To compete for the attention of pollinators, flowers have evolved ingenious methods to entice hungry bees, birds, moths, butterflies, and beetles to inadvertently act as pollen-carrying liaisons between blooms that would otherwise never touch. Their main offerings? Sugar-filled nectar and protein- and vitamin-rich pollen.
The amazing diversity of flowers results from their unique adaptations to lure a range of pollinators. Every aspect of a flower, from the designs on its petals to the timing of its blooming, is vital to the process. As your students observe flowers and pollinators in indoor and outdoor settings, invite them to consider and investigate how this unsurpassed advertising lays the groundwork for pollination. This section describes some of the more apparent features used to draw in customers.
Since most pollinators fly, flower color sends a bold signal to potential partners passing by. Different pollinators may see the same colors differently, and some can't see certain colors at all, but they may be drawn by other characteristics, such as scent. The colors that humans see are not necessarily what bees or beetles see. Regardless of how it is perceived, color is a primary means by which flowers grab attention. Many flowers, such as foxgloves and irises, also feature stripes, spots, or other markings that guide pollinators toward food. (Some of these nectar guides are invisible to humans but quite apparent to hungry bees!) Some, such as Gaillardia (blanket flowers) have concentric rings, providing a target focused on the nutritious nectar "bull's eye." Lilies have ridged petals that similarly guide their guests. Have your students look at a delphinium blossom. Don't those tufts of hairs in the center look like a bee who has already found the flower appealing?
As your students observe who visits which flowers, see what they can uncover about the relationships between flower colors and patterns and the visitors who frequent them. If students notice that some flowers change color over time, invite them to conjecture why. (Color changes can be a way of preventing pollinators from wasting energy on an already-fertilized flower so the other flowers on the plant have a better chance of being visited.) Students in the South can discover that bluebonnets lure bees with a white or yellow spot, which turns red (a color bees can't distinguish) after pollination.
Aromatic blooms signal food to roving bees, butterflies, moths, wasps, and some flies. Certain orchids actually emit an odor evocative of female insects to arouse the males to visit! Other flowers, such as skunk cabbages, smell like rotting flesh to attract insects such as carrion-eating flies or certain beetles looking to lay eggs. Flowers that appeal to a wide range of pollinators often have light aromas, which accommodate a variety of taste buds. Others, such as those that bloom at night, have strong, distinct scents that attract moths and bats in the dark. Many flowers typically pollinated by hummingbirds, such as nasturtiums, don't need to be fragrant because their pollination partners have little sense of smell. Consider inviting students, blindfolded, to try to distinguish among different flower smells. Tough? Honeybees can tease out hundreds of aromas!
Flowers' shapes are important for protecting pollen, attracting or precluding certain pollinators, or ensuring that pollen is picked up and transferred. For instance, butterflies tend to prefer flat, open surfaces with views (e.g., zinnias), while certain bees seem to like those with special petals that serve as landing platforms (e.g., delphiniums). Open, bowl-shaped flowers (e.g., poppies) can be easily seen by and offer warm access to short-tongued insects. The shallow blossoms of milkweeds, phlox, mints, and similar flowers also appeal to short-tongued insects such as honeybees and wasps. The nectar in tubular flowers, such as bee balm, is available to beaks and tongues with a long reach. Drooping, bell-shaped flowers protect their sexual parts from weather and offer food and shelter for honeybees and bumblebees, who can feed while hanging. Some flowers, such as snapdragons, have hinged petals or other mechanisms, to conceal their sexual parts and nectar. They are closed to all but selected pollinators (in this case, certain bees) who have the dexterity, strength, and tenacity to open the flower. What can your students discover or infer about flower shapes and their relationships to different pollinators?
Many of what we call flowers are actually groups of tens or hundreds of tiny flowers in a cluster or along a stem. Imagine what the advantages of this arrangement might be for the flowers or pollinators. The large display of tiny flowers signals loudly to passing pollinators, saving them time and energy. Many such plants bloom and supply food for a long time, keeping pollinators coming back as the flowers open in sequence.
One of the largest families of plants, the Composites, has flowers so tightly packed that they look like one bloom. This family, which includes familiar sunflowers, daisies, and zinnias, has showy outside ray flowers that are exclusively for advertising and hundreds of plain inside disc flowers, ready to be fertilized. These ubiquitous flowers offer up loads of nectar over long periods to hundreds of long- and short-tongued insects. But if they fail to get pollinated, many can take care of business themselves!
Challenge your students to try to find flowers that grow in groups, imagine how the grouping might improve chances of pollination, and use hand lenses to explore these tiny miracles. Consider displays of flower clusters in a clover, dill, or Queen Anne's lace. Or observe plants with flowering spikes, such as loosestrifes or liatrises, over time as they bloom from the bottom up or top down, in sequence.
* bees -- Yellow, blue, purple flowers; there are hundreds of types of bees that come in a variety of sizes and have a range of flower preferences;
* butterflies -- Red, orange, yellow, pink, blue; they need to land before feeding, so like flat-topped clusters (e.g., zinnias, calendulas, butterfly weeds) in a sunny location;
* moths -- Light-colored flowers that open at dusk (e.g., evening primroses);
* beetles -- White or dull-colored, fragrant flowers since they can't see colors (e.g., potatoes, roses);
* bats -- Large, light-colored, night-blooming flowers with strong fruity odor (e.g., many cactus flowers); bats don't see well, but have a keen sense of smell;
* flies -- Green, white, cream flowers; many like simple bowl-shaped flowers or clusters;
* carrion-eating flies -- Maroon, brown flowers with foul odors (e.g., wild ginger);
* hummingbirds -- Red, orange, purple/red tubular flowers with lots of nectar, since they live exclusively on flowers (e.g., sages, fuschias, honeysuckles, nasturtiums, columbines, jewelweeds, bee balms); no landing areas needed since they hover while feeding;
* ants -- Although ants like pollen and nectar, they aren't good pollinators, so many flowers have sticky hairs or other mechanisms to keep them out.
Just how much energy should a flower expend to get pollinated? The oldest method, using wind to transfer pollen, requires little investment in producing flowers, but is not very efficient, since little pollen hits the right destination. Over millions of years, many flowers and pollinators have "co-evolved" to develop more complex relationships. Imagine how this might have happened. A pollinator that is capable of detecting certain colors or scents, or possessing structures that best fit certain flowers, passes these advantages on to its offspring. Over many generations, these traits become well established. Flowers, meanwhile, also evolve with characteristics suiting a variety of -- or particular -- pollinators. Some non-choosy flowers, such as daisies, play host to nearly any pollinator. Others, such as monkshoods, are adapted to be pollinated by just one pollinator (bumblebees). In tropical areas, it's common to find flowers and pollinators exclusively dependent on one another. Although these types of relationships require a lot of energy investment from the plant, they are very efficient.