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| Giant Ragweed, the bane of many fall allergy sufferers. |
While fall is welcomed for the relief it gives from the long, hot summer, it is also a season when allergies run rampant. These allergies are caused by various pollens, and those who suffer might wonder if there are any positives to pollen at all. The answer is yes!
The definition of pollen is a fine, powdery substance consisting of microscopic grains discharged from the male part of a plant. It is necessary for reproduction, and is transported by the wind, insects, or other animals. The grains from each species have a distinct symmetrical shape, surface pattern, and overall structure, and are easily recognizable under a microscope. Due to their high resistance to decay, pollens have been commonly found in both recent and ancient geologic sediments. In fact, the study of plant pollen in both living and fossil form is known as palynology. Since pollen is produced in large quantities at certain times of the year, it is often a significant component of the Earth’s atmosphere.
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| Scanning Electron Microscope (SEM) image of pollen grains, showing their unique shapes. (Credit: Dartmouth College, Electron Microscope Facility) |
Many flowering plants produce all-important nectar, but it is pollen that is the most highly nutritious offering, consumed by virtually all bees, as well as beetles, flies, butterflies, birds, and mammals. Most plants produce pollen in excess, to ensure that at least some of it is used for pollination. Pollination occurs when pollen is transferred from the male anther (fertilizing organ) of a flower to the female stigma (receptive tip of the pistil) of the same flower or one nearby. This is often accomplished with the help of the animal pollinators listed above. This time of year, the plants that produce the most pollen are Giant Ragweed (Ambrosia trifida) and Cedar Elm (Ulmus crassifolia).
Like all ragweed species, Giant Ragweed is in the genus Ambrosia, because it was long ago believed that it would grant immortality or eternal youth to those who consumed it. Giant Ragweed can grow from 6 to 20 feet tall, especially in roadsides and disturbed areas. Flowering occurs from June to October, with inflorescences forming a spike or spikes made up of many tiny flowers at the top of the plant. These flowers are wind-pollinated, releasing copious amounts of pollen in the fall, eventually developing into small burs tipped with several miniscule spines.
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| Giant Ragweed flowers. |
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| Giant Ragweed seeds (burs). |
Unlike many other tree species, Cedar Elms flower from late summer to early fall, usually July through September. Their flowers are tiny, pale reddish-purple, lacking petals, and they appear in clusters at the leaf axils. They are wind-pollinated, and after pollination they develop into flat, oval, winged seeds called samaras, which are an important food source for deer, squirrels, and birds. This fall flowering and fruiting is a distinctive feature of Cedar Elms and helps to distinguish them from other native elm species. They are called ‘cedar’ elms for their propensity to grow in the same habitat as Ashe Junipers, which are colloquially (but incorrectly) called ‘cedars.’
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| Cedar Elm flowers. |
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| Cedar Elm seeds (samaras). |
Pollen plays a critical role in plant reproduction, and is a nutrient-dense resource for bees and other animals. For humans, a vast number of crops depend on pollination for their existence, including one-third of the world’s food crops such as almonds, chocolate, and coffee. Due to their regular reproduction cycles requiring pollen, plants sustain ecosystem services such as moderating temperature by providing shade and releasing moisture into the air, producing oxygen, stabilizing soil, and creating wildlife habitat. That’s how pollen is positive for us and for the environment!
