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Dispersal
Nature Bulletin No. 675-A April 15, 1978
Forest Preserve District of Cook County
George W. Dunne, President
Roland F. Eisenbeis, Supt. of Conservation
SEED DISPERSAL
Plants that scatter their seeds widely have a better chance to survive
than those which do not. Different kinds spread their seeds in various
special ways.
Many seeds ride the wind. Maple seeds spin away like little helicopters
with a single wing. In a strong breeze they can travel a city block.
Those of the elm are small papery disks with a seed in the center. The
ash seed and its wing resembles a canoe paddle. In the basswood a few
seeds are suspended beneath a large flat blade that glides through the
air. Ripe, dry pine cones open and release winged seeds hidden between
the cone scales.
The wisps of cottony fuzz that we see drifting about in spring carry the
tiny seeds of willow, cottonwood and aspen. In a high wind they travel
for miles. The dangling seed balls of the sycamore break up into
hundreds of parachutes of fine fibers each supporting a single seed. The
dandelion, cattail, thistle and milkweed also launch parachutes into the
wind.
When its seeds are ripe, the stem of a tumbleweed breaks off at the
surface of the soil. In a strong wind this large bushy plant goes rolling
and bumping across fields and open country scattering seeds at every
bounce.
The coconut palm thrives on the coasts of tropical seas. Its nut, the
largest of all seeds has a protective, waterproof covering which allows it
to float to a far-distant beach, germinate, and grow a new tree. Dozen of
our Illinois plants on shorelines and floodplains are spread by floating
seeds. The silver maple, giant ragweed, lotus pods with their seeds, and
native pecans, are examples.
Several common plants in our forest preserves "shoot" their seeds,
sometimes for many feet. The ripe seed pods of the violet explode.
When gently touched, the slender capsule of the jewelweed or touch-
me-not snaps into five tight coils throwing seeds in all directions. As the
nut-like pod of the witchhazel tree slowly dries and shrinks, the
pressure forces its two seeds to pop out. Each seed of the jumpseed --
one of the smartweeds -- has a short stem with a core of compressed
pith that shoots the seed like a popgun when jostled.
Many kinds of sweet, pulpy fruits and berries, such as raspberries,
gooseberries, elderberries, grapes, red haws and cherries are eaten by
birds and other animals. The soft parts of the fruit are digested, but the
seeds are able to pass through the intestines undamaged and be
dropped, often long distances away.
Gray squirrels and fox squirrels bury oak acorns and the nuts of hickory
and walnut for winter food. Some are carried considerable distances
away, forgotten, and start new trees. In this way uncultivated fields are
slowly being reforested from nearby woodlands.
The burs are "hitch-hikers, " seeds equipped with hooks or barbed
spines that cling to our clothes and to the hair or fur of animals. Burs
are of many types. Sticktights are the bean-like pods of the legume, tick
trefoil. Spanish needles are shaped like pitchforks. The sandbur, with its
stiff, barbed spines, stabs into the feet of dogs and barefoot youngsters.
Burdock, cocklebur, beggar's lice and bedstraw are other common seeds
that steal rides.
Accidentally, or carelessly, the seeds of many undesirable plants were
brought across the ocean in ships and allowed to escape and grow.
Almost all of the most troublesome weeds in our farms and gardens are
newcomers to America.
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Update: June 2012
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