Every spring, our car windows, decks and sidewalks are coated with coat after coat of yellow powder. A seemingly endless rain of tiny particles streams down from birches, oaks, pines and other trees, sticking to all horizontal surfaces – and causing around 25% of the human population to sneeze.
Spring is the start, but not the end, of pollen season – the ever-present reminder that a lot of plant sex is going on around us, above our heads, along the roads and in our gardens. Yes, without pollen, flowering plants would produce no seeds. So, starting with the spring flowering trees and continuing in the summer with grasses and ragweed, our need for antihistamines, neti pots and air filters is part of life here in Pioneer Valley. .
Of course, pollen isn’t just there to bother and weaken us; it plays a central role in the adaptation of plants to life on dry land. Non-flowering plants, such as mosses, liverworts, and ferns, still use a sexual reproduction system developed millions of years ago in aquatic habitats. Liquid water is essential as the medium through which motile ‘male’ gametes swim, propelled by tiny flagella, to find and fertilize an immobile ‘female’ gamete.
This mechanism is excellent as long as there is enough rain and fog to wet the fertile parts of plants during reproduction, but it is much more haphazard in dry uplands. The evolution of pollen grains allowed the fertile male part of the plant to travel long distances in dry, parched air in a form of suspended animation, until the pollen grain landed on the surface receptive of a flower.
There, the pollen absorbs moisture and begins to grow and divide, producing a structure called pollen tube which allows the sperm to travel to the egg. The trillions of pollen grains crossing the sky are actually tiny dormant plants.
Early flowers probably evolved in concert with insects that carried pollen to another flower, perhaps initially as pollen eaters. Wind pollination seems to have evolved later and is what is called a “derivative” trait, meaning that other closely related plants still depend on insects or other animals.
Wind pollination appears in at least 65 distinct plant lines, suggesting that it is a very important adaptation that offers significant evolutionary value over and over again.
Charles Darwin was intrigued by this adaptation. He wondered how such seemingly wasteful use of plant energy and nutrients could provide enough value to be retained as a trait. This was in light of his study of orchids, where very close ties between bees and orchid species are common, and pollen is clumped into small pollinia rather than spread by wind. Still, more than 10% of angiosperm species are wind-pollinated, probably reflecting the limited number of pollinators in drier steppes and grasslands.
Wide dispersal of pollen is also a way to increase genetic diversity. Obviously, wind pollination works very well for a lot of ecologically important plant types, but not so well for people with allergies.
So why does pollen stimulate allergic reactions that are sometimes quite severe? When the pollen lands inside your nose, the pollen is quite dry. Pollen coats are spiky and menacing – but it’s not the shape that does the damage. Although there may be a few waxy or greasy molecules on the surface of the pollen, the main proteins and lipids known to boost our immune system are actually still inside the pollen grain.
These chemicals have important functions in the growing pollen tube, but can only be released when the grain absorbs water. Studies on cypress pollen have shown that its pollen grains absorb moisture from the nose within minutes and then disintegrate, releasing their contents.
This helps explain why pollen can stimulate an asthmatic response even though the pollen grains themselves are too large to move down the very small passages in the lungs where asthma originates. It is the broken pieces of the pollen that go deep into the lungs.
Another explanation is called the “thunderstorm” hypothesis. Pollen is carried in thunderstorms by rising air currents, gets wet and then explodes, releasing small particles which are then carried into human lungs. It makes sense that pollen absorbing pure water could explode, because the cellular contents of the growing pollen tube are filled with sugars and salts that cause osmotic absorption of water, so much so that in the absence of balancing salts and sugars on the outside, they burst. .
But as I pondered the fate of pollen in our noses and bronchial tubes, I began to wonder about a somewhat more intimate relationship between humans and pollen. Perhaps our mucous membranes are not so different from the receptive surfaces of the flower. Could pollen actually absorb moisture and start germinating in our noses?
How long would it take for a pollen tube to burst? Then the allergenic proteins and fats would be within reach of the white blood cells protecting us from invaders. I searched the internet for pictures and couldn’t find any. Maybe this thought is so off that no one has looked?
Next time I’m sitting in front of a microscope, maybe I’ll take a look. When we sniff pollen, are we pollinated?
Lawrence J. Winship is Emeritus Professor of Botany at Hampshire College and a former board member of the Hitchcock Center for the Environment.
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