(For Grades 3-8)
Ultimately, all life on Earth depends on plants to provide food, shelter, and oxygen for other living things. Consequently, plant reproduction is crucial to all other life on this planet. The first step in plant reproduction is the intricate process called pollination, which occurs when pollen grains, the male germ cell of a plant, reach the stigma, the female reproductive part of the same species of plant. Depending on the plant species, a flower can produce male, female, or both structures. Pollination can also occur within the same flower.
Most flowering plants (90 percent) depend on animals to make the vital pollen-grain delivery. The remaining flowering plants rely on wind and some-times splashing raindrops to ferry pollen, but this is a less precise method. Pollinating animals do the job for a reward: food, usually in the form of nectar. The lessons in this online unit explore the theme of the National Zoo's Pollinarium exhibition: how plant and animal partners interact to accomplish pollination.
As in many processes in nature, timing is important. The female reproductive part of a flower is receptive to pollen only at certain times of the year. Creatures like insects and birds, which move from flower to flower in search of food, are a fast and often guaranteed way for plants to distribute their pollen.
Both the male and the female reproductive parts of a plant are in the center of the flower. The male, pollen-producing part is called the anther, held aloft by a stalk called a filament. The entire male apparatus is called a stamen. Each pollen grain is unique to its species. The female reproductive part of a plant, the stigma, sits on top of a style, or stalk, which leads to an ovary at the base. The entire female plant mechanism is called a pistil.
How does the process of plant pollination by animals work? A pollinator (such as a bee, bird, bat, or butterfly) in search of food visits a plant. The plant has secreted nectar, a concentrated food source, from special glands and tucked it away in its blossom. While crawling around the blossom looking for nectar, the pollinator rubs against the pollen, which becomes attached to different parts of the pollinator's body. When the pollinator visits another blossom, it transfers the pollen grains from its body onto a strategically placed stigma. After it reaches the stigma, the pollen grain grows a tiny pollen tube down the style and into an egg-filled ovary. Eventually, the pollen and the egg form a seed.
Scientists estimate that there are many thousands of animal pollination partners, ranging from invertebrates (animals without backbones) such as bees, butterflies, wasps, flies, and beetles to vertebrates (animals with backbones)such as birds, bats, and other mammals. In North America, most of the pollinators are insects like bees, butterflies and beetles, or vertebrates like hummingbirds and bats. But elsewhere in the world pollinators can be primates (like lemurs), Australian possums, arboreal (tree-dwelling) rodents, or even reptiles like the gecko lizard.
The animal pollinators carry the pollen in different ways. Vertebrate pollinators like birds or bats carry pollen in their feathers or hair. Although invertebrates like bees and butterflies lack hair, they have something just as suitable for carrying pollen: bristles situated on their legs, head, and other body parts. Honeybees have tiny baskets on their legs for carrying pollen back to the hive. When butterflies use their long proboscis, or nectar-gathering appendage, to sip nectar from tubular flowers, they get peppered involuntarily with pollen on the proboscis or the head.
Plants use various techniques to attract their particular animal partners. Flowers are actually cleverly designed reproductive organs that incorporate all kinds of lures. The petals, for example, may serve as a landing platform for a visiting insect. When a bee lands on the lower petal of a snapdragon, its weight causes a stamen to swing down and dust the bee with pollen. Petals of many plant species even have lines or other marks that guide the pollinator to the nectar.
Another type of lure is aroma. A flower's scent must appeal to its pollinator. Many people appreciate the sweet smell of honeysuckle on a midsummer night. At that time, it's at its strongest to draw the honeysuckle's pollinators: nocturnal moths who "smell" with their feathery antennae. While most flowers have a sweet, pleasant fragrance, there are exceptions. One example is the Rafflesia flower, whose "rotten meat" aroma, which is offensive to most humans, is precisely what attracts its pollination partner: the fly.
Plant structures, too, are designed to attract specific pollinating partners. The Queen Anne's lace flower places its nectar right at the base of its tiny flowers where pollinators with a short proboscis (nectar-gathering appendage) such as honeybees, ants, wasps, flies, and beetles can reach it when they crawl on the flower. On the other hand, bumblebees, butterflies, and moths have long proboscises, which enable them to reach nectar in less accessible places. For example, the long shape and curve of the columbine flower complements the long tongue of a bee, butterfly, or hummingbird. By concealing the nectar deep within its trumpet-like blossoms, the columbine prevents animals who are not its pollination partners from taking the nectar and transferring any pollen.
Plants also use colors to attract their ideal animal pollinators. Hummingbirds often, but not always, are attracted to red flowers. As it turns out, red flowers are typically loaded with carbohydrate-rich nectar, which provides almost instant energy for the fast-moving hummingbirds. Insect pollinators see color differently than we do because they are sensitive to ultraviolet (UV) light. UV light makes the reproductive areas of some flowers stand out. To human eyes a buttercup appears as a uniform yellow, but to a bee's eyes the flower's center (where the reproductive structures are) is darker because it reflects UV light. Bees are also attracted to blue and violet flowers. Flowers pollinated by animals who search for food at night are often pale so they'll be visible.
Through natural selection, a process in which living things become better adapted to their environments, some plants have evolved to match a particular animal pollinator. While this may be efficient because the pollinator will always visit the right species, it can also be dangerous for both partners should one or the other become extinct.
On a worldwide scale, animals pollinate more than three-fourths of the staple crop plants that people eat. Scientists estimate that one out of every three bites of food we take is the result of a successful animal-plant pollination system. For instance, consider a hamburger or hotdog with "the works": ketchup, relish, mustard, and onions. Several different bee species pollinated the flowers of the plants that produce these condiments: tomatoes, cucumbers, mustard seed, and onions. Other bees were responsible for the side dishes. For example, hardworking bees pollinated the potato plant that eventually became potato chips and French fries. And for dessert, an endless variety of ice cream flavors, such as strawberry, chocolate, and vanilla, is also the result of successful plant-animal partnerships. A world without pollinators, and thus without flowers, and so many types of food, would bea poor world indeed!
Smithsonian Center for Education and Museum Studies. Plants and Animals, Partners in Pollination. 2010.
Krisanna Machtmes, LSU AgCenter