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.
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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Update: June 2012