Beauty of flowers in eye of beholder

By Gary Raham
Nature Writer and Illustrator

We tend to think of billboards as blights upon the landscape, but flowering plants are some of the oldest billboards of all, and we credit them with great beauty.

Flowers flaunt their form and color for other, alien consumers. Sometimes only creatures that can see ultraviolet (UV) light perceive the complete erudition of flower messages. Many flowers that look pretty uniform in color to us, like the meadow cranesbill (Geranium pratense), display enhanced "landing-strip" markings called nectar guides that, scientists have shown, lead pollinators that can see UV light, like bees, to sweet rewards.

Konrad Christian Sprengel (1750-1816), a linguist, theologian and teacher, first realized the billboard function of flowers and published his research in 1793 under the imposing title "The Newly Revealed Mystery of Nature in the Structure and Fertilization of Flowers." He got depressed when his contemporaries thought his notions were a bit "far out" and moved on to other things, but subsequently scientists have shown the truth of his observations many times over.

Fritz Knoll, a professor at the University of Vienna, designed a simple but very effective experiment in 1926 showing that a hawkmoth uses the nectar guides of the common yellow toadflax (Linaria vulgaris) to locate its lunch. Knoll loosely sandwiched some toadflax flowers between two plates of glass. When a hawkmoth approached the flowers and extended its long tongue (proboscis) to slurp up some nectar, it left marks precisely on the nectar guides-even when the nectar guides were pasted into different positions on the flowers.

Nectar guides induce a reflex reaction in bees and flies called the "head jerk." When a bee lands on a nectar guide, it jerks its head down and extends its proboscis. One diabolical zoologist demonstrated that bees' heads jerk inappropriately when flower parts are scrambled. The dark centers of sunflowers absorb UV light, serving as targets for bee landings and inducing reflexive head jerks. If you remove the petals of a sunflower, rotate them 180 degrees, and paste them together so that the dark, UV-absorbing parts are now to the outside of the remanufactured bloom, the bee will move to the outside edges and try to get nectar where none is available.

What's it like to see like a bee? We can't know for sure, of course, but various experiments have provided some clues. Two differences between human and insect visual perception stand out: shape discrimination and color sensitivity. The color receptors in human eyes are sensitive to red, green and blue light wavelengths. Bees possess receptors sensitive to green, blue and ultraviolet. Thus, bees can't see red and we can't see ultraviolet. Elaborate UV nectar guides are often found on yellow flowers because a yellow background provides the most contrast.

Human eyes focus light through a lens and onto photoreceptors that act like film in a camera. Thus, we see shapes like circles, triangles and squares very well. Insect eyes are composed of many cone-shaped light gathering lenses called ommatidia. For them, the world is broken up into thousands of point sources, almost like the dot patterns in a color picture printed in a newspaper. (For a possible fly's-eye-view of a human face, check out www.rit.edu/~photo/IFS/index-pages/IFS-55.html.) This sort of eye responds well to motion, but can't discriminate between certain shapes. On the other hand, because bees can distinguish up to 265 flashes every second, if a bee watched one of our videos that only has to flash 20 times per second to fool our eyes, it would look like a slide show of still pictures instead of continuous motion.

The flowers sprinkled throughout this article show some of the many UV nectar guide patterns found among flowering plants. You may also want to check out an excellent web site set up by photographer Bjorn Rorslett at www.naturfotograf.com/UV_flowers_list.html.

It turns out that plants may get double duty from the chemicals called DIPS (dearomatized isoprenylated phloroglucinols) that form the UV-absorbing areas on flower parts. These chemicals also serve as repellents for ovary-munching caterpillars. One place DIPS are found in abundance is on the female flowers of commercial hops, used in beer making. Thomas Eisner, one of the scientists who found the repellent characteristics of DIPS said, "If your beer is safe and enjoyable to drink, you ought to thank a flower."

We can thank flowers for many things, including the beauty that we can readily see and the beauty we can only see in the ultraviolet. Who cares if they are primarily advertisements for prospective pollinators? Maybe the many and varied billboards that adorn our cities will be just as aesthetically pleasing someday to aliens with no need to drink beer or remove hair with Burma Shave.