Ragweed is
notorious for causing seasonal allergies. In late summer and early fall an
estimated 23 million Americans suffer runny noses, itchy eyes and respiratory
congestion caused by pollen of ragweed.
This seasonal allergy, also called hay fever, is caused by an
inappropriate immune response.
Our
immune system identifies and eliminates foreign material like an infectious
bacterium or an invading virus.
In some
cases, the immune system mounts a strong attack on a non-pathogenic agent like
ragweed pollen.
This overly vigorous
response includes the release of a powerful molecule called histamine that
causes many of the hay fever symptoms mentioned above.
Treatments for seasonal allergy usually
contain an anti-histamine to counter the effects of histamine.
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| Giant Ragweed Ambrosia trifida |
But ragweed
has a life of its own, beyond being the cause of seasonal allergy, and an
interesting life it is.
Two species of
ragweed are common in North Carolina, Common Ragweed (
Ambrosia artemisiifolia) and Giant Ragweed (
Ambrosia trifida).
These two
ragweed species are in the same family as sunflowers, the Asteraceae.
The name of the genus of ragweed,
Ambrosia, is from the Greek and means food of the gods.
Ragweed seeds are highly nutritious,
containing high concentrations of protein and oil.
Native Americans grew ragweed as a food
crop then abandoned its cultivation when they adopted corn based
agriculture.
The specific name of Common
Ragweed,
artemisiifolia, suggests the
leaves of this plant resembles the highly dissected leaves of
Artemisia, wormwood or sagebrush.
The specific name of Giant Ragweed is
trifida and that refers to the
three-lobed leaves of the plant.
Common
Ragweed grows to a height of about 2 feet.
It has dissected leaves and produces small, green flowers. The flowers, which do not look like
sunflowers despite being in that family, are borne on vertical, candelabra-like
inflorescences. Each inflorescence has
dozens of male flowers that bear pollen-producing stamens. Near the base of the inflorescence are a few
female flowers that have the egg-producing carpel. The flowers of ragweed are small, inconspicuous
and colored green. Plants that have
brightly colored flowers use them to attract pollinators like insects or
birds. Ragweed does not depend on animal
pollinators but uses wind to spread its pollen. Wind pollinated plants tend to
have dull flowers. Each ragweed plant produces
about a billion tiny pollen grains in a season.
This pollen floats on the wind and a few grains pollinate female flowers. This strategy of producing lots of pollen and
casting it to the winds contributes to ragweed’s importance as an allergy
health hazard because some of that pollen is inhaled by sensitive people. Imagine billions of plants each producing a
billion pollen grains and you can see the impact of this plant on human health.
Each ragweed plant has a few female flowers at the base of each inflorescence where they
receive the free-floating pollen. Pollen
lands on stigmas extending from the female flower and a pollen tube grows down
into the flower to fertilize the egg within.
Each flower makes a single, dark brown seed that is a favored food of
wildlife including quail.
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The dissected leaves of Common
Ragweed, Ambrosia artemisiifolia |
 |
Male, pollen producing flowers
of Common Ragweed |
 |
Female flowers of Common Ragweed
with their extended stigmas to collect pollen |
Giant
Ragweed is much taller than Common Ragweed with plants reaching 13 feet in
height.
The stems bear the large,
three-part leaves that give the plant its scientific name.
At the tops of stems are the inflorescences
that are similar to those of Common Ragweed.
The green flowers, both male and female, carry out the same type of
reproduction as the Common Ragweed with massive amounts of pollen released and
brown seeds produced.
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| The three-part leaf of Giant Ragweed, Ambrosia trifida |
 |
Male flowers of Giant Ragweed with yellow,
pollen bearing stamens visible. |
 |
Female flowers of Giant Ragweed with their
stigmas extended. |
 |
| Brown fruits developing from the female flowers of Giant Ragweed. |
Ragweed
plants produce large amounts of pollen because wind pollination is inefficient.
Other wind-pollinated plants, like Bermuda Grass and Oaks, also produce
vast numbers of pollen grains and contribute hay fever suffering.
Ragweed pollen has a molecule on its surface
that makes it a potent allergen that stimulates the human immune system.
The molecule is a protein, an enzyme called
pectinase.
This enzyme breaks down pectin,
a complex carbohydrate called pectin found in plant cell walls.
Pectinase
helps the pollen deliver the sperm nucleus to the egg, allowing fertilization
and more ragweed for next year.
Remember,
ragweed is not trying to make us miserable, it is not trying to give us runny
noses, it is not trying to clog our sinuses.
But, in the fall of the year these things do happen because of the
reproductive strategy followed by
Ambrosia
artemisiifolia and
Ambrosia trifida.
Back to the
question, what good is a ragweed? People
often ask this question of living things they view as problems. What good is a mosquito? What good is a tick? What good is a ragweed? All these organisms had an evolutionary
history before people appeared on the scene. They
lived, reproduced and adapted. The two
ragweed species in this area are highly successful. Ambrosia
artemisiifolia and Ambrosia trifida
fix carbon, provide food for a variety of animals, were once crop plants and
because of the shape of a protein on the surface of their pollen, cause
allergies. None of these traits was the result of design
for a human, utilitarian end. With the gene editing technology CRISPR it is now theoretically possible to deliberately cause the extinction of a whole species. This very thing, intentional extinction, has been discussed to rid the world of a species of a mosquito that transmits one type of malaria. I hope humanity has better sense than to eliminate even more species than we have already done, be it the vector for a terrible disease or the cause of hay fever.
