How does a grain of pollen end up as a seed inside a fruit?

How to use this simulation

1. Press play. The animation walks through seven steps: pollen ripens, a carrier moves it, it lands on the stigma, the tube grows, the tube enters the ovule, double fertilisation happens, and the seed and fruit form. The bar under the flower shows which step you are on.
2. Use Previous step and Next step to move one step at a time. They pause the animation so you can study each picture, and the caption under the bar names what is happening.
3. Pick a pollination agent. Insect and wind both bring pollen from another flower (cross-pollination). The same-flower choice makes the right-hand anther drop its pollen on its own stigma (self-pollination). Watch what changes in step 2.
4. Drag the pollen-grain slider. With 8 grains on the stigma, 6 grow tubes. The readings tell you how many tubes grow, how many seeds form and how many ovules are left over.
5. Drag the ovule slider. Each ovule needs exactly one tube. If there are more ovules than tubes, the extra ovules wither in step 7 and the readout counts them as unfertilised.
6. Drag the haploid-number slider n. The zygote shows 2n and the endosperm 3n in the zoomed ovule, and the readings print the real counts. Try 7 (pea) and see 14 and 21.

A bee, a flower and a lot of cleverness

A bee lands on a mustard flower, dusts its legs with yellow powder, and flies to the next flower. It has no idea what it just did. The powder was pollen, and on the second flower some of it will rub off on a sticky tip. A few days later that flower is a swelling pod full of seeds.

The plant never moved, but its pollen did. A plant cannot walk to a partner, so it sends its male cells out in tiny armoured packages. Pollination is the delivery. Fertilisation is what happens after the package arrives.

Students often mix the two words up, so keep the order straight. Pollination happens outside the ovary, on the stigma. Fertilisation happens inside the ovule. Pollination is the transfer; fertilisation is the fusion.

The parts of the flower that do the work

Only four parts matter for this topic, and the animation labels them. The stamen is the male part: a thin filament carrying the anther, where pollen grains are made. The carpel is the female part: the stigma at the top catches pollen, the style is the stalk that carries the tube down, and the ovary at the base holds the ovules.

Each ovule contains an embryo sac, a small bag of cells with an egg and a central cell. Petals and sepals do not take part in the fusion; they attract carriers and protect the bud.

Self-pollination and cross-pollination

If the pollen lands on the stigma of the same flower, or another flower of the same plant, that is self-pollination. If it lands on a flower of a different plant of the same species, that is cross-pollination. Pea flowers are famous for the first and mustard for the second, which is why Mendel used peas: he could control who fathered whom.

The choice matters for the offspring. Self-pollination keeps the family the same, which is excellent if the parent was well adapted. Cross-pollination mixes two parents, so every seed is a new combination, and a population with that variety copes better when the weather or a disease changes.

Many plants keep both options open. Some flowers shed pollen before their stigma is ready, or have a stigma that rejects its own pollen. These are tricks to push the plant towards crossing.

Insects and wind as carriers

An insect-pollinated flower advertises. It has bright petals, scent and nectar, and its pollen is sticky or spiky so that it clings to the visitor. It makes a modest amount, because a bee delivers it very accurately.

A wind-pollinated flower, such as a grass, maize or paddy, does the opposite. It has small dull flowers, no scent and light, dry pollen that floats. The stigma is large and feathery to comb the air. The plant gambles, so it makes enormous quantities of pollen, since almost all of it will land nowhere useful.

That is why the animation shows some of the wind's grains drifting away while an insect carries its load straight to the stigma. A real maize tassel can release millions of grains for every silk that receives one.

From pollen grain to pollen tube

A pollen grain is not a cell waiting for a miracle. It already contains two cells: a large tube cell and a small generative cell, wrapped in a tough wall. The wall carries a chemical signature, and the stigma checks it. If the grain is the right species, the stigma lets it germinate.

The tube cell pushes out a pollen tube that digs through the style, steered by chemical signals from the ovule. The generative cell divides once on the way and makes two male gametes. By the time the tube tip reaches the ovule, it carries a tube nucleus and two male gametes, each with n chromosomes.

In this model, three out of every four grains on the stigma grow a tube. With 8 grains, 6 tubes grow. That is a model rule for practice, not a measurement of a real species, but the idea is real: not every grain succeeds.

Inside the ovule: the embryo sac

The tube enters the ovule through a tiny gap called the micropyle. Inside is the embryo sac, which in most flowering plants is a bag of seven cells and eight nuclei. Three of them matter here. The egg cell sits near the micropyle. The big central cell holds two polar nuclei. The other cells (two helpers beside the egg and three at the far end) support the process.

Every nucleus in the embryo sac is haploid, with n chromosomes. In the animation, the zoomed ovule shows only the egg and the two polar nuclei, because those are the ones that fuse with the male gametes.

Double fertilisation

The pollen tube bursts and releases its two male gametes. They do different jobs at the same time. One travels to the egg and fuses with it. That gives a zygote with 2n chromosomes, which will grow into the embryo. The other travels to the central cell and fuses with both polar nuclei. That makes a nucleus with 3n chromosomes, which divides to form the endosperm, the food store for the embryo.

Two fusions in one ovule, so it is called double fertilisation. The second fusion involves three nuclei, so it is also called triple fusion. Only flowering plants do this.

With n = 7 (the pea), the zygote has 14 chromosomes and the endosperm 21. The animation prints the same numbers in the readings and on the zoomed ovule, and they update when you move the n slider.

After fertilisation: the flower becomes a fruit

Once the fusions are done, the flower stops advertising. Petals and stamens wither, and the style and stigma shrivel. The ovary swells, because the developing seeds send out hormones that tell it to grow. The zygote divides into an embryo, the endosperm nucleus divides into a tissue that stores food, and the covering of the ovule hardens into the seed coat.

The table follows each part of the flower to its place in the fruit. It is a favourite for matching questions.

How many seeds? One tube, one ovule

Each ovule can be fertilised once, and each tube delivers its two male gametes to one ovule. So the number of seeds is the smaller of the number of tubes and the number of ovules. Count the tubes, count the ovules, and take the smaller one.

With 6 ovules and 8 grains, 6 tubes grow, so 6 seeds form and 0 ovules are left over. The table changes only the pollen. Notice the two regimes: with little pollen, the seeds are limited by pollen; with a lot, they are limited by ovules, and the extra tubes are wasted.

Counting chromosomes

A male gamete and an egg each carry n chromosomes. Their fusion restores 2n. The endosperm gets one male gamete (n) plus two polar nuclei (n + n), so it carries 3n. Every ordinary cell of the plant, including the seed coat and the ovary wall, is 2n.

The table gives four plants with well-known haploid numbers. You can reproduce the first row in the simulation by setting n to 7.

Solved problems

Eight problems in the exam style, solved with the same rules as the animation.

Mistakes that cost marks

Check this list before an exam. Each one has taken marks from someone.

• Writing that pollination and fertilisation are the same. Pollination is transfer to the stigma; fertilisation is fusion inside the ovule.
• Saying the endosperm is 2n. It is 3n, from one male gamete and two polar nuclei.
• Saying both male gametes fuse with the egg. One fuses with the egg, and the other with the polar nuclei.
• Forgetting that the pollen tube has two gametes only because the generative cell divides. The grain starts with two cells.
• Matching the ovary with the seed. The ovule becomes the seed; the ovary becomes the fruit.
• Calling every pollination by wind a waste. It is a high-volume strategy that works for grasses and cereals.
• Using "self-pollination" for pollen moving to a flower on a different plant. That is cross-pollination.

Where you meet it in real life

Farmers use this chapter daily. A rice field flowers for a few days in the morning, and a night of rain washes the pollen away; poor grain filling follows. Orchard growers rent beehives during the flowering season, because more visits mean more pollen on every stigma and more seeds per fruit. A fruit with more seeds is often bigger, because seeds drive fruit growth.

Some fruits do not need fertilisation at all. A banana forms without seeds by a process called parthenocarpy, and seedless watermelons are made by breeding. This works because the ovary can be told to swell by hormones, even though no seed is forming.

Seed banks, plant breeders and anyone crossing varieties by hand use the same ideas: cover the flower so unwanted pollen cannot arrive, then brush on the chosen pollen.

Quick revision

The whole chapter on one page.

• Pollination is the transfer of pollen to the stigma. It can be self or cross, by insect, wind, water or animals.
• A pollen grain grows a tube down the style; the generative cell divides into two male gametes.
• The tube enters the ovule through the micropyle.
• Male gamete + egg gives a zygote, 2n. Male gamete + two polar nuclei gives the endosperm, 3n.
• Ovule becomes seed, ovary becomes fruit, zygote becomes embryo, endosperm feeds the embryo.
• Seeds formed = the smaller of the tubes and the ovules.

Frequently asked questions