Plants face a problem animals do not: their gametes cannot move. A pollen grain has no tail and no way of swimming anywhere. So a flowering plant solves the problem by growing a tube through its own tissue — and then fertilises twice at the same time.
📚 What you need to know
Male gametes are produced in the anther and female gametes in the ovule.
In each pollen sac a diploid mother cell undergoes meiosis to form four haploid pollen grains; mitosis then produces more haploid male gametes.
In the ovule, a single diploid cell undergoes meiosis to produce four haploid cells, of which only one survives and divides by mitosis to produce the female gametes.
Pollination is the transfer of pollen from the anther of one flower to the stigma of another.
Many flowering plants are hermaphroditic, containing both male and female parts, so self-pollination is possible as well as cross-pollination.
After pollination a pollen tube grows from the pollen grain down the style to the ovary, carrying two male nuclei.
One male nucleus fuses with the ovule nucleus to form the zygote; the other forms the embryo’s food store.
After fertilisation the ovule becomes a seed and the ovary develops into the fruit.
Making the gametes
Both sexes use the same two-step pattern: meiosis first to halve the chromosome number, then mitosis to produce more copies of the haploid product.
In the anther are pollen sacs. Each pollen sac contains a diploid mother cell that undergoes meiosis to form four haploid pollen grains, and mitosis then produces further haploid male gametes.
In the ovule, a single diploid cell undergoes meiosis to produce four haploid egg cells, but only one of them survives. That survivor divides by mitosis to produce the female gametes.
Why meiosis matters here. Because the gametes are made by meiosis and then fuse, all pollination is a form of sexual reproduction — even self-pollination, where both gametes come from the same plant.
The structure of a flower
Flowers develop during the reproductive stage of the plant life cycle and contain everything needed for sexual reproduction by pollination. Three structures do the heavy lifting: the anther, where the male gamete is found; the stigma, which receives the pollen; and the ovary, where the female gametes sit.
Learn the two collective names. “Stamen” and “carpel” each cover several structures, and questions often ask for one of them by name.
Structure
Function
Sepal
Protects the developing flower while it is still in the bud
Petal
Colourful, to attract pollinators
Anther
Part of the stamen that produces the male gametes
Pollen
Contains the male nuclei for fertilisation
Filament
The stalk of the stamen that holds the anther up
Stigma
The top of the carpel, where pollen lands
Style
The part of the carpel that supports the stigma
Ovary
Contains the ovules
Ovule
The chamber within the ovary where female gametes develop
Pollination
Pollination is the transfer of pollen from an anther to a stigma. Many flowering plants are hermaphroditic, meaning one flower contains both male and female parts, which opens up two possibilities.
Self-pollination — pollen is transferred between different flowers on the same plant, or even from anther to stigma within the same flower.
Cross-pollination — pollen is transferred from one plant to another. Flowers use shape, colour and scent to attract pollinators and make this happen.
The pollen tube and double fertilisation
Landing on the stigma is not the end of the journey. Once pollination has occurred, a pollen tube grows out of the pollen grain and down through the style to the ovary. The tip of the tube produces enzymes that digest the tissue of the stigma and style, clearing a path.
Two male nuclei travel down that tube to the ovule. This is where plants do something animals never do. One male nucleus fuses with the ovule nucleus to form the diploid zygote. The other fuses with two polar nuclei to produce a triploid nucleus that becomes the embryo’s food store, the endosperm.
Double fertilisation
male nucleus 1 + egg nucleus = zygote (2n) | male nucleus 2 + two polar nuclei = endosperm (3n)
The second fusion is why the food store in a seed is triploid. It is a genuine fertilisation, not just a spare nucleus being used up.
Fertilisation in a plant is the same event as in an animal: haploid male and female nuclei fuse to form a diploid zygote. What differs is the delivery method and the fact that it happens twice.
Worked examples
WORKED EXAMPLE
Explain why self-pollination is still classed as sexual reproduction. [2]
Point 1 — how the gametes are made
The male and female gametes are both produced by meiosis, so they are haploid and genetically varied.
Point 2 — what happens next
The gametes fuse at fertilisation to form a diploid nucleus, which is the defining feature of sexual reproduction, regardless of whether the two gametes came from the same plant.
2 marksSelf-pollination produces less variation than cross-pollination, but it is not asexual.
WORKED EXAMPLE
Describe what happens from pollen landing on the stigma to the formation of a seed. [4]
Point 1 — the tube
A pollen tube grows from the pollen grain down through the style towards the ovary, with enzymes at the tip digesting the tissue ahead of it.
Point 2 — the journey
Two male nuclei travel down the pollen tube to the ovule.
Point 3 — double fertilisation
One male nucleus fuses with the ovule nucleus to form a diploid zygote; the other fuses with two polar nuclei to form the triploid food store.
Point 4 — afterwards
The ovule develops into the seed and the ovary develops into the fruit.
4 marksOvule to seed, ovary to fruit — that pair is worth memorising as one line.
💡 Exam tip
Keep pollination and fertilisation apart. Pollination is transport; fertilisation is fusion of nuclei.
Say the male gamete is the nucleus inside the pollen grain, not the pollen grain itself.
Give the ploidy levels: zygote 2n, endosperm 3n. Numbers score marks.
“Ovary” means two different things in plants and animals. Make clear which you mean.
Both anther and ovule use meiosis then mitosis. Getting the order right matters.
For flower diagrams, label the collective terms stamen and carpel as well as the individual parts.
⚠ Common mix-up
Pollination is not fertilisation. Pollen can land and never fertilise anything.
Self-pollination is not asexual reproduction. Gametes still fuse.
The pollen tube is not the male gamete. It is the route the nuclei travel down.
Only one of the four cells from meiosis survives in the ovule. Do not claim four egg cells result.
The stigma is not the ovary. The style connects them.
Endosperm is triploid, not diploid. Three nuclei were involved in making it.
Up next: Pollination — how insect-pollinated and wind-pollinated flowers are built differently, and the tricks plants use to avoid pollinating themselves.
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