10 Facts About Pollen

Most people consider pollen to be the sticky yellow mist that blankets everything in spring and summer. Pollen is the fertilization agent of plants and the essential element for the survival of many plant species. It is responsible for the formation of seeds, fruit, and those pesky allergy symptoms. Discover 10 facts about pollen that may surprise you.

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Pollen Comes in Many Colors

Colorized Pollen — A scanning electron microscope image of pollen grains from a variety of common plants. William Crochot - Source and public domain notice at Dartmouth Electron Microscope Facility

Though we associate pollen with the color yellow, pollen can come in many vibrant colors, including red, purple, white, and brown. Since insect pollinators such as bees, can't see red, plants produce yellow (or sometimes blue) pollen to attract them. This is why most plants have yellow pollen, but there are some exceptions. For instance, birds and butterflies are attracted to red colors, so some plants produce red pollen to attract these organisms.

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Some Allergies Are Caused by a Hypersensitivity to Pollen

Pollen is an allergen and the culprit behind some allergic reactions. Microscopic pollen grains that carry a certain type of protein are typically the cause of allergic reactions. Though harmless to humans, some people have a hypersensitivity reaction to this type of pollen. Immune system cells called B cells produce antibodies in reaction to the pollen. This overproduction of antibodies leads to the activation of other white blood cells such as basophils and mast cells. These cells produce histamine, which dilates blood vessels and results in allergy symptoms including a stuffy nose and swelling around the eyes.

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Not All Pollen Types Trigger Allergies

Since flowering plants produce so much pollen, it would seem that these plants would most likely cause allergic reactions. However, because most plants that flower transfer pollen via insects and not via the wind, flowering plants are not typically the cause of allergic reactions. Plants that transfer pollen by releasing it into the air, however, such as ragweed, oaks, elms, maple trees, and grasses, are most often responsible for triggering allergic reactions.

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Plants Use Trickery to Spread Pollen

Plants often employ tricks to lure pollinators into collecting pollen. Flowers that have white or other light colors are more easily seen in the dark by nocturnal insects like moths. Plants that are lower to the ground attract bugs that can't fly, such as ants or beetles. In addition to sight, some plants also cater to insects' sense of smell by producing a rotten smell to attract flies. Still, other plants have flowers that resemble the females of certain insects to lure males of the species. When the male attempts to mate with the "false female," he pollinates the plant.

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Plant Pollinators Can Be Large or Small

When we think of pollinators, we usually think of bees. However, a number of insects such as butterflies, ants, beetles, and flies and animals such as hummingbirds and bats also transfer pollen. Two of the smallest natural plant pollinators are the fig wasp and the panurgine bee. The female fig wasp, Blastophaga psenes, is only about 6/100 of an inch in length. One of the largest natural pollinators happens to be the black and white ruffed lemur from Madagascar. It uses its long snout to reach the nectar from flowers and transfers the pollen as it travels from plant to plant.

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Pollen Contains the Male Sex Cells in Plants

Pollen is the male sperm producing gametophyte of a plant. A pollen grain contains both non-reproductive cells, known as vegetative cells and a reproductive or generative cell. In flowering plants, pollen is produced in the anther of the flower stamen. In conifers, pollen is produced in the pollen cone.

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Pollen Grains Must Create a Tunnel for Pollination to Occur

In order for pollination to occur, the pollen grain must germinate in the female portion (carpel) of the same plant or another plant of the same species. In flowering plants, the stigma portion of the carpel collects the pollen. The vegetative cells in the pollen grain create a pollen tube to tunnel down from the stigma, through the long style of the carpel, to the ovary. Division of the generative cell produces two sperm cells, which travel down the pollen tube into the ovule. This journey usually takes up to two days, but some sperm cells can take months to reach the ovary.

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Pollen Is Required for Both Self-Pollination and Cross-Pollination

In flowers that have both stamens (male parts) and carpels (female parts), both self-pollination and cross-pollination can occur. In self-pollination, sperm cells fuse with the ovule from the female part of the same plant. In cross-pollination, pollen is transferred from the male portion of one plant to the female portion of another genetically similar plant. This helps in the development of new species of plants and increases the adaptability of plants.

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Some Plants Use Toxins to Prevent Self-Pollination

Some flowering plants have molecular self-recognition systems that help prevent self-fertilization by rejecting pollen produced by the same plant. Once pollen has been identified as "self", it is blocked from germination. In some plants, a toxin called S-RNase poisons the pollen tube if the pollen and pistil (female reproductive part or carpel) are too closely related, thus preventing inbreeding.

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Pollen Refers to Powdery Spores

Pollen is a botanical term used as long ago as 1760 by Carolus Linnaeus, the inventor of the binomial nomenclature system of classification. The term pollen referred to "the fertilizing element of flowers." Pollen has come to be known as "fine, powdery, yellowish grains or spores."

Sources:

"Causes of Environmental Allergies." National Institutes of Allergy and Infectious Diseases. National Institutes of Health. Updated 22 April 2015. (http://www.niaid.nih.gov/topics/environmental-allergies/Pages/cause.aspx).
"Disorders of the Immune System." National Institutes of Allergy and Infectious Diseases. National Institutes of Health. Updated 17 Jan. 2015. (http://www.niaid.nih.gov/topics/immunesystem/Pages/immuneDisorders.aspx).
"Fig wasp". Encyclopædia Britannica. Encyclopædia Britannica Online. Encyclopædia Britannica Inc., 2015. Web. 10 Jul. 2015 (http://www.britannica.com/animal/fig-wasp).
"Pollen." Dictionary.com Unabridged. Random House, Inc. 10 Jul. 2015. (Dictionary.com http://dictionary.reference.com/browse/pollen).
"New Clues in the Plant Mating Mystery." University of Missouri-Columbia. National Science Foundation. 15 Feb. 2006. (http://www.nsf.gov/news/news_summ.jsp?cntn_id=105840).