parts of the petal or pistil also often make fragrant compounds, and often in different quantities. Rose pollen, for instance, contains a fragrance profile separate from the rest of the flower, but fragrances from the rest of the flower usually overpower pollen smells.
Have you ever noticed that garden scents are often stronger after a rain when the air is humid? Rohwer says this is because the scent molecules are able to travel farther and are easier for your nose to pick up when the air is more humid. The plants may also release more fragrant compounds under humid conditions.
Stretching to smell heliotrope, sweet peas or roses is almost instinctual. Sniffing springtime lilacs is an unavoidable pleasure. Each year, I grow from seed night-blooming moonflower, a relative of morning glory, because of its perfume-like fragrance. I've also grown datura (Angel's Trumpet) because of its marvelous fragrance. And my yard is full of native basswoods, which sweetly scent the entire township when they're in early summer bloom.
If you'd like to plant more fragrant flowers this year, I discovered a list of the 150 most fragrant plants at the website www.organicgardening.com/library/fragrance.html.
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PUTTING DOWN ROOTS:
A Delightful Blend of
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