What's behind different floral scents

Wave after wave of scent has filled the air since my garden awakened in spring. Most prominent have been the aromas from daffodil blossoms, plum, flowering currant, and now dame's rocket, pinks and roses.

Of course, it's not for us that flowers waft those sometimes delectable aromas. Evolutionarily speaking, we don't return the favor with anything more than the carbon dioxide that we -- and all other animals -- breathe out, and that plants use for photosynthesis.

Flowers release their aromas to attract pollinators. As such, floral aromas might mimic countless other kinds of aromas, depending on just what creature a particular flower is trying to attract.

Some of those floral aromas are actually unpleasant to us. Skunk cabbage (smelly and inedible) is a good example, but there are worse -- or better -- examples. The arum lily of South Africa, for example: From its spathe, a spike-like inflorescence of many small flowers rising up from what looks like an upended purple skirt, wafts the smell of rotting flesh. This aroma is perfect for attracting the carrion beetles that pollinate this plant. Heat generated inside the inflorescence heightens the morbid effect and helps pump the aroma out into the atmosphere.

But on to more pleasant aromas -- for instance, flowers that mimic pheromones, the scents that female animals give off to signal their readiness to mate. Of course, those ersatz pheromones coming from flowers are directed at insects, because they are the ones flowers want to attract to perform pollination.

Floral deception

More than mere scent may be needed to keep an insect on a flower. The mirror orchid, for example, deceives the male bees that pollinate it by not only smelling like receptive female bees, but also by looking like them.

After one or two flowers, any smart bee is going to realize that he's not dealing with a real female and give up trying. So to keep up the deception, each mirror orchid plant smells slightly different; it takes a half dozen or so before a bee catches on, and by then the flowers have gotten what they wanted.

Even more intricate in its deception is the bucket orchid of Central America, which splays out little "buckets" filled with a quarter-inch of perfumed liquid. Each of the 20 or so species of this plant has a slightly different perfume -- although the differences are undetectable to us -- according to the species of iridescent bee it means to attract. In the flurry of activity around a bucket, an occasional bee falls in. As the bee squeezes out through a narrow tube, it incidentally pollinates the orchid flower.

People pleasing

What makes a scent pleasing to humans? Perhaps, as with bees, some aromas conjure up pleasant primeval associations. At any rate, we like them enough to put great effort into capturing and bottling them.

The first essential oil, attar of rose, was bottled by the Arabian physician Avicenna about 1,000 years ago. Two hundred years later, Eleanor of Aquitaine had 2,500 pounds of violets crushed to make 1 pound of violet attar.

Humans soon learned to preserve a scent better by combining it with a fixative. Fixatives originally were musk extracted from the genital area of deer and ambergris from sperm whales, but now synthetics are also used.

As an alternative to the elaborate extraction and fixing of floral aromas, consider planting a rose outside your back door, then stepping outside and putting your nose to it. Even better, plant a spectrum of scented blossoms for sensory thrills from late winter to late fall.