Ready for buzzing: side view of silverleaf nightshade (Solanum elaeagnifolium)
Ready for buzzing: side view of silverleaf nightshade (Solanum elaeagnifolium)
Flowers live forever in poems, paintings, photographs, and other works of art. But they would appear too fragile and short-lived to be of much use in a laboratory compared to fruit flies, bees, or little white mice. Yet flowers studied as part of scientific investigations have forever changed the way we view our world.
Let’s return to those daring expeditions of the eighteenth and nineteenth centuries, with the botanists and their illustrators collecting and drawing specimens found along the coasts of southern Africa and Australia. These men obviously had daring and productive adventures discovering and collecting new specimens for European museums. But what did they accomplish? All students and followers of that indomitable Swedish explorer-scientist Carl Linnaeus, plant collectors and botanical artists alike, followed his Linnaean method, identifying and cataloging specimens based on the number of sexual organs in each flower. The botanists also noted additional peculiarities such as petal shape, sepal numbers, the presence or absence of nectar glands, and the way that baby seeds (ovules) attached themselves to ovary walls. Such floral features, Linnaeus noted, were far more conservative, less varied, and more dependable than leaf, stem, and bark characteristics.
Over just a few decades, the botanists receiving those dried plant specimens and illustrations came to two unexpected findings. First, most of the plants collected in the southern hemisphere were distinct from those encountered and classified north of the equator. It was as if two creators had been at work. When cataloging the plants of the southern hemisphere, curators needed to publish new scientific names including those for genera and species. Furthermore, the newly discovered species were often placed in entirely new plant families and orders.
The second revelation was of the geographies of these new plant families. In particular, there was the weird family known as the Proteaceae (the macadamia nut family). Using the Linnaean method, botanists easily recognized these plants because their flowers resembled stylized hairpins, elephant heads, miniature long-barreled guns, witches’ faces with pouched mouths, or little swans with curved necks. Their flowers came in many colors and were often united in massive flowering branches resembling clubs, cobs, or tiaras. The very name Proteaceae was used to remind readers that the Greek god Proteus assumed many forms. Today, the familiar living or dried flowering branches of banksias, waratahs (Telopea), and proteas (Protea) command high prices in your local florist shop.
We now know that species in the family Proteaceae are native to every continent and large island in the southern hemisphere. But how did they get there? Imagining connections between now sunken “land bridges” or stepping-stone islands between Africa and South America seemed unlikely. Nor do the seeds of plants of this family, as a rule, float or survive long in seawater. The most logical theory had to wait until the early-twentieth century.
Alfred Wegener (1880–1930) wasn’t much of a botanist, but was respected as a great polar meteorologist and an early geophysicist. He theorized that the modern distribution of banksias in Australia versus proteas in South Africa was likely due to the long-term gradual effects of continental drift. Continents were not fixed in one place for all of geological history, according to Wegener. Like thick ice floes on a river, they collided, grinded, and bumped into one another and/or rafted apart over long periods. Many scientists vehemently despised Wegener during his life for his radical ideas, but today a new generation of geophysicists have demonstrated that plate tectonics, and moving continents, are an ancient and modern fact of our planet’s geological history.
In your international travels, you may have noticed that wildflowers are not uniformly distributed around the world. Certain kinds of flowers, even entire plant families, are found only on one continent but not others. Flowering plants have contributed to the field of biogeography, in which plant geographers study the natural distributions of plants as they exist now, where they originated, and how they moved in the past, including their slow but inexorable passage on the continents, riding along atop the raftlike tectonic plates.
Before the 1680s, the words plant sexuality formed an oxymoron. As noted earlier, the ancient Babylonians knew how to hand-pollinate date palms to produce fruit crops, even if they didn’t understand the intimate details. Their techniques persist to this day, although some date orchards now employ a pollen “cannon” to blast pollen grains onto female flowers. Otherwise, most people presumed that flowers turned into fruits spontaneously, the same way that tadpoles turned into frogs. After the invention of magnifying lenses and microscopes, such beliefs faded away, thanks to the work of London physician Nehemiah Grew and his associates. His book The Anatomy of Plants was published to wide acclaim in 1682.
Grew is remembered for his discovery that the stamens in flowers are the “male organs” of seed plants and their pollen grains are equivalent to the sperm produced by animals. His observations were largely mechanistic as he deciphered how plants functioned, much as one would examine a clock or a similar device. His research stimulated the young science of reproductive botany carried on by Marcello Malpighi (1628–94) and Christian K. Sprengel (1750–1816).
Imagine how this botanical understanding would come to change agriculture and horticulture over the centuries. If you did not like the crops you traditionally grew, you could make new ones by crossing existing varieties. All you needed was a hand lens and some feathers or a toothpick to transfer pollen from one blossom to another. You no longer needed to wait for an infrequent mutation to appear. You had the option of seeing what would happen if you crossed two different species in the same garden. Many of the garden flowers that we love and grow today began as hybrids between different species. What were these first crosses? They included the soulangeana magnolias, polyanthus primroses, almost all of our tulips, and thousands of hybrid rosebushes. Not everyone approved. Pious people insisted that godless scientists were defying the laws of creation by making plants without the blessings of their deity.
Blasphemous or not, the science of genetics originated from crossing flowers for a scientific purpose. It all began in an Austrian monastery garden more than 160 years ago. In St. Thomas’s Abbey, a monastery in Brno, Austria-Hungary (now the Czech Republic), an Augustinian monk made careful observations and predictions while crossing flowers of the common garden pea (Pisum sativum).
Gregor Johann Mendel (1822–84) is recognized as the father of modern genetics, the inheritance of physical traits from parents to offspring in plants and animals. Between 1856 and 1863, Mendel expanded the monastery’s five-acre garden. His abbot didn’t want the friars working on sexual reproduction in animals, so Mendel and his assistants switched to plants, less controversial experimental subjects. During this time, Mendel crossed around twenty-eight thousand pea plants. Mendel’s crosses over many pea generations tracked the inheritance of seven physical traits including plant height, pod shape and color, seed shape and color, along with flower position on the stem, and flower colors of white and violet. He noted how these physical traits varied naturally among his pea plants, but wanted to know what happened in the next and succeeding generations.
An example of one of Mendel’s simple experiments follows. Like the flowers of most plants, pea flowers are bisexual. Male and female organs are found together in the same blossom. Therefore the flower of a garden pea usually pollinates itself shortly after the petals open. Mendel interrupted this self-pollination by collecting pollen from one parent (the donor or father pea blossom) and rubbing it on the receptive pistil of a flower on a second pea vine (the seed mother). He marked these crosses with short pieces of identifying yarn. If this cross resulted in a pea pod, he let it mature and dry. He counted the peas and noted their color, yellow or green, and if they were wrinkled or smooth. He planted them to find out which traits they had as mature plants.
Mendel took pollen from a violet flower and dabbed it onto a white flower. He gathered and sowed the seeds from this cross. One hundred percent of the offspring plants, the first filial (F1) generation, had violet flowers. Somehow, the white-colored flowers were suppressed. Gardeners, and scientists of the day, believed in “blending inheritance,” that the flowers of the next generation should have been mauve or pink, the intermediate color between the parents, or that the progeny should have shown a ratio of 50 percent violet flowers and 50 percent white flowers. This clearly never happened with Mendel’s peas.
Mendel allowed the next generation (F2) to self-pollinate. That yielded 705 violet-flowered plants and 224 with white flowers, a ratio of 3.15 of violet to white. One in every four pea plants had purebred recessive traits (white); two of four were hybrid (purple); and one of four had dominant traits (purple again). He concluded that the pea-plant traits were either expressed (visible traits) or latent (hidden but still there). The dominant traits (for example, flowers with purple petals) were inherited, but did not change when different plants were crossed. On the other hand, the recessive traits (for example, white petals) became latent, but reappeared in the second generation.
From his numerous crosses and fastidious record-keeping, Mendel made two generalizations relating to the curious 3:1 ratio that he repeatedly found. This observation became his Law of Segregation (of alleles), and the Law of Independent Assortment. College biology students recognize both of them today as Mendel’s Laws of Inheritance.
Where would our knowledge of genetics be without the use of plants to provide answers (in the form of seeds) within one or two growing seasons? (Remember that the use of short-lived fruit flies to study inheritance didn’t begin until the twentieth century.) Mendel’s story of one gene controlling one plant character (e.g., flower color) was not the end of the story, as we know now from the work of German botanist Carl Correns (1864–1933) and his four-o’clocks, giving genetics its concept of incomplete gene dominance.
Charles Darwin was a contemporary of Mendel’s but appeared to be unaware of the monk’s groundbreaking but obscure publications. Nevertheless, he shared Mendel’s interest in the flowers of annual, domesticated plants, but Darwin’s research went further. He was plainly fascinated by variation of floral forms throughout what Victorians called the Vegetable Kingdom. The blossoms of hundreds of species passed under Darwin’s microscope. When he couldn’t grow a flower, he set off to find wild plants around his countryside home. Who could have predicted that the native orchids and primroses of an English countryside would provide hard evidence for Darwin’s evolutionary theories?
It’s time to retire our warm and fuzzy views of Darwin contemplating a chimpanzee (or the chimp contemplating Darwin, which appeared in mocking newspaper illustrations of the time) and his nostalgia for the giant tortoises and the finches of the Galápagos Islands. These charismatic vertebrates certainly played roles in his science-changing theories, but we must emphasize that over forty years of his adult life were spent in and around his country estate in Kent (now one of London’s southernmost suburbs). Following the publication of On the Origin of Species in 1859, Darwin wrote a series of books based on home-based research that bolstered his new theories. Seven of those books were plant studies, and three of them dealt almost exclusively with the form, function, and breeding of flowers.
Plants illustrate important concepts in On the Origin of Species. When Darwin referred to “the struggle for existence,” he based his research on his own garden plots and woodlots, not on large exotic birds or beasts. He found that if you crowd too many seeds into the same garden plot, most of the sprouting plants die or fail to mature and flower. Today, we call this “survival of the fittest,” and plant ecologists are still studying the effects of overcrowding. Darwin had a far different vision of what flowers could teach us compared to Linnaeus. Darwin used floral sexual characters to identify species, but unlike Linnaeus he was convinced that flower forms had specific functions encouraging a plant’s survival. More important, in a Darwinian universe, floral architecture changed over time as environmental conditions and pollinators came and went.
Why was the first book that Darwin published after On the Origin of Species all about orchid flowers? While this book has an exhaustively long title—On the Various Contrivances by Which British and Foreign Orchids Are Fertilised by Insects and on the Good Effects of Intercrossing (1862)—that title does describe the contents within the book. Darwin begins his analyses on a half dozen wild orchid species growing a short distance from his home. After these flowers, he turns to the tropical blooms in his greenhouse. It doesn’t matter whether he’s dissecting the flower of a long purple (Orchis mascula) collected from the local woodland or a large Cattleya from Mexico. All the flowers reveal much the same story, and Darwin provides valuable lessons in floral biomechanics. All those strange organs that are fused together in orchid flowers have a unique way of sending and receiving their pollen on the bodies of visiting insects. Pollen grains are jam-packed into ovoids, shrink-wrapped, then attached to sticky bases that become glued to the back or tongue of a visiting bee or butterfly. The fancy doodads in orchid flowers may look special, but all of them are actually derived from standard petals, stamens, and pistils.
Evolution in orchids represents Darwin’s concept of “modification by descent.” With natural selection determining which flower will set the most seeds, the orchids we know today wear various contrivances that must have been absent in their ancestors. The flower forms that we see in our greenhouses, prom corsages, and wedding bouquets promote cross-pollination, which was a radical idea for Darwin’s readers. As orchid collections became a Victorian fad in the nineteenth century, people presumed that the weird flowers must all self-pollinate in the wild because, in all orchids, male and female organs fuse together. No, Darwin says. When orchid organs unite within the same flower, they promote outbreeding, not inbreeding (selfing).
By 1876, Darwin hammers home the lessons of 1862 with The Effects of Cross and Self Fertilisation in the Vegetable Kingdom. Now he’s gone down the garden path, or his beloved Sandwalk, his thinking path at Down House, and is concentrating on annual plants, just like Mendel before him, but Darwin is experimenting on dozens of short-lived vegetables and bedding flowers. The Victorians preferred annuals to perennials as they could be quickly massed into colorful patterns in their gardens. When the petunias and marigolds died, the beds were turned over for new seeds or plants. It was a time when bedding plants were being developed from all over the world. Darwin hand-crosses or self-pollinates California poppies (Escholtzia), mignonettes (Reseda), sweet peas (Lathyrus), ragged robins (Clarkia), nasturtiums (Tropaeolum), pheasant’s eyes (Adonis), pinks (Dianthus), pocketbook flowers (Calceolaria), etc., and waits for their seeds.
Do these plants always make the same number of seeds based on cross- or self-pollination? No, you tend to obtain more seeds after cross-pollination. Do the offspring of self- and cross-pollinations show equal rates of survival in overcrowded beds? No, the seedlings grown from cross-pollinated seeds tend to be fitter. Once again, this was a radical notion in Darwin’s day, when breeds of cattle, pigs, dogs, pigeons, and poultry were standardized by crossing parents and offspring. It was also a time in which families maintained their wealth by encouraging marriages between first cousins. For much of the nineteenth century an Englishwoman lost all her property, and right to inherit, as soon as she married. Darwin was well aware of the genetic dangers of inbreeding, in all animals, including people. (Even so, he married his first cousin Emma Wedgwood, of Wedgwood porcelain fame, and together they had ten children, none with birth defects, and seven of whom survived to adulthood.)
Darwin quickly followed the second book with a third based on yet another walk in the woods. The Different Forms of Flowers on Plants of the Same Species (1877) begins with the mating habits of primroses and cowslips (Primula spp.). This leads us to the two or three reproductive forms found in each population of bluets (Hedyotis), lungworts (Pulmonaria), loosestrife (Lythrum), flax (Linum), and buckwheat (Fagopyrum). Once again, the pressures of natural selection appear to favor plants that make flowers that avoid self-pollination and retain forms that permit only cross-pollination. Ah, could this also help answer why the majority of animals on this planet have only two genders?
Of course, not all research on flowers has been so universal and earth-shattering. As in most scientific studies, the biological study of flowers increases our knowledge in small but useful increments. And sometimes it has been very practical, helping us to increase the yields of useful plant-derived products.
When we explored floral pollination in an earlier chapter, the subject of wind-pollinated flowering plants was largely neglected. But such plants and their flowers are important, even if somewhat green and not as showy as most other flowers. About 10–20 percent of the flowering plants in any habitat cast their reproductive fates to the breezes by broadcasting ridiculously large numbers of pollen grains into the air. Further, the wind-pollinated cereal and grain crops (e.g., rice, wheat, millet, rye, barley, and corn) feed the world, providing essential carbohydrate calories that keep just over 7 billion people from gnawing starvation. Wind-pollinated flowers are fascinatingly complex, far more interesting than you might at first realize.
Jojoba (Simmondsia chinensis) is a nondescript, native–Sonoran Desert plant that blooms in early February. You may have first met this plant while taking a shower. Most people know the word jojoba as a shampoo ingredient. Jojoba fruits, uniquely, contain a liquid wax identical to sperm-whale oil, with its unique properties. (But no whales died to give you nice clean hair.)
Like some of the plants Darwin studied, jojoba is dioecious. There are equal numbers of boy bushes (pollen makers) or girl bushes (oilseed makers). Neither sex bothers to make petals, and breezes, not bees or hummingbirds, move pollen from male to female flowers.
The reproductive biology of this lowly shampoo plant is every bit as exciting as any whodunit. I’d known about the pioneering biomechanical studies of researcher Karl Niklas of Cornell University. I was also familiar with fossil plants from the Carboniferous period that Niklas modeled, along with his ideas about how these ancient plants sieved pollen from the air like a modern snow fence. I telephoned Karl. To my delight, he would look at jojoba, and we collaborated on these studies. I collected fresh female jojoba flowers and pollen. A few days later, the green flowers were “flown” in a wind tunnel in Karl’s laboratory. A stroboscopic light flashed four hundred times a second while a movie camera filmed the action. Smooth-flowing, nonturbulent air passed across the flower. Unlike most aerodynamicists Karl didn’t use smoke in his wind tunnel to track flow lines (you’ve seen car commercials with smoke passing over the hood of a luxury sports car). Instead, Karl used a tiny, custom bubble generator. One-millimeter-diameter, helium-filled soap bubbles were released upstream of our female jojoba blossom. We jiggled a wire holding jojoba pollen, and thousands of pollen grains floated toward the female flower.
A few days later, the film had been made into eight-by-ten-inch prints. We couldn’t believe our eyes. The shiny, miniature soap bubbles faithfully tracked the smooth airflow, then beautifully demonstrated when the flow broke up into turbulence as it hit, then passed by, the leaves, stem, and flower farther downwind. The rabbit-ear leaf arrangement caused something unexpected to happen: the airstream abruptly shifted ninety degrees toward the flower. The leaves directed a shower of pollen at the female flower. When the leaves were removed, the directional pollen shower was lost. Watching our movie clips revealed how pollen grains had multiple chances to impact the styles. We observed pollen-containing air swirling in tight eddies, called von Kármán vortices, over the tridentlike styles for almost a minute. This “clever” wind-pollinated flower had improved its lottery-like odds of being pollinated. Due to slowly accumulated changes in its floral and leaf morphology during its evolutionary history, the pollen grains remained aloft in the vicinity of its sexual parts, enhancing pollen capture. Never again would I think of wind-pollinated flowers as uninteresting. Nature, it seems, always has surprises for the prepared observer.
Through their long evolutionary histories, wild plants such as jojoba, and the grasses we know as cereal crops, have developed strategies to loft their pollen into the breezes so that some of it, although a tiny fraction, makes its way to female flowers, with their sticky stigmas, waiting downstream.
I was careful not to rip the delicate turkey oven-roasting bag on the spines of the cactus, since it would ruin our experiment. What was I doing roaming the Sonoran Desert with a plastic oven bag? The plastic (food-grade, heat-resistant nylon) in these bags is perfect—relatively inert, so it won’t outgas contaminants that would have confounded our fragrance studies. I was in a remote Arizona-desert cactus patch to collect sweetly aromatic floral scent molecules. In the distance was my prize.
A few individual plants of queen-of-the-night cactus (Peniocereus greggii) sent their camouflaged, thin, gray stems up the branches of creosote bushes and ironwood trees. You have to know where to find them because you aren’t likely to spot these cryptic cacti otherwise. When in bloom they’re spectacular and unmistakable. Rumored to bloom for only one night each year, individual plants actually bloom for several nights during the intense desert heat of May and June. Peniocereus is almost a mythical plant among cactus and succulent lovers. I tugged the oven bag over an enlarged bud, one that I believed was going to open in about two hours, then close early the next morning. I adjusted the bag around its stem and attached some flexible, plastic tubing. My partner in scent, Dr. Robert Raguso of Cornell University, did the same thing at another cactus bearing equally promising buds. We connected battery-powered air pumps to the tubes attached to the oven bags. This volatile-trapping technique has been worked out by Rob and other chemists. Outdoor air is drawn through the oven bag over the delicate Peniocereus blooms releasing their delicate fragrance molecules. The floral-scented air is drawn across chemically activated material inside short glass tubes, “odor traps,” where the molecules get stuck. Later, we wash the cactus smell from the traps with organic solvents and freeze the samples until we are ready to identify the odorant molecules using a gas chromatograph/mass spectrometer.
Only a mile or so away, along a desert wash, close to agricultural fields just across the Mexican border, were flowering clumps of sacred datura (Datura wrightii). Unlike the “queen,” the Datura plants bloom for months, often producing fifty or more big, trumpet-shaped flowers per plant. They are the nectar sources for hawk moths, and the leafy food for their big, green caterpillars. The large hawk moths (Manduca sexta and M. quinquemaculata) are the familiar tomato hornworms despised by most gardeners. Unfortunately, its usually “squish on sight” for these marvelous animals. The adult moths are equally spectacular, flying miles every night in search of the nectar deep within the creamy-white blossoms. As we watched, the husky moths hovered and unrolled their slender, eight-inch-long tongues, probing the blooms. Others, including the smaller white-lined sphinx (Hyles lineata) were forced to land, crawling inside to reach the nectar with their much shorter tongues.
We were here to find out if true chemical mimicry existed between the queen-of-the-night cactus flowers and the sacred-datura trumpets. Both plants bear large, white, nectar-rich desert flowers pollinated by the same kinds of moths. They looked alike, seemingly mimics of one another, so shouldn’t they also smell alike? We were going to find out. A few weeks later, back in his Cornell University laboratory, Rob took one of the queen’s glass scent traps from his deep freeze and let it come to room temperature. With a few tablespoons’ worth of hexane solvent he washed the scent molecules into a laboratory vial. He did the same for traps containing Datura scent. Under a stream of nitrogen gas, he concentrated their scent molecules. Using a glass syringe, Rob injected the scent-laden solvent into a scientific oven called a gas chromatograph (GC). Inside, the injected fragrance molecules move along a coiled-glass capillary column, with which they interact. The oven starts at room temperature and is programmed to continually rise and then hold at a high temperature. The lightest molecules, such as hexane, come tumbling out the end of the capillary coil first and hit the gas chromatograph detector, then the heavier fragrance molecules follow in order of increasing size and reactivity.
On a computer screen we could see the separated chemicals as needlelike peaks appearing on the GC trace. After being detected in the gas chromatograph oven, some molecules from each peak are diverted into an instrument call the mass spectrometer. Inside, the molecules are torn apart, and their resulting ions are revealed. Using a gas chromatograph coupled to a mass spectrometer, chemists routinely identify unknown chemical compounds, including pesticide residues on vegetables or priority pollutants in water samples. In all, we saw only eight distinct peaks (chemicals) in Peniocereus, but more than a dozen for sacred datura. Surprisingly to us, but likely not to the moths, the queen and the Datura scents were not chemical clones. The dominant scent chemicals we found in Datura were complex blends of terpenoid, benzenoid, aliphatic, and nitrogen-containing compounds, while Peniocereus had only benzene-containing compounds, eight of which were also present in Datura. Perhaps a bit of scent mimicry was taking place. To my largely untrained nose, Peniocereus smells faintly like wintergreen, and indeed, one of its chemicals, methyl salicylate, is also present in oil of wintergreen, or wintergreen breath mints. Stranger yet, we tested another Sonoran Peniocereus cactus (P. striatus) and enigmatically found it to be scentless, a rarity for any flower. It likely attracts its moth pollinators solely by the shape and white color of its flowers on moonlit nights.
Raguso has tested many of these scent compounds individually and in combinations on Manduca moths flying inside laboratory wind tunnels and in field tests. Using scientific experimentation, we collected, identified, and finally bioassayed (field-tested with living animals) the scent molecules released onto the breezes by the blooming desert trumpets. Moths flew in to inspect our scented artificial blooms.
In the first chapter we learned that bees see much differently from humans. The ultraviolet (UV) wavelengths that burn our skin at the beach are the same region of the spectrum that some flowers use as a “secret” communication channel with their pollinators. If we could see like a bee, we’d experience flowers as they do. We would be shocked, perhaps terrified, by how different the world of flowers and plants looks to bees. Bees are more myopic than we are, since they have only five thousand four hundred ommatidial cells (special cells that enable bees to see) per compound eye, compared to the 120 million rods and cones lining human retinas. Not surprisingly, bees have difficulty discerning fine details. Think of our own ability to detect the separation of closely spaced parallel lines on a printed page. This is the so-called minimum visual angle. For a worker honey bee, this angle is one or two degrees, while in a human eye, the visual resolution is a fine 0.01 degrees, making our eyes about sixty times better than a bee’s in resolving power. However, bees’ eyes still function extremely well for them. Why? Bees are small and tend to be close to the flowers and other natural objects that they must see for their survival (e.g., nest mates, predators at the hive).
Using a camera and a special UV filter, we can come closer to experiencing how bees see flowers during their foraging visits. With the right film or digital camera and UV-passing filter, you can experience how some UV-marked flowers might appear to hungry bees. A flower photographed in UV was presented in chapter 1.
Time-lapse (TL) photography is used to make apparent the imperceptibly slow movements of petals and other structures unfurling from tight buds into open flowers. Think of the amazing sequences of red roses in the film American Beauty, which were created by TL master Louie Schwartzberg of BlackLight Films in Los Angeles. The opening of a rose or other blooms in thirty seconds of faster-than-life playback is magical. Audiences never tire of seeing the majesty and beauty of flowers revealed by this photographic marvel. I’ve used DSLR cameras outdoors and in makeshift indoor studios to record the opening and closing of saguaro and the queen-of-the-night flowers. Although it’s a fascinating technique used by artists and scientists alike, the subject is too broad to discuss here in greater detail.
In my first scientific paper, with Dr. Claris E. Jones, published while a sophomore at California State University, Fullerton, we described how modified flowers could control the behavior of free-flying wild bees. In this set of experiments, our Organization for Tropical Studies work group located a legume (Caesalpinia eriostachys) in full bloom, to test our ideas. The small tree was growing at the Guanacaste field station, an old cattle ranch in the northwest province of Costa Rica. It was the dry season, of February–March, and the neighboring Tabebuia trees were giant masses of yellow or pink blossoms. Our little Caesalpinia wasn’t as impressive but was abuzz with clouds of noisy, native bees. Groups of digger bees (e.g., Centris and Gaesischia) whizzed past our heads to and from the nectar-rich flowers. We planned to see if we could trick the bees, altering their behavior, by messing with their little, yellow flowers. That was one of our problem-solving exercises during the eight-week field course.
By using our large, heavy, but portable Sony television camera and magnetic tape unit, we already knew some of the flower’s secrets. As a “caesalpinaceous legume” (a subfamily in the pea family), our flowers had four side petals and a fat, uppermost one called the banner petal. Like all banners, this one was for advertising, a miniature billboard, a floral signpost, if you will, for bees. It was a uniform canary yellow with squiggly, red lines at its base. These lines are guides that show the bees where to probe to find the hidden, sweet nectar, the correct place to drink.
Under the UV gaze of the now-antique black-and-white television camera, we saw the small, yellow blooms as the bees did. The side petals looked bright white, indicating their intense UV-light reflection. The bees saw the lateral petals as a mixture of reflected yellow wavelengths plus UV. Physiologists call this color “bees’ purple,” even if we don’t know exactly how it looks since humans are UV blind. The slender stamens tipped with their plump, orange anthers, along with the upright banner petal, were the same color. However, they appeared black to our panchromatic video camera. Black, the absence of color, indicates that the dark parts of our flowers strongly absorbed the incident UV rays. Further, they reflected only the yellow wavelengths of light, which is how the bees saw them.
We got to work dissecting flowers and modifying others, figuring out what we might test. One of our experiments involved gluing a yellow side petal over the banner with a bit of Elmer’s glue. The incoming bees seemed confused. They landed but had a difficult time finding the hidden nectar. We also twisted the flowers to one side, so the UV-absorbing banner was now horizontal. To our surprise, the bees turned and landed sideways, then probed for nectar. We also turned flowers completely upside down. Scientists can be mischievous at times. The bees weren’t fooled by our manipulations, instead they executed a quick barrel roll maneuver in midair just before they landed. Upside down on the flowers, they quickly drank the nectar below the banner petal.
Later, we learned that similar floral manipulations were conducted decades earlier by plant ecological pioneers Frederic Clements and Frances Long on alpine flowers near the Carnegie Alpine Laboratory on the slopes of Pikes Peak in the Colorado Rockies. Clements later conducted research at the University of Arizona Desert Laboratory on Tumamoc Hill in Tucson. Gently pestering bees in the name of science seems to have a long and respected history.
One eastern-US bumble bee species, Bombus impatiens, has almost become the white-mouse or laboratory-rat analog for insect behaviorists across the United States. Bombus impatiens occurs naturally east of the Mississippi River, but has been cultured by the millions, then transported to pollinate commercial greenhouse tomato and pepper crops. You can pick up the telephone and order one or more bee colonies. For about $80 you’ll get a small starter colony (a queen and her twenty or thirty daughters) handily delivered to your university laboratory by FedEx. Getting bees doesn’t get any easier. Much is now known about this bumble bee since researchers have studied and published accounts of its behavior for the past decade. You still must care for your indoor bees, like pollinator pets, giving them pollen and sugar syrup, and providing them with real flowers and places to fly.
Our team of insect behaviorists, pollination biologists, and neurobiologists (at the University of Arizona, and the University of Nevada at Reno; Drs. Dan Papaj, Anne Leonard, Wulfila Gronenberg, and I) decided to test some of the tenets of bumble bee learning and their memory abilities. We especially wanted to learn how bumble bees make their decisions about whether to forage for nectar or for pollen on a given trip, and if they switch their preferences on successive days, or as the colony workforce grows. We were determined to construct artificial, robotic flowers, and to fly, train, and behaviorally test the bees in our respective Tucson and Reno laboratories.
University of Arizona doctoral candidate Avery Russell and I watched closely as the pale-yellow-and-black worker Bombus impatiens walked through the clear plastic tunnel leading out of the colony where she had emerged as an adult about a week earlier. She approached the Y-tube junction and turned left. A little farther along the corridor, she entered a wooden and plastic chamber that held Solanum flowers with their stamens poking through a central opening. These flowers had artificial petals made of either bright yellow or blue paper, supposedly colors preferred by bumble bees in tests by other researchers. On the way out of her nest, the worker bee walked under a sensor. A tiny, harmless two-by-two-millimeter radio-frequency identification device (an RFID chip) containing a miniature circuit and antenna was previously glued to her back, as was on many of her techno-bee nest mates.
Passing under the radio-frequency sensor, her eight-digit, unique ID code (let’s call her bee 54) rang out; the signal was sent to a nearby laptop computer. I watched as her number appeared in the spreadsheet. At least we now knew the whereabouts of bee 54. During her time in the foraging arena, her visits to the blue versus yellow faux flowers would be recorded by a video camera. On her return trip to the nest, laden with a full honey stomach of sugar water, her foraging time would be recorded by the trusty RFID system, just as if she were a human factory worker punching an old-fashioned time clock. Bees have a finely tuned visual system and are capable of rapidly learning associations of floral shape, colors, scents, surface textures, and possibly even electric fields left behind as footprints by previous bee visitors. Our group is using 3-D scans of living flowers, including Solanum, to fabricate plastic, exact-scale models of these blooms for presentation to our bumble bees this time in Reno by Anne and her grad student Jake Francis and post doc Felicity Muth. Our artificial flowers will be more realistic, while odorless and uniformly colored, for our further investigations of bee learning.
Work with bumble bees and floral colors has been conducted since the early 1900s, but most scientists have focused on nectar rather than pollen. We are interested in whether workers become nectar or pollen specialists and especially the neglected aspects of pollen collection. In the experiment above, we determined that bumble bees did learn to recognize our yellow and blue artificial flowers while pollen foraging. We have also used RFIDs as laboratory time clocks for bees, exploring questions about bumble bee fidelity to certain flowers and colors, and for recording the comings and goings of free-flying bumble bees outdoors. Our bees remembered these rewarding color associations in the short term, but they didn’t remember them once back in the colony even a day later. Hmm, perhaps the bumble bees were dreaming of real flowers, ones brimming with nectar, not our crude, construction-paper, make-believe flowers.
A female bumble bee (Bombus impatiens) sonicates pollen from the anthers of a deadly nightshade blossom (Solanum tridynamum) in the laboratory
Forensic botany uses leaves, fibers, wood, but especially pollen grains to help solve violent crimes or to settle legal disputes. In recent years, it has been widely accepted by the courts in other countries and to a lesser extent in the United States. Laboratories with resident pollen identification experts (palynologists) are using pollen, along with other plant evidence including DNA samples, to help solve mysteries. Botanical sleuthing has been featured in some of the immensely popular US television shows including CSI, Law & Order, and Cold Case. Pollen grains are composed of a natural biopolymer and are highly resistant to decay. Pollen can even be retrieved from inside solid rocks millions of years old. These microscopic, dustlike particles easily find their way into clothing, hair, nasal passages, under fingernails, and into stomachs of those murdered or the perpetrators. DNA can be extracted from pollen for yet more precise determinations. Pollen grains can remain unnoticed, bearing silent witness to events, long after fingerprints, DNA, and other equally perishable evidence have faded away.
In Christchurch, New Zealand, in 1997, a young woman was pulled into an alleyway and raped. Shaken by the traumatic attack, she was still able to describe her assailant to the police. Shortly thereafter, a possible suspect matching her description was taken into custody. The male suspect admitted being in the area but claimed he had stopped to help her, suggesting that this was why she remembered his face. No DNA trace evidence was recovered, but the police noticed dirt stains on his clothing. He claimed this was from his working on his car. Nevertheless, the police wisely took his clothing as evidence and sent the soiled fabric to the palynology laboratory of the New Zealand government’s Geological Survey.
The alleyway where the rape took place was lined on one side by a row of flowering shrubs of wormwood (Artemisia arborescens), a plant native to the Mediterranean region and infrequently planted as an ornamental in New Zealand. Some of the plants appeared as if they had been broken or flattened during a struggle. A lot of pollen was mixed in with the soil particles on the suspect’s clothing, and 77 percent of it was Artemisia pollen. Investigators combed the area near the suspect’s home and other areas but couldn’t find any Artemisia plants except at the crime scene itself. The pollen evidence was presented by an expert botanical witness at the suspect’s trial, and he was convicted, receiving an eight-year prison sentence.
We think that fragrance and memories are linked because smells plug directly into the primitive part of our mammalian brains known as the limbic system. Part of human memories and emotions also reside in this area of the brain. Natural scents may consist of one molecule, or usually many kinds. Sniff a garden rose and thirty or more different chemical compounds, including dominant ones such as geraniol and geranyl acetate, are pulled into your nose. These fragrant molecules enter the warm and humid channels inside our nasal sinuses and become trapped. Here, neuronal processes occur that nearly instantaneously produce the sensations of smells in our brains. We easily distinguish between flower aromas and other scents. Within our noses, a pair of dime-size, mucus-covered patches are packed with millions of odor-sensing cells. These cells have minute, hairlike structures carrying receptor sites. Here, the scent molecules temporarily bind, causing electrical impulses to race to the brain, which registers them as familiar scents. Ah, the scent of a rose, a sweet violet, or the unforgettably powerful scent of a Stargazer lily; we recognize them all. Amazingly, after a century of research by olfaction scientists, we still don’t understand the exact details about how the scent molecules produce their signals, what happens after they bind to their receptor sites—what actually goes on behind the scenes on bee antennae or inside our noses.
Currently we have two competing and dissimilar theories for how olfaction works in our noses at the molecular level. The oldest, traditional model is the stereochemical theory of olfaction (called odotope). Scent molecules are thought to bind in a lock-and-key fashion. Their unique, 3-D molecular shapes fit into receptor sites on the olfactory sensory cells. The sense cells are turned on and register this particular sent in electrical signals sent to the olfactory-bulb region of the brain. Instantly we recognize the scent of a rose! This theory is the one accepted today by most scientists. The details, however, are unclear. Truly, we don’t fully understand what happens in the olfactory sense cells at the deepest levels.
A newer (first proposed by Malcolm Dyson in 1928) theory for how scents are detected and recognized is being championed with new data and insights by perfumer and biophysicist Luca Turin. This opposing theory of olfaction is called the vibrational theory of smell. Proponents such as Turin believe that individual scent molecules resonate at a particular frequency that is different from that of other scent molecules. The vibrational frequency switches on the correct receptor. A customized molecular-shape fit alone may not explain how we smell things. The vibration of the scent molecules produces the signal to the olfactory-bulb region of the brain, causing the recognition of a single scent, or blends of scented molecules. Other quantum-level biological effects may ultimately help explain some conundrums in photosynthesis, or vision, and magnetic-field reception in migrating animals. The atomic-level vibration of scent molecules varies widely from molecule to molecule among odorants. We can think of their resonant vibrations as almost like a musician striking a tuning fork while the corresponding receptive sensory cells fire in synchrony with those specific vibrations.
Exciting recent work using fruit flies and honey bees may help to confirm the resonance theory of olfaction, at least in these odor-specialist insects. I was fortunate to collaborate in an experimental test of the resonance theory of olfaction using honey bees in the University of Arizona laboratory of Dr. Wulfila Gronenberg and his students. Luca Turin sent us samples of highly purified aromatic compounds. Turin had replaced some of the hydrogen atoms in these fragrant molecules with deuterium atoms. This would make the chemicals behave the same in chemical reactions but produce unique quantum vibrations. They would have exactly the same molecular shape but exhibit different vibrations. If, therefore, worker-bee olfaction operates by shape recognition, then our honey bees should not be able to tell the normal chemicals from their deuteurated cousins. If, instead, the bees’ sense of smell is based on recognition by their underlying molecular vibrations, they would be able to tell the chemicals apart in our laboratory assays.
We used a feeding bioassay called the proboscis extension response (or PER): bees will stick out their tongues (the proboscis) when their feelers (antennae) touch nectar in a flower, or sugar water in the laboratory. Each bee circles and is brought close to a tube delivering one scent at a chosen concentration together with sugar water. The bee will then stick out her tongue and be rewarded with a drop of sugar water. Upon repeated trials (simultaneous paired presentation of odor and sugar reward) the bees learn to associate the odor with the reward and will then extend their tongue to the odor alone. I like to joke with my behavioral colleagues that the bees are sticking their tongues out at us. We tested all of Luca’s normal molecules (i.e., those containing hydrogen atoms) and their deuterated substitutes. After only a few trials, and at statistically significant levels, the bees differentiated between the paired test substances. They hadn’t been fooled. We assume that they were discriminating between the odorants based on differences in their molecular resonance. This is only the second molecular shape-versus-vibration test using insects, after earlier tests with fruit flies. Previous results with human guinea pigs weren’t as conclusive. Therefore, we don’t yet know the complete answer to this fascinating question of how humans, or bees, smell flowers, which is all part of the intrigue and fun of doing original scientific research. It’s my hope that this chapter has provided not only a historical perspective of studies by famous scientists (e.g., Mendel, Darwin), but also a personalized glimpse into some of my own flower and bee research over several decades. I’m sure that many more exciting discoveries with flowers, and flower-loving animals, lie ahead for both scientists, citizen scientists, and inquisitive individuals everywhere.
