Flower Structure, Pollination, and Fertilization

A flower is the reproductive organ of a flowering plant, and pollination and fertilization are the two steps that turn its ovules into seeds. In brief: pollination is the transfer of pollen from an anther to a stigma, and fertilization happens later when a male sex cell from the pollen fuses with a female sex cell in an ovule. A bee visiting an apple blossom can carry pollen from one flower to another and make seeds possible.

Flowers produce the fruits and seeds that feed people and animals. This guide explains the parts of a flower and their jobs, the types of pollination, what happens during fertilization in flowering plants, how to dissect a flower and identify its parts and the mistakes that cause confusion.

What is a flower?

A flower is a short shoot with modified leaves arranged to carry out sexual reproduction. Flowering plants, or angiosperms, are the most diverse group of land plants. Their flowers range from the tiny blooms of grasses to large, colorful lilies.

Not every flower has both male and female parts. Some flowers have only one type, and some plants have male and female flowers on separate individuals. Flowers with both are called bisexual or perfect flowers.

What is the structure of a flower?

A typical flower is built from four rings of parts attached to the receptacle, the swollen tip of the flower stalk.

Sepals and petals

The outermost ring contains the sepals, which are usually green and protect the bud. Inside them are the petals, which are often bright and scented and attract pollinators. Together they are called the perianth.

Stamens: the male parts

Each stamen has an anther on a stalk called the filament. The anther contains pollen grains, and each pollen grain carries the male sex cells.

Carpels: the female parts

A carpel has three regions. The stigma is the sticky top that receives pollen, the style is the stalk that connects the stigma with the ovary, and the ovary at the base contains one or more ovules. Each ovule holds an egg cell and can become a seed. The carpel or group of carpels is also called the pistil.

What is pollination?

Pollination is the transfer of pollen from an anther to a stigma. It is not the same as fertilization, because the sex cells have not yet fused.

• Self-pollination: Pollen lands on a stigma of the same flower or the same plant. It needs no pollinator but gives less genetic variation.
• Cross-pollination: Pollen goes to a flower on a different plant of the same species. This increases variation, which can help a population cope with change.

The carriers of pollen are called pollinators. They include insects such as bees, butterflies and moths, birds, bats, wind and, in some cases, water.

How are flowers adapted for pollination?

Insect-pollinated flowers usually have large colored petals, scent and nectar. Their pollen grains are sticky or spiky and cling to the insect’s body. The stigma is sticky as well, so that grains brought by the insect stay on it.

Wind-pollinated flowers, such as those of grasses and hazel, are usually small and have no bright petals or nectar. Their anthers hang outside the flower, and the pollen is light and produced in very large amounts. The stigmas are often feathery and extend outside, which helps to catch pollen from the air.

How does fertilization happen in flowering plants?

After a compatible pollen grain lands on a stigma, it germinates and grows a pollen tube. The tube grows down through the style toward the ovary and enters an ovule through a small opening.

1. The pollen tube carries two male sex cell nuclei.
2. One nucleus fuses with the egg cell. The result is a zygote, which will develop into the embryo.
3. In flowering plants, the second nucleus fuses with other nuclei in the ovule to form a tissue that nourishes the embryo, the endosperm. This double event is called double fertilization.

After fertilization, the ovule develops into a seed, and the ovary usually develops into a fruit. The petals and other parts often wither and fall away. Seed and fruit formation are covered in the next article.

Worked example: dissecting a flower

A simple dissection helps to learn the parts. Use a large, simple flower such as a lily or a tulip, a hand lens, tweezers and a white sheet of paper.

1. Look at the flower from the outside and count the sepals and petals.
2. Remove the sepals and petals gently and place them on the paper in order.
3. Locate the stamens around the center. Count them and look at the anthers with the lens for yellow pollen.
4. Remove the stamens, leaving the carpel in the middle.
5. Identify the stigma, style and ovary. Cut the ovary lengthwise to see the ovules.

For a lily, a typical count is six petal-like parts and six stamens, with one carpel. Counts vary among species, and so a record of the numbers is more useful than a fixed expectation. The positions are always the same: male parts surround the female part in the center.

Insect-pollinated and wind-pollinated flowers compared

How was plant sexuality discovered?

Ancient people already knew that date palms need male trees nearby to produce fruit. The German botanist Rudolf Jakob Camerer showed experimentally around 1694 that plants need pollen to make seeds.

Later, Joseph Gottlieb Kölreuter carried out many crossing experiments in the eighteenth century. Charles Darwin studied the benefits of cross-pollination and the relationships between flowers and insects. The detailed description of double fertilization was published by Sergei Nawaschin in 1898.

Why do pollination and fertilization matter?

Many crops depend on animal pollination, including apples, almonds and pumpkins. Farmers rent honeybee hives during flowering so that enough pollen is transferred. A decline in wild pollinators can therefore affect harvests, and it is a concern for conservation.

Plant breeders also use these processes. They remove anthers from one parent and dust pollen from another onto the stigma by hand to produce new varieties. Understanding flower structure helps gardeners to plant varieties that flower at the same time so that cross-pollination can occur.

Which mistakes are common?

Mistake 1: Using pollination and fertilization as synonyms

Pollination is the transfer of pollen. Fertilization is the fusion of sex cells, which may happen hours or days later, after the pollen tube has grown.

Mistake 2: Swapping stamens and carpels

Stamens are male, and they produce pollen. Carpels are female, and they contain ovules. Pollen is made on the anther, and it is received on the stigma.

Mistake 3: Thinking that the pollen grain itself fuses with the egg

The pollen grain grows a tube that delivers male nuclei. Only these nuclei fuse with the egg cell.

Mistake 4: Assuming bright petals mean wind pollination is impossible

Some plants use both methods. As a rule, bright petals and nectar suggest animals, but the best evidence is how pollen is actually carried.