Pollination: Flowering Plants, Pollinators, and the Wonder of it All
Pollination is a familiar term to almost everyone. We understand that flowers must be pollinated, usually by various insects, for the plant to create fruits and seeds. We have seen headlines asserting that insect pollinator populations are declining, threatening those essential processes. We know that pollen is that yellow dust that covers our cars in early spring, and we blame it for our seasonal allergies. For many of us, that’s about as far as it goes. But taking the time to look deeper can lead to some amazing discoveries.
Individual plants have developed their own, often unique and highly specialized structures and pollination techniques over millions of years. Plant-pollinator interdependence has evolved to become the central facilitator of both plant reproduction and pollinator nutrition. A thoughtful look at this adds clarity to why maintaining bio-diversity is so important to the health of the earth and its people. So let’s take a brief, and hopefully insightful, look at the fundamentals.
The Basics of How it Works
Pollination is the reproduction system used by flowering plants. Specifically, it is the act of transferring pollen from the male “anther” to the female “stigma” of a flower to make seeds and enable reproduction. Pollen are fine dust-like particles that develop within the anther and collect on its surface. Pollen grains are unique in their shape and the sculpturing of their tough exterior surfaces. For successful pollination, the pollen must be transported to a stigma of the same plant species at the right time.
When pollen lands on a receptive stigma, one of the pollen grain’s two internal cells germinates and creates a pollen tube, essentially a tunnel through the stigma and style, forming a path to an ovule inside the flower’s ovary. The second cell in the pollen grain divides into two sperm cells which travel down the pollen tube to the ovule. One sperm cell unites with the egg, creating the embryo, and ultimately, a seed. The other unites with a cell in the ovule to create endosperm, which provides nutrients for the embryo and in some cases for the seed’s early growth. Ovaries may have one or many ovules. Only fertilized ovules develop into seeds. If all the ovules aren’t fertilized by individual pollen grains, fewer seeds develop and the resulting “fruit” is likely to be oddly-shaped. The realization that the seeded fruit and vegetables we eat are actually plant ovaries may be slightly off-putting, but is an interesting fact nonetheless.
Today we recognize more than 250,000 flowering plants that require pollination for reproduction. About 80% of these require biotic pollination, via animals. Of the 20% of plants that are pollinated abiotically, 98% are wind pollinated and 2% water pollinated.
Plant and Flower Types
A little bit of botany goes a long way with most of us, but a few basic terms are instructive.
Complete and Incomplete flowers refer to the overall flower structure:
- Complete flowers have sepals, petals, pistils and stamens on each flower, as in the drawing above:
- Both male and female reproductive parts
- Sepals protect the flower buds during development
- Petals lure pollinators to the flower to promote pollination
- Incomplete flowers are missing one or more of those features
Perfect/imperfect flowers refer only to the sexual flower parts:
- Perfect flowers have both male (staminate) and female (pistillate) parts on the same flower. They are referred to as hermaphroditic and represent about 90% of flower types.
- Imperfect flowers are missing one set of reproductive parts
Imperfect flowers reside on one of two plant types:
- Monoecious plants (above photos) have both sexes on the same plant on separate imperfect flowers. Examples include corn, melons, cucumbers, squash, pumpkins, pecans, begonia and clematis.
- Dioecious plants have separate male and female plants. Hollies and willows are examples.
Plants may be Self-Pollinated or Cross-Pollinated:
- Self-pollinated means fertilized with their own pollen
- Cross-pollinated means fertilization occurs between pollen and ovules of different plants of the same species.
Cross-pollination is predominant. There are benefits to variation in helping plants adapt in a changing environment. Flowers prevent accidental self-pollination in several ways, including varying the timing of pollen release and stigma receptivity, the spatial arrangements of anthers and stigmas, and by separating male and female flowers on the same plants. Amazingly, some plants that normally have self-pollination barriers in place can change structure and chemistry to accept their own pollen if cross-pollination doesn’t occur in a timely way due to weather or lack of pollinator activity.
Water pollination can take place both underwater and on the surface. In both cases, large amounts of pollen are released and depend on currents or breezes to bring it in contact with a receptive stigma.
Ribbon weed is an example of surface water pollination. It is dioecious, having separate male and female plants. Female flowers, tethered to the mother plant reside on the water surface, creating a dimple in the water. Male flowers are released from the male plant to float on the surface, relying on breezes or drift to find their way to a dimple. The flower slides down into the dimple, colliding with the female flower, causing pollen to be catapulted to the stigma. There is a lot of chance involved in successful water pollination, which may explain the relatively small number of water pollinated plants.
Conifers and about 12% of flowering plants are wind-pollinated. Oak, birch and cottonwood trees and cereal crops, grasses and ragweeds are examples. Wind pollinators don’t waste energy on colorful or scented flowers. Their anthers generate huge amounts of lightweight, smooth pollen that is easily wind transportable. Their stigmas are feathery and sticky to catch the floating pollen as it disperses relatively unpredictably. Wind-pollinated plants may be monoecious or dioecious.
Corn illustrates the wind pollination of a cereal plant. It is monoecious. The top tassels are the male anthers and the silks growing out of the husk are the female stigmas. Corn pollen is heavy and elevated. When released, it drifts toward the ground, its length of travel influenced by the weather (wind and rain). Each silk is the stigma for one kernel. Silks are covered in sticky hairs to help them catch passing pollen. Typically the silks emerge a day or two before pollen release and remain receptive for about 6 days.
When a pollen grain lands on a silk, it has to enter and pass down through the silk to the cob. The pollen has the usual two cells, the first creates a pollen tube through the silk to the ovule. The second cell follows the first to the ovule where it splits into two sperm cells, one of which fertilizes the ovule forming the seed embryo. The second forms the endosperm which surrounds the seed as it develops. Within 4 days of pollination, the silk detaches and dries up.
The silks from the tip of the cob are the last to emerge from the husk and can be buried under the existing silks, making fertilization difficult to achieve and explaining why the leading end of the corn cob is often populated with undeveloped kernels. A corn cob covered with fully developed kernels has had all of the 1,000 or so silks in the husk fertilized by 2000 or so pollen grains per silk released by surrounding tassels and delivered by the wind. Not efficient, but surprisingly successful, given the seeming randomness of the process. It also explains why corn is best planted densely by grouping in multiple short rows, rather than spread out in fewer long rows.
Biotic or animal pollination
About 80% of flowering plants — including 35% of our food crops — are animal-pollinated. Approximately 200,000 animal species act as pollinators, including about 3500 species of native bees, 1000 species of hummingbirds, as well as bats, small mammals and all manner of insects.
Humans think of flowers as a pretty landscape feature or home decoration. Their real purpose in nature however, is to lure pollinators to the plant so that they will blunder into the anthers, picking up pollen and depositing it on receptive stigmas as they are guided to the nectar, and in some cases pollen, that they need for their own survival. Some plants are pollinated by a variety of pollinators, others by a single type.
Different flower characteristics attract different pollinators; for instance:
- Beetles, among the earliest pollinators going back to the Mesozoic era, tend to like bowl shaped flowers of plain or dull coloring, open during the day. They seek scents varying from none to fruity to something resembling decaying flesh and moderate amounts of nectar. Examples are magnolia, pond lily, goldenrod, and spirea.
- Birds, of which about 2,000 species globally feed on nectar, prefer tubular flowers with curving petals that create a welcoming entry, perching supports and bright red, yellow and orange coloring. Scent is unimportant. Flowers should be open during the day, have a large quantity of nectar and a pollen structure that dusts the birds’ head and back as it forages for nectar. Hummingbirds are favorites to watch as their long, slender bills reach deep into flowers for nectar, withdrawing with their faces dusted in pollen.
- Butterflies are daytime pollinators that like brightly-colored flowers with landing platforms and scents that simulate the scents that they produce to attract the opposite sex.
- Flies are attracted to pale and dull colors with strong often putrid scents. They are common pollinators of tunnel-like and complex trapping flowers. Examples are jack in the pulpit, pawpaw and skunk cabbage.
- Moths typically feed in late afternoon or at night on clustered white or pale flowers with strong, sweet scents that offer a landing platform and hidden nectar that they can reach with their proboscis. Morning glory, tobacco, yucca, and gardenia are examples.
- Bee Pollinators Bees are the best known and most important pollinators as far as our food supply is concerned. They tend to be attracted to bright, white, yellow and blue flowers. Their vision is UV sensitive so flower features that look pale to humans may stand out to them. Nectar guides are a case in point where lines on flower petals visually lead the bee to the well that contains nectar and often a landing platform.
Snapdragons (see photo on left above) have a bee-visible landing area. Their sexual parts and nectar are hidden, but when the right sized bee lands on the designated spot, the petals open, revealing the nectar stash and pollination organs inside.
Colonizing bees also collect pollen, inadvertently in their role as pollinator, and intentionally in their role as workers in the hive, since pollen is an important food source. Colonizing bees make 12 or more trips per day to visit several thousand flowers, usually one type of flower per trip, as they forage to bring food to the hive to raise their next generation.
Composite flowers are a class of flower that is widespread and shares common traits among its members. Composite flowers are the flat, open flowers that include sunflowers, asters, dahlias, zinnias, chrysanthemums and black-eyed Susans. What appear to be their petals are actually “ray flowers.” which are sterile and are missing one of the sexual parts. The center is made up of many “disc flowers,” which are multiple petals formed into tubular shapes, each a complete flower itself, including nectar. With broad landing areas and nectar within easy reach, composite flowers are accessible to a wide variety of insects, so are great pollinator attractors.
The Take Aways
If this article communicates anything, I hope it is that there is more to pollination than most of us notice. The co-evolution and inadvertent symbiosis of plants and pollinators that has evolved over millions of years into the functioning reproduction system that supports life on our planet is both mind-boggling and awe-inspiring. That it is under threat is disturbing. The mutual dependence of many plants and specific pollinators on each other brings the need for maintaining diversity in both plants and organisms into clear focus. And hopefully, it motivates us to do our part to protect and promote natural processes in the future.
It has been eye-opening for me to look more closely at the flower structures in our gardens and even at the “weeds” on our lawn. Observing the match between the pollinators that approach each flower, and the flower design that meets pollinator capabilities and needs offers a clear visual demonstration of the adaptability and mutual dependence of both. In fact, it vividly illustrates the mutual dependence of all living things on each other.
There is a lot written about how to make our microenvironments more pollinator-friendly. The Garden Shed has a sprinkling of articles that touch on the topic. The August issue will feature an article on insects, for example. And there will be more to come. I hope you will investigate it and join us in becoming lifelong learners and promoters of pollinators, pollination and, more broadly, the natural way of things.
The Sex Life of Flowers, (Meeuse and Morris, 1984).
Botany for Gardeners, (Capon, Timber Press, 2010).