Thursday, September 30, 2010

Pollen Menace or Miracle?


Ah-choo! That sound, combined with watery, itchy eyes and a drippy, irritated nose, heralds the arrival of spring for millions of people. Their allergy usually results from an atmosphere laden with pollen. The BMJ (formerly British Medical Journal) estimates that 1 in 6 people in the industrialized world suffers from seasonal pollen allergies, also called hay fever. That number is hardly surprising considering the staggering amount of pollen that plants release into the air.

Scientists estimate that the spruce forests in just the southern third of Sweden release about 75,000 tons of pollen each year. A single ragweed plant, the bane of North American hay-fever sufferers, can produce a million grains of pollen a day. Carried by the breeze, ragweed pollen has been found 2 miles [3 km] above the earth and up to 400 miles [600 km] out to sea.

But why does pollen trigger an allergic reaction in some people? Before we consider that question, let us take a close look at pollen and see the amazing design found in these minute grains.

Tiny Grains of Life

Pollen, says The Encyclopædia Britannica, is "formed in the anther, or male apparatus, in seed-bearing plants and transported by various means (wind, water, insects, etc.) to the pistil, or female structure, where fertilization occurs."

In flowering plants, pollen grains are made up of three distinct parts—a nucleus of sperm cells and two layers that make up the wall or shell of the grain. The tough outer layer is highly resistant to disintegration and able to withstand strong acids, alkalies, and even intense heat. Nevertheless, with few exceptions, pollen is viable for only several days or weeks. The tough shell, though, may last for thousands of years without decaying. Hence, pollen grains can be found in abundance in the earth's soil. In fact, scientists have learned much about the earth's botanical history by studying pollen found in soil samples taken from various depths.

That history can also be quite accurate, thanks to the distinctive designs found on the outer shell of pollen grains. Depending on the type of pollen, the shell may be smooth, wrinkled, patterned, or covered with spines and knobs. "Thus, for purposes of identification, the pollen of each species is as reliable as a human fingerprint," says professor of anthropology Vaughn M. Bryant, Jr.

How Plants Pollinate

Once a pollen grain comes in contact with the stigma, a part of the pistil in female plants, a chemical reaction causes the pollen grain to swell and to grow a tube that reaches down to the ovule. Sperm cells from inside the pollen grain then travel down the tube to the ovule, causing a fertilized seed to form. When the seed is mature, it simply needs to settle in the right environment in order to germinate.

While some seed-bearing plants grow as either male or female, most produce both pollen and ovules. Some plants self-pollinate; others cross-pollinate by transferring pollen to other plants of the same species or of a closely related one. Those that cross-pollinate "often avoid self-pollination by shedding their pollen either before or after the stigmas on the same plant are receptive to it," says Britannica. Others have the chemical wherewithal to detect the difference between their own pollen and that of another plant of the same kind. When they detect their own pollen, they inactivate it, often by blocking the growth of the pollen tube.

In an area where there is a variety of vegetation, the air may be a veritable cocktail of pollens. How do plants sift out the pollen they require? Some employ complex principles of aerodynamics. Consider pine trees, for example.

Harvesting the Wind

Male pinecones grow in clusters and, when mature, release clouds of pollen to the wind. Scientists have discovered that female pinecones, in cooperation with the pine needles surrounding them, channel airflow in such a way that airborne pollen swirls and falls toward the reproductive surfaces of the cones. In receptive females these surfaces become exposed when the scales open slightly, separating from one another.

Researcher Karl J. Niklas conducted extensive tests on the aeronautical wizardry of pinecones. In the magazine Scientific American, he wrote: "Our studies reveal that the unique shape of the cone produced by each plant species results in idiosyncratic [distinctive] modifications of the airflow patterns . . . Similarly, each type of pollen has a distinctive size, shape and density, causing the pollen to interact with the turbulence in a unique way." How effective are these techniques? Says Niklas: "Most of the cones we studied filtered their 'own' pollen from the air but not that of other species."

Of course, not all plants pollinate by harnessing the wind—much to the relief of allergy sufferers! Many make use of animals.

Seduced by Nectar

Plants that are pollinated by birds, small mammals, and insects usually employ things like hooks, spines, or sticky threads to attach pollen to the body of the foraging pollinator. A hairy bumblebee, for example, may find itself hauling off some 15,000 pollen grains in a single load!

Bees, in fact, are the preeminent pollinators of flowering plants. In return, plants reward bees by giving them both sweet nectar and pollen to eat, the latter providing proteins, vitamins, minerals, and fats. In an extraordinary act of cooperation, bees may visit over 100 flowers on a single trip, but they will collect pollen, nectar, or both from just one species until they have gathered enough or until supplies run out. This remarkable, instinctive behavior helps to ensure efficient pollination.

Fooled by Flowers

Instead of offering sweet treats, some plants rely on elaborate deceptions to coax insects to pollinate them. Consider the hammer orchid, which grows in Western Australia. The hammer orchid's flower has a lower lip that, even to the human eye, almost perfectly resembles the plump, wingless female thynnid wasp. The flower even emits a chemical copy of the sex pheromone, or sex attractant, of the real female wasp! Poised at the end of an arm just above this alluring decoy are sticky bags filled with pollen.

A male thynnid wasp, lured by the scent of the imitation pheromone, will grab the decoy and try to fly off with "her" in his grasp. As he takes off, however, his momentum flips him and his intended up and over, right into the sticky pollen sacks. After realizing his mistake, he releases the decoy—which is conveniently attached to a hinge, allowing it to fall back into place—and flies off, only to be fooled again by another hammer orchid.* This time, however, he pollinates the orchid with the pollen he picked up on his previous encounter.

But if female thynnid wasps are active, males will invariably choose one of them, not the impostor. Conveniently, the orchid blooms several weeks before female wasps emerge from their underground pupae, giving the flower a temporary advantage.

Why the Allergies?

Why are some people allergic to pollen? When tiny pollen grains lodge in the nose, they get trapped by a layer of sticky mucous. From there they move to the throat, where they are swallowed or coughed out, usually without any ill effects. Sometimes, though, pollen excites the immune system.

The problem lies in pollen protein. For some reason the immune system of an allergy sufferer views the protein of certain pollens as a threat. The body reacts by setting off a chain reaction that causes mast cells, which are found in the body's tissues, to release histamine in inordinate amounts. Histamine causes blood vessels to dilate and become more permeable, so that they leak fluids that are rich in immune cells. Under normal circumstances, these immune cells migrate to the site of injury or infection, where they help to rid the body of harmful invaders. For allergy sufferers, however, pollen triggers a false alarm, which translates into irritated, dripping nostrils, swollen tissue, and watery eyes.

Researchers believe that people inherit the tendency to be allergic from their parents, although the tendency may not relate to a specific allergen. Pollution could also be a sensitizing factor. "In Japan a direct relation was found between sensitivity to pollen and proximity to areas with high levels of diesel exhaust particles in ambient air," said the BMJ. "Animal studies suggest that these particles increase allergic sensitisation."

Happily, for many sufferers, antihistamines can ease their symptoms.# As the name suggests, these drugs oppose the action of histamine. Despite pollen's irritating effects, however, one cannot help but be deeply impressed by the ingenuity evident in both the design and the dispersal of these tiny particles of life. Without them, planet Earth would be a barren place indeed.

* The flower is called the hammer orchid because the decoy (the labellum) pivots up and down on a hinge, which allows it to swing like a hammer.

# In the past, antihistamines tended to induce drowsiness and a dry mouth. Newer formulations have reduced these side effects.

Information from:

Ghost, OUT!

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