Theme: Pursuing Pollinators
Flower Courtship
Alluring Advertisers
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.
Colors/Patterns
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.
Scents
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!
Shapes
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?
Numbers
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.
Pollinator Preferences
- 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.
Evolutionary Excellence
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.
Author: Eve Pranis
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© 2008
National Gardening Association