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Pollen and Pollination
Nature Bulletin No. 603-A   May 8, 1976
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation

POLLEN AND POLLINATION
Almost everybody loves a flower. Commonly the word "flower" suggests showy colors, symmetrical designs, fantastic shapes, fragrance and nectar. However, these pleasing features are mainly lures to attract insects upon which plants depend to carry their pollen from one flower to another. This is pollination, a function essential to the life line of the plant and the production of seeds and fruit which we use as food. Many other plants, such as grasses and our common trees, have wind-borne pollen. Even though their flowers are inconspicuous, colorless and odorless, they have pollen -- and that makes them flowers.

Pollen is the yellow stain on a youngster's nose after he sniffs a dandelion. It is the shower of golden dust from blooming ragweeds in late summer which causes most of our hay fever cases. From midsummer through the first heavy frost, air samples are taken and daily pollen counts are published. Pollen can be blown by west winds from Illinois across Lake Michigan or carried hundreds of miles out to sea. In general, wind-borne pollen is abundant, light and dry; while insect-borne pollen is sticky, heavy, and produced in small quantities.

A pollen grain is the male sex cell of a flowering plant. On the mature stigma of a flower of the same species it germinates and a pollen tube grows down through the style to unite with an ovule, or egg cell, which then develops into a seed. For example, a single strand of the thread-like silk of an ear of corn, sometimes over a foot long, is a greatly elongated style. A pollen tube pushes its way down this entire length in a matter of a few days to fertilize one kernel of corn. In contrast, some oaks require almost a year for their pollen tubes to grow one-eighth of an inch.

The great majority of the world's flowers are cross-pollinated. They can be divided up according to the way pollen is carried from one plant to another. We have bee flowers, moth flowers, fly flowers, beetle flowers and hummingbird flowers. In some of our water plants, such as eel grass, pollen is floated from one flower to another. In the tropics there are even bat flowers.

Bee flowers are usually blue or yellow -- colors which are brightest to a bee's eye. Some of them provide a special lip or landing field with guide lines leading to the nectar stores deep inside. As the bee sips the nectar, the body hairs pick up pollen and transfer it to the next flower visited. Butterflies are often attracted by red or orange flowers -- colors which bees cannot see. Moths, unlike bees and butterflies, hover over flowers at dusk and night, preferring those that run to shades of white and very heavy fragrance. Some hawk moths have tongues several inches long. Fly flowers are mostly dull-colored and have rank odors. Some smell like spoiled meat, fish oil or stale tobacco.

The pollen grains from various plants differ greatly in size, shape and surface markings. Some, like those of the pines, have air bladders which make them unusually light and buoyant. Grass pollen is smooth with a single pore, sunflower pollen is spiny, and pigweed pollen, when magnified, looks like the markings on a golf ball.

Pollen grains are protected by thin, glassy, plastic covers which are highly resistant to decay. Actually, these tiny, fragile-looking granules are more durable than any other part of a tree. As the wind blows them about and they settle in peat bogs or the beds of ancient lakes, a record is preserved of the kinds of plants that grew here in the past. By identifying and counting these pollen grains, we can read the changes which have taken place over vast periods of time.

Which came first, the bee or the flower?


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