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|Say it with flowers -- quite a few plants do.
Flowers are advertisements armed with reproductive equipment. Some send glaring signals noticed by innocent bystanders such as ourselves and are admired or imitated by painters, poets, and perfumers.
Because we appreciate the colors, forms and fragrances of some flowers, we think we have gotten the message. In fact, much of the communication between plants and animals is easily missed, the code broken only by botanists and ecologists devoted to deciphering information intended for insects and birds.
Every flower species you encounter is worthy of contemplation. How does it work? Who is receiving the signals that mark each bloom? How does a newborn insect learn to use a complicated floral apparatus and recognize it again? On your next wildflower-watching expedition, get close to your subject, whether or not you know its name, and try to read its messages.
Animals come to flowers for something useful.
Mostly, they are seeking food, but other relationships have evolved so that blossoms offer shelter, mimic egg laying sites, or serve as egg laying sites. An animal choosing these offerings benefits the plant by carrying its gene-bearing pollen to another blossom of the same species. Then, cross-fertilization will lead to a varied population with enough genetic tools to survive the future.
Pollen grains are tiny spheres with tough outer walls often decorated with ridges, warts and spines that make them easier to carry. Along with DNA, a pollen grain is loaded with good food -- proteins, fats, starches, and vitamins.
Many plants lay out their pollen like fried eggs on a Shoneys Breakfast Bar. Bright yellow pollen masses that stand out against a contrasting background --white, blue or purple -- entice honeybees, especially, who feed it to their newly hatched larvae in the form of "bee milk."
Some grains, of course, escape the combing and grooming, and stick to the bellies, backs, or faces of their insect vehicles, so they are flown to other flowers where they might germinate and fertilize an ovule.
The first insect visitors to flowers were probably beetles, foraging for a pollen meal and chewing petals in the Mesozoic era. Magnolia flowers, disorderly structures with no nectar but an abundance of pollen, are considered to be a primitive type.
It takes considerable energy for a plant to produce pollen, so some have developed ways to short their hungry guests while still drawing them to the table. Flower petals with yellow flecks or flares near their base imply a pollen mother lode, though the actual offering may be miserly.
Some, like Pale Touch-me-not; Jewelweed (Impatiens pallida) or Common Wood Sorrel (Oxalis montana), have small, distinct spots that seem to imitate the orderly spacing of anthers (the plant organs that hold pollen.) Iris and Beardtongue (Penstemon sp.) petals have coarse hairs that may also mimic the stalks that support anthers.
The yellow, pollen-packed anthers of Shooting Star (Dodecatheon meadia) hang downward in a tight, bright cone, a display that would tempt many of the pollen-eating insects. Shooting Star will only give up its pollen to its most dependable pollinators, the bees. They latch onto the cone and with precise vibration of their thoracic muscles, shake the pollen from small pores at the tips of the anthers.
Nectar is an even more attractive treat made available by plants. Most of the insects that visit flowers -- bees, butterflies, moths, ants and flies -- have mouthparts that can lap or suck up the sugar water that lies in tiny pools under the reproductive organs of a plant.
Nectar takes less energy to produce and is not as vital to a plants future as pollen, but is even more effective as bait.
A few plants, like English Ivy (Hedera helix,) advertise their nectar as blatantly as they do pollen, showing it off as glistening globules. Grass-of-Parnassus (Parnassia asarifolia) has a ring of fake nectaries beckoning probing visitors. Most plants must hide their nectar from the evaporating power of the sun and wind. How, then, can they call attention to it? Signs posted on the petals point the way. The purple stripes on the petals of Spring Beauty (Claytonia virginica), Wild Geranium (Geranium maculatum,) and all the violets (Viola sp.) direct visitors to the nectar reservoir. The spots and dots on mint petals (Laminacea), like Hedgenettle, and the dashes and contrasting colors on pea flowers (Fabeacea) and Great Lobelia (Lobelia siphilitica) indicate the entrance of a pathway to plenty. Once inside, further aides may guide an insect. Colors may become lighter as the visitor travels farther down the throat of a bell-shaped or tubular bloom, and insects that travel toward the light are well rewarded.
Other markers are invisible to human eyes, but impossible for pollinators to ignore. We are blind to markings that reflect ultraviolet (UV) light. Periwinkle (Vinca minor) appears to us to have a pale white ring at its center. Special UV photography has revealed a much bolder ring, which would stand out to bees, as their eyes are very sensitive to the short wavelengths of UV light. The pure-yellow corolla of Marsh Marigold (Caltha palustris) is strongly figured to a bees eyes, appearing to have a large, circular UV pattern in its middle. The German researcher, Hans Kugler, studying various types of flower shapes and patterns from the 1940s through the 1960s, found nectar or pollen guide marks present on 50 percent to 88 percent of the flowers in each category he examined.
After landing on a petal, honeybees show an added reaction to the marks. As soon as one reaches a nectar guide, it suddenly stops, instinctively drops it head down, and begins probing for sugar with its sucking mouthparts.
There may be further visual guide marks that our eyes cannot distinguish. Botanists have recently suggested that minute sculpturing of a petals surface may also orient bees toward their food.
The over-all color of flowers seems to be a significant signal, as well. Hover flies have shown a preference for white and yellow, but are not stimulated by blue colors. Bee flies have shown an attraction to blue-violet and ignore yellow, gray and red. Green flowers are often fly pollinated.
Color preferences also vary in different butterfly species. Some butterflies distinctly prefer yellow and blue; others prefer blue or purple and shun yellow. Most can see red. An early theory suggested a tendency for butterflies to use flowers colored like their own wings, relating those colors with mate recognition and protective camouflage.
Bees have the very broad range of color vision, but petals or guide marks that are violet, blue, ultraviolet, or yellow will especially attract them. They are blind to red, however.
Unlike most insects, birds are quite sensitive to red. A number of plants offer red winter fruits to birds, which will very effectively distribute the seeds along with a dose of fertilizer. Red flowers attract nectar-sucking birds, as everyone with a hummingbird feeder knows.
The size of the color display appears to have an effect also. Wild Hydrangea (Hydrangea arborescens) and sunflowers (Helianthus) amplify their color signal with a ring of large sterile blossoms that surround a group of smaller, duller "working" flowers. Dogwoods have tough, white bracts, which most people think of as petals, functioning the same way.
Honeybees, at least, have been studied enough to know that they prefer certain flower shapes. A British study in 1970 confirmed a decisive attraction for star-shaped or lobed forms over circles.
Generally speaking, insects are more often excluded by a shape than attracted to it. Flowers with long, narrow tubular bodies typically offer nectar only to long-tongued animals like birds, butterflies or moths. Open, disk-shaped flowers can be used by many kinds of insects, from beetles to bees to butterflies, but they will not be equally effective at the job of pollination.
Every worker bee is equipped with something like 6,000 smelling sensors on its antennae, another powerful tool for finding food. Plants, the food bearers, have developed an array of olfactory messages for their mutual benefit. German tests with 80 plant species proved that 77 had scent marks, implying they are even more common than visual marks. Visual marks are often saturated with scents, even those only visible as UV patterns. There may be a gradient, so that the intensity of a scent increases as the animal approaches the nectaries. Or, distinctly different odors may be found within a blossom, so that a smart insect can quickly learn which scent is associated with reward.
Some butterflies and many moths can be trained to identify scents, implying they could use odors for guidance in feeding. Others are known to search visually. Fragrant night-blooming plants are usually moth-pollinated.
Flies definitely know their odors, and seem particularly attracted to the strong fragrance of flowers in the parsley and mustard families. Plants also pull tricks on flies, playing upon their attraction to rotting flesh and dung, which they need for egg laying. Species of Asarum, kin to our Wild Ginger (Asarum canadense) and Arisarum, which is close to Jack-in-the-pulpit (Arisaema triphyllum) have been shown to attract fungus gnats with their dark purple, flesh-like coloration and rank smell.
Around the world, the strange flowers of Aristolochia vines, like our Dutchmans Pipe, have been studied, as they are known to attract blood midges and other flies into an inescapable trap. Lured by smell, they enter the flower, finding nectar but no escape. The second day the flower produces no odor, and pollen is shed upon the backs of the imprisoned flies. The entrance of the flowers opens again, allowing the dusty flies a chance to find another blossom.
When The Partys Over
Once pollination or fertilization has occurred, a plant may send clear signals to its insect benefactors that their service is no longer needed at that particular site. In many flowers, within minutes of fertilization, nectar dries up, the flower begins to fade, or close, or, in the case of White Clover (Trifolium repens), droop down. Often, the petals change color, and botanists have verified that a plants scent fades or changes, too. All these visual and chemical signals send the pollinators to places where their work will do some good for themselves and the plants they serve. Amazing.
Ecologist Ed Clebsch has overseen three research projects regarding pollination ecology in Tennessee: a study of the pollination of the rare mint, Cumberland Rosemary (Conradina verticillata); the documentation of natural hybridization between Crimson Bee Balm (Monarda didyma) and Wild Bergemot (Monarda fistulosa); and a bumblebee-chasing study within the famous hybrid swarm of azaleas on Gregorys Bald in the Great Smoky Mountains National Park.
Is enough known about the means by which our native Tennessee flowers carry on the business of pollination?
Clebsch offers an emphatic "no."
Dr. Alan Heilman, recently retired from the University of Tennessee (U.T.), has an idea about the lack of effort. "It takes a lot of fieldwork, a lot of sitting to determine who pollinates what. And most botanists arent good entomologists. The heyday for that sort of thing was the later part of the 19th century. Some of the best early work was the three-volume Handbook of Flower Pollination by Paul Erich Otto Wilhelm Knuth. It was published in English in 1908."
As a high school student in Pittsburgh in the 1940s, Heilman began experimenting with micrography, photography through a compound microscope. He had an even earlier interest in photographing wildflowers, using a Kodak Recomar -- a large, demanding camera that would handle either 9x12 cm sheet film or 35 mm film. The discipline required in manipulating such equipment stayed with him, and he taught biological illustration after joining the U.T. Department of Botany faculty in 1960.
As an educator, Dr. Heilman has been a great advocate for the appreciation of the beautiful and strange features of plants, and the fantastic ploys and relationships between plants and insects. Heilman taught plant anatomy, and, for 30 years, a course that confronted broad environmental issues. The loss of insects -- pollinators-- due to environmental change is one of the issues he has presented. "These things we never see have such an important effect."
He has also performed the tedious task of close-up photography of plants and their varied parts. Heilman still leads an annual photography workshop and a pollination walk at the Great Smoky Mountain Wildflower Pilgrimage. His photographs have rarely appeared in The Tennessee Conservationist.
Heilman recommends at least three books for anyone seeking a better understanding of the language of plants and its significance: The Natural History of Pollination by Michael Proctor, Peter Yeo and Andrew J. Lack; Insects and Flowers by Friedrich G. Barth; and The Forgotten Pollinators by Stephen L. Buchmann and Gary P. Nabhan.
(Bob Fulcher is a regional interpretive specialist for Tennessee State Parks.)
Updated May 1, 2000; Send comments to Department of Environment and Conservation.