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Pollen Grain: Definition, Structure & Function

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  • 0:35 What Are Pollen Grains?
  • 0:57 Structure of Pollen Grains
  • 2:53 Pollination in Angiosperms
  • 4:09 Pollination in Gymnosperms
  • 4:36 Lesson Summary
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Lesson Transcript
Instructor: Wendy McDougal

Wendy has taught high school Biology and has a master's degree in education.

A pollen grain is a microscopic body that contains the male reproductive cell of a plant. It is crucial in a plant's fertilization process. Learn more about this tiny structure and take a short quiz at the end.

What are Pollen Grains?

Spring is a time of new growth and fresh beginnings. Days get longer, the sun gets warmer, and flowers bloom in a magnificent show of color. And as a result of these lovely blossoms everywhere, many of us find ourselves in sneezing fits. In order for flowers to propagate every year, they must be pollinated. And pollination sends millions of tiny pollen grains through the air, many of which end up in our nose.

But pollen does not exist simply to make us miserable. Pollen grains represent the male portion of the reproductive process in plants and trees. These tiny bodies are swirling in the air and on the legs of insects so that they can join the female part of the plant to create a new seed. This important process is known as fertilization. As we will discover, pollen plays a crucial role in the plant world.

Structure of Pollen Grains

Pollen grains are microscopic structures that vary in size and shape. Some are tiny orbs, while others are egg-shaped. Although too small to see individually, they can be seen by the naked eye in large quantities. You have probably noticed a bright orange-yellow coating on your car during springtime that closely resembles cheese powder from the macaroni and cheese box.

Assorted Pollen Grains (Sunflower, Morning Glory, Hollyhock, Primrose, Castor Bean)
Pollen Grains

Viewed through a microscope, a pollen grain hardly looks real. An extremely durable body, it has a tough outer coating. This hardy coat offers great protection from the harsh outdoor environment. This is important because inside this tough shell lie two cells: the tube cell, which will eventually become the pollen tube, and a generative cell, which contains the male sperm nuclei needed for fertilization.

Let's zoom in to take a closer look. There are three main components of a pollen grain. The inside of the grain is made up of cytoplasm. This fluid medium houses the aforementioned living cells, keeping them moist and alive. The outer shell is made up of two layers. The inside layer is aptly named the intine (think interior). It is composed partly of cellulose, a common component in the cell walls of plant cells.

Pollen Grain Diagram
pollen

The tough-as-nails outer layer is known as the exine (think exterior). This highly sophisticated and complex outer layer is rich in a compound known as sporopollenin. Waterproof, resistant to deterioration and very stiff, this shell is basically one of nature's most advanced polymers. It ensures that the tender cells inside have a strong chance of survival.

In addition, often times the exine has folds, creases and spikes rising from its surface. Like extra armor, these features add to the protective nature of this layer. They also play an important role in the mobility of the grains, making it more likely that they will stick to the legs of insects as well as catch the wind.

Pollination in Angiosperms

Pollen is produced and transported in slightly different ways depending on whether the plant is an angiosperm or a gymnosperm. Angiosperms are flowering plants, and gymnosperms are non-flowering. Let's first take a look at pollen in angiosperms.

In angiosperms, pollen is produced by the anther, which sits on the filament in the center of the flower. Look closely and you will most certainly see the fine yellow powder on each anther.

Pollen on Anthers of a Flower
Anther with pollen

In order to complete fertilization, pollen must make its way to another plant. Since pollen cannot move on its own, it must rely on other methods of transport.

In flowering plants, transport is done mostly by insects and other animals. Bees are certainly one of the most important pollinators in the plant world. The pollen gets stuck on their legs, and as they move to another flower, the tiny grains stick to the stigma of that flower.

Bee Pollinating a Flower
Honey bee pollinating a flower

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