How Pollination Helps Flowering Plants Make Seeds

The male parts of a flower are called stamens. Each stamen usually has a filament supporting an anther, where pollen grains develop. Pollen carries the male gametes, or the cells that will contribute genetic material during fertilisation.

The female structure is called the carpel. Its sticky top is the stigma, which receives pollen. Beneath it is the style, leading to an ovary that contains one or more ovules. Each ovule holds a female gamete and can develop into a seed after successful fertilisation.

Flowers vary enormously. Some contain both stamens and carpels, while others contain only one type. Different species also place these structures in different positions. The arrangement often matches the animal, wind pattern or other method that moves pollen.

Animal-pollinated flowers often use colour, scent, nectar and shape to attract visitors. As an insect, bird or other animal feeds, pollen sticks to its body. Some of that pollen may later touch the stigma of another flower of the same species.

Wind-pollinated plants do not need bright petals or nectar for the same purpose. They often produce large amounts of light pollen and expose their anthers and stigmas to moving air. Much of the pollen misses its target, but producing many grains increases the chance that some will arrive.

Self-pollination occurs when pollen reaches a stigma on the same flower or plant. Cross-pollination moves pollen between different plants of the same species and can create greater genetic variation. Many plants have timing, shape or chemical mechanisms that reduce self-pollination.

After a suitable pollen grain lands on the stigma, it can grow a pollen tube down through the style. Male gametes travel through the tube towards an ovule. Fertilisation happens when male and female gametes join, producing a cell that can develop into an embryo.

The ovule then develops into a seed, while the ovary often develops into a fruit. Fruits protect seeds and may help disperse them through wind, water or animals. Because botany enjoys correcting grocery shops, tomatoes, peppers and cucumbers are fruits in this biological sense.

Follow the sequence:

• •Anthers produce pollen.
• •Pollen reaches a suitable stigma.
• •A pollen tube grows towards an ovule.
• •Male and female gametes join.
• •Ovules become seeds and the ovary may become fruit.

Flowering plants have an unusual trick that animals do not share, called double fertilisation. A pollen tube delivers not one but two male gametes into the ovule. One joins with the egg cell and forms the zygote, which grows into the embryo, the tiny young plant inside the seed. The second male gamete joins with a different cell in the ovule, one that contains two nuclei. That combination develops into a tissue called endosperm, which acts as a food store for the developing embryo.

Endosperm matters far beyond botany. In cereal grains such as wheat, rice and maize, most of what you eat is endosperm, packed with starch and some protein. White flour is made largely from ground wheat endosperm, while wholemeal flour also keeps the outer layers and the germ, which is the embryo. The white flesh of a coconut and the liquid inside it are also forms of endosperm. So a large share of the food eaten by people around the world exists because of this second fertilisation event inside flowers.

In 1862 Charles Darwin examined an orchid from Madagascar, Angraecum sesquipedale, which keeps its nectar at the bottom of an extraordinarily long tube, often around thirty centimetres. He reasoned that the flower could only be pollinated by an insect able to reach that nectar, and so predicted that a moth with a tongue of similar length must live on the island. Many people found the idea far-fetched, since no such insect was known at the time.

In 1903, after Darwin's death, a hawkmoth with a very long tongue was described from Madagascar, and its scientific name includes the word praedicta in honour of the prediction. Much later, the moth was filmed visiting the orchid. The example shows how flowers and their pollinators can shape one another over many generations. A longer nectar tube favours moths with longer tongues, which favours even longer tubes, because the flower benefits when the moth must press its head against the pollen to drink.

Vanilla shows what happens when a plant leaves its pollinators behind. The vanilla orchid comes from Mexico and Central America, and when growers took it to other tropical colonies in the nineteenth century, the vines flowered but almost never produced pods, because local insects could not pollinate them. Inside each flower, a flap of tissue separates the pollen from the stigma. In 1841, on the island of Réunion, a twelve-year-old enslaved boy named Edmond Albius worked out how to lift the flap with a thin stick and press the two parts together by hand.

His quick method made vanilla farming possible across the tropics, and most of the world's vanilla is still pollinated flower by flower in much the same way. Each flower opens for only about a day, so workers must check the vines every morning. Other crops rely on managed animals instead. Almond trees in California need pollen from a different variety to set nuts, so growers plant alternating rows of compatible trees and bring in well over a million honeybee hives each spring, one of the largest managed pollination efforts in the world.

Some flowers keep their pollen locked inside tube-shaped anthers that open only through tiny pores at the tip, a little like salt shakers. Tomatoes, potatoes, aubergines and blueberries are among the thousands of plants built this way. A visiting bee cannot simply brush the pollen off. Instead, a bumblebee grips the flower and vibrates its flight muscles rapidly without flapping its wings, shaking a cloud of pollen out onto its body. You can sometimes hear the pitch of its buzz change as it does so. This is called buzz pollination, and honeybees do not perform it.

That detail shaped modern greenhouse farming. Tomatoes grown indoors lack wind and wild bees, so growers once shook every flower cluster by hand with electric vibrating wands, a slow and costly job. From the late 1980s, commercially reared bumblebee colonies, first widely used in the Netherlands and Belgium, took over the task, and they are now standard in greenhouses across much of the world. Growers can check the work, because a visiting bumblebee leaves small brown bite marks on the flower. Exporting the bees has a downside: escaped colonies of one European species have spread through parts of South America, competing with native bumblebees.

Pollination transfers pollen to a stigma, while fertilisation happens later when gametes join inside an ovule. The resulting ovules can become seeds, and the ovary may become fruit. Keep those stages separate, and flower reproduction becomes a clear journey from pollen movement to the next plant generation.