Pollen from a Plant's Point of View

Pollen has one purpose for the plant. It makes more plants. Sex is a real challenge for creatures that can't move and meet one another, so plants have found a few good ways to move their genes from one flower to another, even when they are far apart.

Pollen is a white, yellow, or brown powder-like substance that is produced on the anthers of the flower, the male part of the plant. Pollen is used to fertilize the plant via the process of pollination, which is defined specifically as the transfer of pollen from the male anthers to the flower's stigma, the female part of the plant. Pollination produces fruit and seed. Pollen contains the plant's genetic code.

There are two types of pollen: light, dusty pollen and heavy, sticky pollen. These are useful because they allow pollination to happen in different ways.

Light and Dusty Pollen

Light and dusty pollen grains can be airborne and blown in the wind for long distances. Plants with tiny or inconspicuous flowers, such as grasses, corn, and many trees, generally have light, dusty pollen that blows on the wind. Some of the pollen lands on the stigma of a compatible flower, just by chance. The anthers have to make a lot of pollen, since most of it will never reach a female flower. If you ever visit a corn field while the plants are pollinating, you can actually see the pollen in the air, on the leaves, and on the ground. This is very expensive for the plants, but they do not have to produce colourful flowers or nectar to attract insects. Most of the pollen falls on the ground; only the tiniest fraction actually reaches another flower, and insects are neither attracted to the flowers, nor fed by their pollen.

Heavy and Sticky Pollen

Heavy and sticky pollen cannot blow in the wind and therefore pollination must be carried from flower to flower by insects. These plants provide nutritious pollen for the insects to eat, and in return the pollinators carry some of the pollen directly to other flowers. This is a far more economical arrangement compared to wind pollination, because the plant only needs to create a small amount of pollen. Such plants must make sure that one insect does not take too much pollen in one visit, without leaving anything for the next insect visitor. Many flowers force insects to come in contact with pollen while foraging for nectar by positioning the anthers in such a place that the insects will brush up against them. The pollen grains will stick the body of the insect, especially if it is hairy and electrostatically charged, as bees are. Some parts of the body cannot be easily groomed, keeping the pollen from being put in pollen baskets. This ensures the transfer of pollen when the insects visit other flowers.

Flowering plants requiring animal pollinators have to create big colourful flowers to attract insects, and they have to create sweet nectar to lure the insects inside. Their pollen is often also a more nutritious food source. All of this costs the plant a lot of energy, but it saves on the amount of pollen that it has to produce. Only a tiny amount of pollen is needed, since the insects act as an efficient delivery service.

Nearly three-quarters of all plants use this strategy, so it must be a winning partnership.

Pollen from an Insects' Point of View

For pollinating insects, pollen is a perfect food. Rich in protein, it is one of the most nutritious substances in nature. Some bees collect pollen in special "baskets" on their bodies, to bring back to the hive for later use, or for winter food. If it were available in greater quantity, we would probably eat it ourselves, but the average flower produces only enough to feed a few tiny insects. Pollen is a vital food source to pollinators and also bee larvae. Pollen is very rich, containing 7-35% protein, as well as starch, vitamins, enzymes and pigments. Some plants do not produce nectar, and their pollen can be rich in lipids, including sterols, which are the building blocks of hormones and pheromones.

Pollinators are attracted to flowers with bright colours and strong scents. They are also attracted to nectar, a sugary liquid produced at the bottoms of flowers. This source of carbohydrates is like fast food for insects on the move. Nectar is their fuel during the foraging job. For most insects, pollen is secondary to nectar as a floral food reward.

Insects tend to get more pollen from older plant varieties. Many modern ornamental flowers are bred to have full, double-petaled shapes, which obstruct the pollen-bearing center parts. Some of these varieties are so difficult for bees and flies to access, that they cannot produce seeds without human hand pollination! Some modern varieties have less fragrance than the original varieties, or colours and shapes that are unattractive to insects. As well, there are some F1 hybrid flowers that have no pollen, such as pollen-less sunflowers, meant for cutting but not seeds. There is no need to avoid such flowers completely, but if you want to encourage native bees in your garden, be sure to plant some old-fashioned, single-flowered, richly-fragrant plants to provide food for your pollinators.

Pollen from a Gardener's Point of View

Traditionally, the gardener's view is that plants produce flowers, which either bear fruit or mysteriously fall off. This view must change to include the pollinator as a key part of the garden, and a full partner in the process of growing vegetables and fruit. Always a silent partner, it is the pollinator that has determined success for the cucumber grower, the melon or squash gardener, the berry or apple picker, and even the "lucky" gardener whose tomatoes are earlier, more prolific, and better shaped than their friends'. Even self-pollinating species benefit from the good vibrations of a bumblebee "buzz-pollinating" as it holds tightly to the flowers, shaking them and ensuring that they're well pollinated.

Inside each female flower, each embryonic seed is fertilized by a different grain of pollen. A cucumber with 300 seeds needs 300 grains of pollen to fertilize it completely. Have you ever seen a cucumber or zucchini that looks like a half-inflated balloon? Insufficient pollination left part of it unfertilized, so the fruit didn't bother to form completely. The same problem can be seen in misshapen strawberries or raspberries, and lumpy tomatoes or peppers. Some fruit, such as melons and squash, simply fall off prematurely if they aren't fully pollinated.

Pollen from an Allergy Sufferer's Point of View

Yes, pollen has its dark side. But fortunately for pollinators, it is only the wind-pollinated type of plant that causes allergies. Any pollen from a large-petaled flower is almost always heavy and sticky, to attach itself to a visiting bee that will carry it to another flower. Also, insect-pollinated flowers do not make much pollen. Wind pollinated flowers, such as grasses, maple, birch, and ragweed are to blame for allergies. They produce enormous quantities of lightweight pollen that carries easily with the wind, for miles. There is a common myth that plants such as goldenrod and sunflowers cause allergies, but this is false; those insect-pollinated flowers just bloom at the same time of year as the real culprit, ragweed.

Allergies caused by pollen are commonly referred to as hay fever. Hay fever is the most common allergic disease.1 One can only be allergic to the light and dusty pollen that is airborne, from wind-pollinated plants (trees, shrubs, weeds and grasses). The only way to have an allergic reaction to pollen from insect-pollinated plants is by direct contact.

Pollen normally enters the body via the nose and throat. The body's immune system produces antibodies that fight against the antigens. Powerful inflammatory chemicals are produced, which act on tissues in various parts of the body, for example the respiratory system, causing allergy symptoms. Different kinds of pollen appear in the air during different times of the year, making pollen allergies seasonal. Some examples of trees that release pollen into the air are the maple, willow, birch, ash, oak, and pine. Examples of grasses and weeds include ragweed, russian thistle, sageweed, plantain, and nettle.

It is difficult to avoid airborne pollen. Pollen counts, given by local weather reports, is a measure of how much pollen is present in the air. It is a general guide for when it is better to stay indoors to avoid contact with pollen.

Pollen from a Scientist's Point of View

As plants that rely on just wind, or even water currents, to move their pollen to the female reproductive flower parts grow, some of their pollen inevitably ends up in lakes, accumulating in the sediments. The structure of the pollen has characteristics that allow it to be identified, if not to species level, then to genus or family level (see Figure 1). Pollen is often well-preserved in sediments, due to its natural polymer called sporopollenin, one of the most inert organic substances known.2

The information in sediments includes the remains and traces of biota that lived in the lake and in the catchment area, and they are generally laid down in chronological order. These "records" of the past are very useful when we lack written historical data. We can access the information by taking sediment cores, which are sectioned and dated (using radiocarbon or other methods). Samples are prepared for microscopic analyses, and this allows researchers to identify and count the assemblages of pollen.

By understanding the present conditions that allow a tree or plant to grow (climatic conditions, soil constituents, geology, chemistry, etc.) we can correlate the pollen assemblages we find in core sections to reconstruct histories of climate, for example, or other variables of interest. For example, human disturbance is often noted by a sudden increase in ragweed pollen (Ambrosia species), because as European settlers arrived in North America, they cleared the land for development, and ragweed quickly dominates such landscapes.3

Pollen analyses, under the term palynology, are useful in many applications of ecology, archeology, even in forensic studies. Relatively recently, pollen analyses in Bosnia's mass graves were used to convict war criminals involved in Srebrenica atrocities. If interested in this particular aspect, you can learn more here.

 

Quiz

1. Do ragweed plants need insect pollinators?

2. Does goldenrod cause hayfever allergies?

3. Which can pollinate further, corn or pumpkins?

 

Answers

1. Ragweed has very light, dusty pollen that blows on the wind. You can tell because the flowers are small and not showy, so they aren't meant to attract insects, and especially because the pollen causes allergies. People who suffer from hayfever are breathing ragweed pollen. In late summer, the small plants create so much wind-borne pollen that everyone winds up breathing it. Only a tiny amount ever lands on another ragweed flower to do its intended job.

2. Goldenrod is often blamed for allergies, but it has heavy, sticky pollen that never travels on the wind. Hayfever sufferers are really allergic to ragweed pollen, but the bright, showy goldenrod flowers are more obviously visible. It's just a coincidence that they bloom at the same time of year.

3. Corn has light, dusty wind-blown pollen. Pumpkins have large, yellow, nectar-rich, insect-attracting flowers, and heavy pollen that can only be spread by insects. The question is: which travels further, the wind or the insects? The answer is that the native squash bee might only travel a maximum of about 100 meters during its daily feeding, but the wind can blow pollen up to three kilometers away.


 

  1. Medicinenet.com http://www.medicinenet.com/script/main/art.asp?articlekey=20277
  2. Smol, J.P. 2008. Pollution of Lakes and Rivers: A Paleoenvironmental Perspective – 2nd Edition. Blackwell Publishing, Oxford. 383 pp.
  3. Ibid.
  4. Photo credit: Marie Majaura, Pinus mugo (pine pollen). Wikimedia Commons (http://commons.wikimedia.org/wiki/File:Pinus_mugo1-1.jpg)