t the right
you see something amazing, despite the fact that what's pictured is a sprig of a very
common weed I just snipped from the path leading to my door. This is a cluster of fruits
and flowers of the plant called Wild Geranium, Geranium carolinianum
this plant is also called the Carolina Storkbill because the the fruit consists of that
long, dark object you see arising from the center of three old flowers (one young flower
with its pink petals is at the lower right), which reminds some people a stork's long
In this species, the base of each "bill" expands into five baglike
things (the five carpels of the earlier ovary). When the fruit is ripe, the baglike things
break away from one another and the bag's "handle," which runs up the
"bill," violently recoils, making the "bag" snap upward. During this
process, the "bag's" seed is tossed away from the plant. In other words, each
flower has five built-in "catapults" that physically toss the seeds into new
territory. In the picture, two flowers show one of their "catapults" just after
it's snapped upward. In the middle flower, inside the calyx, you can see two "
unsnapped bags" waiting for their turn to be catapulted.
All seed-dispersal mechanisms are not this amazing, but, all-in-all, figuring out how
plants spread their seeds into new territory is a fascinating subject. Here are the main
ways most, but not all, plants do it:
WIND is the main transporter of most fruits and seeds of wild plants invading
our backyards. Here are some adaptations enabling fruits and seeds to travel on the wind:
tufts of hairs grow on many fruits entering our backyards, the best-known example
being the dandelion, with its fluffy-white "parachute." It's been shown that a
breeze of only two mph can keep a dandelion achene (fruit) alight, so obviously a good
spring storm can easily carry them for hundreds if not thousands of miles. Asters,
goldenrods, milkweeds, clematis, and willows are other common plants producing fruits with
tufts of hairs that latch onto the wind. The picture at the right is the fruiting head of
a Purple Thistle, Cirsium carolinianum, of the Composite Family. This is a
common weed along many roadsides and in abandoned lots, with purple flowering heads
appearing in the summer and the white fruiting heads shown here appearing in the
- "Wings" appear on fruits of a number of backyard trees,
including maples, ashes, elms, birches, and pines. The winged fruits at the right are
those of the Red Maple, Acer rubrum. While winged fruits don't float in the air
the way dandelion achenes do, they certainly can spin to the ground a good distance from
the parent. Kids often call maple fruits "helicopters" because of their
spinning. Of course the taller the tree is, the farther its winged fruits can spin away.
This means that any tree robust enough to grow very tall will send its genetic material
into new territory faster than smaller trees -- a good example of how "survival of
the fittest" works.
- Being very small is the simplest and most successful manner of being dispersed by
wind. On dry fall days when stiff winds usher in a major weather front, mosses, lichens,
algae, and fungi produce diffuse clouds of super-lightweight spores. Spores have been
found in abundance two miles high in the upper atmosphere, and can travel for thousands
- Bladder-like fruits are those in which
the fruits are enclosed in an inflated, papery structure which can be rolled over the
ground or snow by wind. You may not see this kind of fruit in your backyard unless you
grow Chinese-Lantern Plant or Tomatillos in your garden, or you have a weedy area nearby
where Ground Cherries grow. You see a Ground Cherry fruit at the right. The orange-framed
insert shows part of the papery husk (the inflated calyx) cut away to show the tomato-like
fruit inside it. In fact, Ground Cherries are very closely related to tomatoes. Many
Mexican dishes use tomatillos, the green fruit inside the husk.
- Tumbling is a wind-disseminating strategy you may not see unless you
live near an open, weedy area. Here the seed-bearing part of the plant or the entire plant
snaps off at the base and rolls with the wind, dropping seeds as it tumbles. The most
famous example of this is the Tumbleweed, but Russian Thistles also do it. The
flower-bearing part, or inflorescence, of a very common grass called Agrostis is
shown at the right. When this grass's seeds are mature the inflorescence breaks from the
plant and on windy days can be seen rolling across fields. If the wind is really stiff,
the whole inflorescence lifts into the air and can be carried for miles!
- Catapulting is a special kind of
wind-scattering seen in several common backyard garden plants such as irises,
evening-primroses, poppies, and larkspurs. At the right an open iris capsule is shown. The
capsule had been three-sided, and now the three capsule walls have split apart, each wall
taking its red seeds with it. When the wind blows, the flat capsule walls will catch in
the wind and the capsule will shake. This shaking will then throw the seeds about so
vigorously that the threadlike "stems" holding the seeds onto the walls will
snap and the seeds will catapult a fair distance onto the ground.
ANIMALS are the other main transporters of plant fruits and seeds into our
backyards. Here are the main ways they accomplish that:
are eaten by animals, which carry the seeds inside their bodies elsewhere, until they
come out, either by regurgitation through the mouth, or by being dropped in the feces.
This is often the case with berries and other fleshy fruits in which the hard seeds or
stones are embedded in soft tissue, as with cherries. In eastern North America it's
amazing how quickly Red-cedar invades abandoned fields because birds flying overhead
bombard their seeds there. Birds often swallow fruits whole, the pulps are removed in
their stomachs, and the naked seeds are disgorged from the mouth and fall to the ground.
Of course only animals who swallow seeds whole are good "dispersal agents.".
Grosbeaks are birds who crack the seeds' hard coverings and eat the embryo and storage
material so that nothing alive gets planted. The legume at right has matured and opened up
in such a way that its bright red seeds are placed very conspicuously out where they
almost seem to say "Take me!" to any bird passing by. This fruit is
from a Coral Bean, Erythrina herbacea, growing near my home, so this kind of
spectacular attempt of plants to entice animals to carry their seeds into new territories
is not at all uncommon in nature.
- Caching, pronounced "cashing," is seen among backyard squirrels who
bury acorns and other nuts for winter use. When these caches are forgotten or the cacher
dies, the buried seeds find themselves nicely planted, ready to sprout in the spring. Many
rodent species besides squirrels cache smaller fruits, but their activities usually take
place at night, so we don't notice them.
- Clinging is seen among fruits equipped with hairs or spines specially
adapted for latching onto passing animals -- or backyard- naturalists' trouser legs.
Tick-trefoils produce legume-type fruits covered with tiny, hooked hairs that adhere to
fuzzy surfaces like Velcro. Fruits of Spanish Needles (pictured here) are mounted with
needle-like spines that are themselves provided with barbs, usually backward-projecting. Take a walk in the fall around my place and
it's hard to escape returning with a lot of Spanish Needles stuck to your socks and
trousers. There are several species of Spanish Needles. The one pictured is Bidens
bipinnata, a common weed in eastern North America. At the left is an immature
cocklebur fruit about the size of a pea. It's Xanthium strumarium, a fairly
common European weed in parts of North America. Notice the hooks on the spine tips. When
these spines penetrate a rabbit's fur, a dog's long tail hairs or a naturalist's socks,
it's pretty hard to get the fruit free. If a spine penetrates your skin and one of those
tiny hooks curves around your flesh, pulling the spine out is no fun!
- The mistletoe strategy is effective because mistletoe fruits are gummy and
sticky. This stickiness causes the fruits' tiny seeds to adhere to bird beaks. When the
birds finish eating, they fly to a tree branch, wipe their bills on the branch's bark and,
, a mistletoe seed finds itself exactly where it needs to be, for now it
can send its "roots" into the tree, and become a semi-parasite.
- Unspecialized fruits can also travel. Maybe you can remember walking across a
muddy lawn, then when you were inside cleaning your shoes you noticed that crabgrass seeds
were embedded in the mud. Such tiny seeds as well as spores travel on the muddy feet of
other animals, too. It's been found that many kinds of fungal spores travel within the
plumage of birds, including migrants, so feather-bound spores can hitch rides for
thousands of miles. Even insects carry spores on their bodies, including spores of
disease-causing organisms. Dutch elm disease, cucumber wilt, and many other diseases are
transmitted this way.
- Humans transport vast numbers of wild fruits and seeds, both
intentionally and unintentionally. The picture at the right shows some little brown
"stick-tights" I found stuck on my socks after a walk in the fields. Each brown,
triangular thing, about 4/16-inch across (10 mm) is a section of a fruit from a plant
called Tick Trefoil (Desmodium paniculatum). Later when I sit cleaning my socks
and thumping the things in the grass, I'll be sowing Tick Trefoil. All human
plant-sowing is not so obvious. If you sow your lawn with bluegrass, there just might be a
few weed seeds in it. If you hire a truck to carry in fill dirt to rid your lawn of a low
spot, that fill dirt may bring some fascinating invaders. If you buy a Twinkie, the
railroad car that carried the flour with which to make your Twinkie may have transported a
few weed seeds on its runners, from one side of the country to another... Well, the
avenues by which humans can spread seeds are practically endless.
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