JUDY JESSOP: Pollen May Be Annoying, But It Is Extremely Important
Lightweight floating pollen that puffs with each step, speckling the air we breathe, causes havoc with those of us that have allergies.
It is, however, the only way that many trees and plants achieve pollination. Wind pollinators must dump huge amounts of pollen into the air.
This strategy is necessary to raise the possibility of some grains finding the right flowers, so germination can take place. Ragweed is at the top of the list of allergy, producing wind pollens. Others such as oak, birch, maples and grasses may also trigger allergies. Now that summer has arrived, though, and many wind-pollinators are finished, allergy symptoms are settling down.
Not all pollen triggers hay fever. In general, those of us with such allergies can be friends with all the plants that depend on insects, birds and other animals to pollinate their flowers. Such plants have heavy, sticky pollen that clings to any volunteers drawn to sip sweet nectar. The major ways that pollen is transported in our region of the state is by insects, birds and the wind.
Flowers that depend on insects use many different strategies to attract them. Some flowers generalize -- any insect will do, so their structure may be quite simple, with no specialized features.
The tulip is a good example. It has a large wide cup of petals with long anthers holding the pollen. Any insect crawling over the anthers, as it looks for a sweet reward, will pick up a bit of sticky pollen to transport onward.
Some flowers specialize to attract specific insects, such as bees. These flowers often have a lobe that acts as a landing pad and deep-throated petals. The pollen-laden anthers are located at the top of the flower throat, dusting the backs of the bees as they enter. Bees are most sensitive to the violet end of the color spectrum, so many bee-pollinated flowers have blue or violet markings.
Another strategy, used by some flowers, is to display showy brackets, or modified leaves, that attract attention (such as the dogwood). These bright white flags also provide landing platforms for insects as they sip nectar while pollinating the tiny flowers nestled in the center.
Flowers can develop some interesting relationships with their pollinators. The plants of the yucca family, for instance, depend on a single species of moth for successful pollination.
The female yucca moth gathers a ball of pollen and stuffs it into the cup-shaped stigma of each flower. She then lays a few of her eggs in the pollen. When the moth eggs hatch, the larva consume a small number of the developing seeds, but this loss is outweighed by the benefit of having such an effective pollinator.
Another interesting aspect of this symbiotic relationship is that if the moth lays too many of her eggs on the stigma, the flower will abort and drop off to the ground -- the plant's way of selecting against moths that over-lay to the detriment of the yucca's regeneration.
Many flowers depend on hummingbirds for pollination. These flowers are often deep throated, and the hummingbird hovers while reaching deep inside for nectar.
Bird-pollinated flowers have no landing platform, since hummers need not land to feed. Anthers are often dangled to dust the bird's head as he or she feeds on the nectar. These flowers are often red, orange or deep pink, colors that attract the birds but are inconspicuous to most insects.
The nectar of hummingbird flowers is relatively weak, about 25 percent sugars with high levels of sucrose. Insect-pollinated flowers normally produce highly concentrated nectars dominated by fructose and glucose.
As I mentioned earlier, many of the trees and plants that pollinate in early spring depend on the wind to spread their pollen. These trees and plants may be either gymnosperms (nonflowering) or angiosperms (flower-producing).
Pine trees are an excellent example of gymnosperms. These trees do not have flowers. Instead, they reproduce by developing cones, which are modified leaves. Some of these cones are male, producing pollen, while others are female, with each scale of the cone housing two eggs.
The male cones are present only in early spring and now litter the ground. Pine trees are particularly conspicuous when pollinating because the pollen is yellow, and here in the Sandhills, we have lots and lots of pine trees -- the reason that at times even the air swirls in yellow puffs.
Interestingly, pine pollen is not allergy-producing to most people. It just gets blamed because there is so much and it is so yellow.
Other wind pollinators, the angiosperms, produce flowers, but the flowers are not scented, and they don't produce nectar. The flowers are also very simple and often do not even have petals. The red maple is an example. Often these plants bloom before the leaves emerge to allow wind-borne pollen easy access to the flowers.
The Precious Cargo
Pollen is amazing stuff, each grain a complicated little travel capsule. Its important occupants are two male sperm cells.
The outside wall (called the exine) is very tough material composed of waxes and proteins. It protects its precious cargo from solar radiation and dehydration, as the cell bumps around in search of the right flower for pollination. Exines have either pores or special ridges where the case is thinner -- an important feature when the pollen finds the right mate.
Packed inside the exine is an inner layer of cellulose (the intine) much like a cell wall. Inside this layer are a few cells. One of these cells is very specialized. It is a reproductive cell with two nuclei. One nucleus will produce a pollen tube, and one will divide into two sperm cells.
In angiosperms, every grain of pollen that successfully reaches the flower of its own species still has work to do. When it arrives on the stigma (the part of a flower that receives pollen), the tube cell in this grain of pollen begins to elongate, exiting the outer wall of the pollen grain through a pore or ridge.
The tube continues growing down into the center of the flower toward the unfertilized eggs. This single-celled tube can be quite long in some plants. In corn, for example, the tube must grow as much as 12 inches to reach the eggs.
As the tube grows, the two sperm cells travel down the tube toward the tip. When the tip of the tube reaches an ovule (female reproductive cell), it bursts, releasing the two sperm cells.
One of these sperm cells will unite with the egg, making a fertile seed. The other will unite with another cell, which produces food that is stored around the seed. The food store is high in starch, protein and oils. Such grasses as corn are so high in stored food that they have become a main staple of the human diet.
In gymnosperms (such as our pines), during the period of time that pollen is being released to the wind, the scales of female cones open temporarily to receive pollen, and the pollen grain is drawn directly into the egg. The female cones then close during fertilization and maturation.
There are many intricate, amazing and important things that are going on during pollination -- each single microscopic pollen grain a complex structure. Yet one pinch of pollen powder holds thousands of grains. It may at times wreak havoc with those of us with allergies, but without it where would we be?
Contact Judy Jessop by e-mail at email@example.com.
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