Module 2

Pollinators and Plants in Partnership

Purpose:

• To understand the anatomy of flowers that make them attractive to pollinators.
• To understand plant reproduction and the role of pollination.

Background

Some flowers announce their presence with bold and vibrant hues, while others remain modest and drab. Some are simple and open in form, but others feature tricky entries or convoluted mazes.

Flowers have long inspired humans with their beauty and fragrances, and we've bestowed them with symbolic meanings. Myths and symbolism aside, the real job of flowers is to ensure that plants produce off-spring.

Animals can roam about and seek mates with whom to reproduce, but imagine the challenge for a plant, rooted firmly to the ground, to achieve the same end. Over millions of years, flowers have evolved a remarkable range of strategies to guarantee that male pollen is transferred to female flower parts so fertilization and seed production can occur.

Relying on wind to move pollen, as grasses and many trees do, is the oldest method of ensuring pollination. But a more efficient — and fantastic — means is by luring unsuspecting animal partners to inadvertently make the transfer as they search for food.

Insect wiggling its way into snapdragon flower.


Over millions of years,
flowers have evolved a
remarkable range of strategies
to guarantee that male pollen is
transferred to female flower parts.





It's easy to identify the different parts
of the flower on lilies (top) and amaryllis.
Illustrations and photos by Suzanne DeJohn/NGA.

Insects — especially beetles, ants, flies, bees and wasps, butterflies, and moths — are the predominant animal pollinators. They have physical characteristics that make them extremely efficient in locating flowers and transferring pollen from one flower to another. (From "The Secret Life of Flowers," Growing Ideas: A Journal of Garden-Based Learning, National Gardening Association 10 (3) September 1999.)

Of course, animals don't do the work of pollination for nothing, so plants offer rewards. Animals use flowers as sources of food for themselves and their offspring. First of all, animal-pollinated flowers produce nectar, a sugar-based substance that also contains vitamins, amino acids, and other nutrients. The amount of nectar a flower typically produces relates to the needs of its pollinators. Second, pollen itself is a good source of protein for many animals. Finally, a few plants reward their pollinators with fatty oils, resin or wax.

The typical flower contains the necessary parts for enticing pollinators and producing seeds. The center of a flower usually contains the female, pollen-receiving pistil. The stigma at its tip is often sticky, feathery, folded, or otherwise designed to trap pollen. When they're ready to accept pollen, stigmas ready themselves for the transfer. They may be pushed upward by the long style that supports them, lean toward the male parts, or become stickier.

At the base of the pistil, the generally hidden ovary protects ovules (eggs), which become seeds when fertilized. The male parts, or stamens, typically surround the pistil. They may be quite long, to maximize exposure to wind and pollinators; hidden inside the flowers, to force pollinators to touch the stigmas on their way in or out; or able to lengthen and shorten over time, as needed.

The stamen is made up of the filament that supports the anther which produces and releases huge quantities of pollen.

Animal-pollinated plants have large, irregular pollen grains with lots of tiny hooks, spines, and craters on the surface. A rough texture and sticky surface ensure that the pollen will stick to a visiting animal's hair, scales, feathers, or appendages and then stay there until the animal visits another flower. At the next flower the pollen will be rubbed off onto the strategically placed stigma.

The pollen descends down the style of the sigma and fertilizes an ovule, leading to seed production. Once fertilized, the ovary wall takes in moisture and swells, becoming the fruit, which surrounds and protects the developing seeds.

The petals, which are typically the most noticeable parts of flowers, are designed to attract and provide platforms for insects, bats, birds, and other roving pollinators. The base of many petals contains nectaries, which produce the nectar. Since this food treasure is typically tucked deeply in the flowers, pollinators are coaxed into touching the flower's reproductive organs, and thus transferring pollen, in their search for nourishment. (From "Digging into Flowers: Pollen, Petals, Pistils, & Other Parts," Growing Ideas: A Journal of Garden-Based Learning, National Gardening Association 10 (3) September 1999)

In this indispensable partnership, flowers and pollinators are utterly dependent on one another for survival. And in turn, we depend on this process for much of the food we enjoy.

References

Digging into Flowers from Growing Ideas: A Journal of Garden-Based Learning

Parts of the Flower, Information sheet 9. Africanized Honey Bees on the Move, Africanized Honey Bee Education Project, University of Arizona

Pollination: The Art and Science of Floral Sexuality by Nancy C Pratt and Alan M. Peter, ZooGoer, July/August 1995.

"The Secret Life of Flowers," Growing Ideas: A Journal of Garden-Based Learning, National Gardening Association 10 (3) September 1999.

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