It’s May, the month of flowers! After a long winter there’s nothing more wonderful than seeing the world in bloom. A lot of the flowers in gardens were cultivated by humans to have a shape, size, and color we find pleasing. For wildflowers, their pretty looks have another purpose.
Plants need to be fertilized before they can make seeds. Flowering plants (angiosperms) have “female” parts that produce ovules and “male” parts that make pollen. Some flowers have both male and female parts on the same flower while others have separate flowers for each. Fertilization occurs when pollen is carried to ovules. Some plants self-fertilize or let their pollen drift on the wind. The rest rely on pollinators.
Pollinators, such as bees and butterflies, are animals that carry pollen from one plant to another. Pollinators like flowers because they can collect food from them in the form of nectar or pollen. Flowers like pollinators because they help with genetic exchange and fertilization.
Over millions of years of evolution, flowers have evolved to be just the right size and shape to attract their preferred pollinators. At the same time, pollinators have evolved beaks, proboscises, and other features that make it easy to feed from their favorite flowers. This process where the evolution of one species affects the evolution of another is called coevolution. Not all coevolution is as friendly; many parasites coevolve with their hosts in a never-ending evolutionary arms race. Fortunately for plants and their pollinators, they have a relationship where both sides benefit (usually).
All sorts of organisms can serve as pollinators. Each type of pollinator comes with its own set of flower preferences, so by looking at the features of a flower you can tell what might be pollinating it. Many flowers are visited by several different pollinator types, but some are highly specialized to just one.
Bees love bright colors like yellow and blue, but you won’t see a red bee-pollinated flower. Bees have a hard time seeing red so tend to pass up flowers with ruddy hues. They probably don’t miss red much, because bees can see an additional color: Ultraviolet. Flowers pollinated by bees often have markings only visible in ultraviolet, making them even more attractive to bees. A sweet or minty smell also draws in bees, letting them know that there is a nectar or pollen reward waiting for them.
Bee-pollinated flowers come in a variety of shapes and sizes. Some are wide and bowl-like, providing a large landing area for bees. Many are tubular, making the bee crawl past pollen-covered anthers to get to the nectar within. Others have complex shapes, like a snapdragon. The weight of a bee landing on a snapdragon petal causes the flower to open, allowing the bee inside. The flower won’t let in insects that are too big or small!
Butterflies also enjoy bright colors, but red can be on the menu. Butterflies draw up nectar through their proboscises, long tubular mouth-parts. Butterfly-pollinated flowers often have a cylindrical base shaped to fit a proboscis. There may be many small flowers grouped together, causing the butterfly to linger longer on the cluster as they move from flower to flower.
Butterfly-pollinated flowers don’t have well-defined landing platforms. Pollen often hangs on anthers protruding past the petals so a butterfly will be dusted while it probes with its proboscis. Butterflies are not very attracted by smells, so flowers visited by butterflies usually have moderate scents at best.
Bats eat more than just bugs. Some bats love to dine the on nectar or pollen of flowers that bloom at night. Nectar quantity is much more important than the way a flower looks for bats. Bats can’t see in color, so the flowers they pollinate tend to be shades of pale green, yellow, or just plain white. Bat-pollinated flowers don’t always look very impressive to humans and they don’t tend to smell amazing either. They use a strong musky scent to attract bats.
A bat is a lot heavier than a bee. Bat flowers are usually on the end of sturdy stems or branches to allow bats to land safely or hover with enough room for their flapping wings. Bats pollinate a lot more flowers than many people realize, including common food plants like bananas, guavas, and mangoes.
Hummingbirds really love flowers, and flowers love being pollinated by them. Flowers pollinated by hummingbirds (or other birds) are bedecked in red, yellow, or orange colors. They are shaped like tubes, cups, or funnels, depending on the bill shape of their preferred pollinator. Some have landing platforms that the bird can perch on while feeding. Hummingbirds eat while hovering, so their flowers often hang down.
Many birds don’t have a strong sense of smell (though research has shown that they’re much more capable than we’ve realized), so flowers that are pollinated by birds don’t have much of a scent. What they do have is nectar, and lots of it! The nectar is hidden deep within the flower and the bird’s head gets dusted with pollen as it forages.
You can catch more flies with honey than vinegar, but there are things that flies are even more wild about. The plants that cater to fly pollination use these preferences to their advantage.
Flies don’t need fancy ultraviolet landing strips. What brings a fly to a flower is the smell. Flies are attracted to rotting meat, dung, blood, and decay. The flowers that attract flies emit these scents. These flowers may be funnel-like to allow easy access but others are complex, designed to trap the fly until they’ve picked up enough pollen to carry to another plant. Their colors aren’t too impressive: White, or sometimes brown or purple.
There are a lot more ways that plants get pollinated. Some flowers rely on non-flying mammals like mice. Others have partnered with moths, ants, wasps, beetles, or even lizards. A few plants have adapted to allow water to carry their pollen from one flower to the next.
The only purpose of a flower is to attract pollinators, or otherwise facilitate fertilization. It's lucky for us that we happen to find them alluring too!
- Kate Dzikiewicz, Paul Griswold Howes Fellow