A flowering plant faces the same problem as any animal — get two haploid nuclei to fuse — with one enormous handicap: it cannot move. Everything odd about plant reproduction, from bright petals to a pollen tube that has to grow through solid tissue, is a workaround for that single limitation.
📘 What you need to know
Male gametes are made in the anther and carried inside pollen grains; female gametes are made in the ovule, inside the ovary.
A diploid cell in each pollen sac undergoes meiosis to give four haploid pollen grains; mitosis then produces the male gamete nuclei.
In the ovule a diploid cell undergoes meiosis to give four haploid cells, of which only one survives and divides by mitosis to give the female gamete.
Pollination is the transfer of pollen from an anther to a stigma. It is not fertilisation.
After pollination a pollen tube grows from the pollen grain down through the style to the ovule, carrying two male nuclei.
Double fertilisation: one male nucleus fuses with the egg to form the diploid zygote; the other fuses with two polar nuclei to form the triploid food store.
After fertilisation the ovule becomes the seed and the ovary becomes the fruit.
Where the gametes come from
The pattern is the same as in animals but with an extra step at the end, so it is worth setting out carefully.
In the anther are pollen sacs. Inside each sac a diploid mother cell divides by meiosis to produce four haploid pollen grains. Those grains then divide by mitosis to produce the actual male gamete nuclei — two of them per grain, which matters later.
In the ovule, a single diploid cell divides by meiosis to give four haploid cells. Three of them break down and only one survives. That survivor divides by mitosis to produce the female gamete, along with the other haploid nuclei found inside the ovule.
Watch the order: meiosis first, then mitosis. Students often assume that because meiosis makes gametes in animals, it must make them directly in plants too. In plants meiosis makes haploid cells, and mitosis turns those into gametes. A question that mentions “mitosis in the anther” is not a trick — it is testing exactly this.
The parts of a flower
You need to be able to draw and annotate an insect-pollinated flower. The structure is far easier to remember once you see that it is built in rings, working inwards: sepals on the outside, then petals, then the male parts, with the female parts in the centre.
Stamen and carpel are collective names, not individual structures. Losing a mark for writing “stamen” when the question wanted “anther” is one of the most avoidable errors in this topic.
Structure
Function
Sepal
Protects the flower while it is still a bud
Petal
Large and coloured in insect-pollinated species, to attract pollinators
Anther
Part of the stamen that produces pollen grains, which carry the male gametes
Filament
The stalk that holds the anther in position
Stigma
Top of the carpel; the surface on which pollen lands
Style
Supports the stigma and provides the tissue the pollen tube grows through
Ovary
Encloses and protects the ovules; becomes the fruit after fertilisation
Ovule
Contains the female gamete; becomes the seed after fertilisation
Nectary
Produces a sugary solution that rewards visiting insects with energy
From pollination to fertilisation
Pollination gets the pollen onto the stigma. That is all it does. The male gamete is still stuck at the top of the carpel, several centimetres from the ovule, with solid tissue in between. Solving that is the job of the pollen tube.
After a compatible pollen grain lands, it grows a tube down through the style towards the ovary. The tip of the tube releases enzymes that digest the tissue ahead of it, clearing a path. Two male nuclei travel down the tube to the ovule.
The second fusion is why a seed comes with its own packed lunch. The embryo and its food supply are created in the same event, which is a genuinely elegant piece of biology.
Double fertilisation, and what happens next
Fertilisation in a flowering plant happens twice, in the same ovule, at the same time. One male nucleus fuses with the female gamete to produce the diploid zygote, which will become the embryo plant. The second male nucleus fuses with two polar nuclei to produce a triploid nucleus, which develops into the food store the embryo will live on until it can photosynthesise.
Double fertilisation
male nucleus + egg → zygote (2n) | male nucleus + two polar nuclei → food store (3n)
After fertilisation the structures are renamed, and this is a favourite one-mark question: the ovule becomes the seed and the ovary becomes the fruit. The pollen tube nucleus, having done its job of guiding the tube, degenerates once the tube reaches the ovule.
Why plant fertilisation still counts as sexual reproduction. Both gametes were produced by meiosis, and two haploid nuclei fuse to give a diploid nucleus. That is the definition, and it holds whether the pollen came from the same plant or a different one.
Worked examples
WORKED EXAMPLE
Distinguish between pollination and fertilisation in a flowering plant. [2]
“Distinguish” means give a matched pair of statementssame feature, contrasted — not two unrelated factsPollinationthe transfer of pollen from an anther to a stigma; no nuclei have fusedFertilisationthe fusion of a male nucleus with the female gamete inside the ovule, forming a diploid zygote1 mark for each correctly contrasted statementthe giveaway phrase is “no nuclei have fused” — it shows you know why they are different
WORKED EXAMPLE
A plant species has 14 chromosomes in its leaf cells. State the number of chromosomes in a male gamete nucleus and in the food store of a newly formed seed. [2]
Leaf cells are diploid, so 2n = 14 and n = 7male gamete nucleus is haploid = 7 chromosomesThe food store is triploid, from three haploid nucleione male nucleus + two polar nuclei = 3 × 77 and 21“triploid” is the clue that you need three lots of the haploid number, not two
💡 Exam tips
Practise the flower diagram until you can produce it from memory with eight labels. Diagram questions are among the most reliable marks in this topic.
Never use stamen or carpel when the question wants a specific part. Name the anther, filament, stigma, style or ovary.
State clearly that meiosis comes first, mitosis second when describing gamete production in plants.
If a question gives you a chromosome number, check whether it is asking about the 2n zygote or the 3n food store.
Keep ovary (the plant organ) and ovule (the structure inside it) completely separate in your writing. Examiners will not give you the benefit of the doubt.
⚠️ Common mix-ups
Treating pollination as fertilisation. Pollination is transport. Fertilisation is fusion. There is often a gap of hours or days between them.
Saying the pollen grain is the male gamete. The grain is a container; the gametes are the nuclei inside it.
Forgetting the second fusion. Double fertilisation is the distinctive feature of flowering plants and is regularly worth a mark on its own.
Swapping which structure becomes what. Ovule to seed, ovary to fruit. If you can remember that a pea is a seed and a pea pod is a fruit, you have it.
Assuming the pollen tube “swims” or is pushed. It grows, digesting a path with enzymes released at its tip.
Writing that plant reproduction is asexual because there is only one plant. Self-pollination still involves meiosis and the fusion of gametes, so it is sexual.
Up next: Pollination — how flowers persuade an insect or the wind to do the job of carrying pollen, and the mechanisms they use to avoid pollinating themselves.
Want this explained one-to-one?
Book a free session with an experienced IB Biology tutor and get your trickiest topics made simple.
