Flowers, Pollination and Fertilisation

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.3 — The structures of insect-pollinated and wind-pollinated flowers and their adaptations
• 3.4 — Pollen tube growth and fertilisation leading to seed and fruit formation

Specification section 3(a) Reproduction — also taught in Sexual and Asexual Reproduction, Seeds and Germination, The Human Reproductive Systems, The Menstrual Cycle.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A plant has a problem that no animal has. Sexual reproduction requires a male gamete from one individual to reach a female gamete in another — and a plant cannot walk across the field to deliver it. Everything about a flower is an answer to that, and there are two quite different answers in common use. One is to hire a courier. The other is to throw the pollen into the wind and rely on numbers.

The parts of a flower, and what each one is for

1. ANTHER — produces the pollen grains, which contain the male gametes. Anther and filament together make up the STAMEN, the male part.
2. FILAMENT — the stalk holding the anther in position.
3. STIGMA — the surface that receives pollen. Stigma, style and ovary together make up the CARPEL, the female part.
4. STYLE — the stalk connecting the stigma to the ovary; the pollen tube grows down through it.
5. OVARY — contains the OVULES, each holding a female gamete. After fertilisation the ovary becomes the fruit.
6. PETALS and SEPALS — petals attract pollinators where there are any; sepals protected the flower while it was a bud.

POLLINATION is the transfer of pollen from an anther to a stigma. It is worth stopping on that definition, because it is only the delivery — the pollen has arrived on the outside of the female part and nothing has yet fused. Fertilisation happens later, and there is a real distance still to cross.

Think of it like a courier service against a leaflet drop

If you must get one document to one specific address, you pay a courier: it costs something, you have to make the delivery worth their while, and almost every package arrives. If instead you print half a million leaflets and drop them from a plane, each one costs almost nothing, nobody has to be persuaded of anything — and the overwhelming majority land on roads, rivers and rooftops. Both strategies work. The courier route is why flowers have colour, scent and nectar, all of which are payment. The leaflet route is why a single grass plant makes millions of pollen grains and no petals at all: it is not trying to attract anything, and decoration would be a waste.

Insect-pollinated flowers — every feature earns its place

1. LARGE, BRIGHTLY COLOURED PETALS, and often a scent — these attract the insect, which is the whole strategy.
2. NECTARIES producing nectar — the payment. The insect comes for food and pollination is incidental to it.
3. ANTHERS INSIDE the flower, held firmly — so the insect must brush past them and pick pollen up on its body.
4. STIGMA INSIDE the flower, and sticky — so pollen already on the insect is wiped onto it and held.
5. POLLEN in small amounts, and sticky or spiky — it must cling to the insect and does not need to travel loose.

Wind-pollinated flowers — the same list, answered the opposite way

1. SMALL, GREEN OR DULL petals, or none at all, and no scent and no nectar — there is nothing to attract, so all of that would be wasted material.
2. ANTHERS HANGING OUTSIDE the flower on long filaments — so that the pollen they release is exposed to moving air, which catches it and carries it away.
3. STIGMA OUTSIDE the flower, large and feathery — a big net to catch pollen out of moving air.
4. POLLEN in enormous quantities, and small, light and smooth — most grains will be lost, so the strategy depends on producing far more than is needed.
5. The flowers are often held high on the plant and open before the leaves in spring, so nothing blocks the air.

Now the part almost everyone rushes. Pollen has landed on the stigma, and the female gamete is inside an ovule, inside the ovary, at the bottom of the style. The pollen grain has no way to travel. So it grows a POLLEN TUBE — a thin tube that pushes down through the tissue of the style, carrying the male gamete nucleus inside it, until it reaches the ovule and enters through a small opening. Only then does the male gamete nucleus fuse with the female gamete nucleus. That fusion is fertilisation, and it happens deep inside the ovary, not on the stigma where the pollen landed.

What each part becomes afterwards

1. The fertilised OVULE becomes the SEED — containing the embryo and a food store.
2. The OVARY becomes the FRUIT, which surrounds and protects the seeds and helps to disperse them.
3. The petals, stamens and stigma have done their jobs and wither away.
4. That is why an apple has pips inside it, and why a pea pod is a row of seeds inside a swollen ovary wall. A fruit is not a botanical category of sweet things — it is a former ovary, which is why a tomato, a cucumber and a chilli are all fruits.

One more distinction, because it is examined and it is easy to blur. SELF-pollination is pollen reaching a stigma on the same plant; CROSS-pollination is pollen reaching a stigma on a different plant of the same species. Only cross-pollination brings together genetic material from two different individuals, so it produces far more variation — which is why so many flowers have arrangements that make self-pollination difficult, such as ripening their anthers and stigmas at different times.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Pollination

The transfer of pollen from an anther to a stigma. It is the delivery of the male gamete to the female part of a flower — not the fusion of the gametes.

The parts of a flower and the function of each

Anther

Produces pollen grains, which contain the male gametes.

Filament

The stalk that holds the anther in position. Anther + filament = the stamen.

Stigma

Receives pollen grains. Sticky in insect-pollinated flowers, feathery in wind-pollinated ones.

Style

Connects the stigma to the ovary; the pollen tube grows down through it.

Ovary

Contains the ovules. After fertilisation it becomes the fruit.

Ovule

Contains the female gamete. After fertilisation it becomes the seed.

Petals

Large and brightly coloured to attract insects; small, green or absent where the wind does the work.

Sepals

Protected the flower while it was in bud.

Statement 3.3 — insect against wind, feature by feature

Statement 3.4 — from pollination to seed, in order

1. A pollen grain lands on the stigma — this is pollination.
2. The pollen grain grows a pollen tube, which grows down through the style.
3. The male gamete nucleus travels down inside the pollen tube.
4. The tube reaches the ovule and enters it through a small opening.
5. The male gamete nucleus fuses with the female gamete nucleus — this is fertilisation, and it produces a zygote.
6. The fertilised ovule develops into a SEED; the ovary develops into a FRUIT.

Pollination against fertilisation — the single most examined distinction on this page

A. POLLINATION is the TRANSFER of pollen from an anther to a stigma. It is delivery, it happens on the outside of the carpel, and nothing has fused.

B. FERTILISATION is the FUSION of the male gamete nucleus with the female gamete nucleus. It happens later, inside an ovule, after the pollen tube has grown down the style.

Self against cross-pollination

A. SELF-pollination: pollen reaches a stigma on the SAME plant. Reliable, but the genetic material comes from one individual, so there is little variation.

B. CROSS-pollination: pollen reaches a stigma on a DIFFERENT plant of the same species. Less reliable, but it combines genetic material from two individuals, so the offspring are genetically varied.

What becomes what — a line examiners award marks for verbatim

• Ovule → SEED
• Ovary → FRUIT
• Zygote → embryo, inside the seed
• Petals, stamens and stigma → wither and fall away
• A fruit is a former ovary, which is why tomatoes, cucumbers, peppers and pea pods are all fruits

Model answer [4 marks]

Describe how fertilisation occurs in a flowering plant after a pollen grain has landed on the stigma. [4]

The pollen grain grows a pollen tube, which grows down through the style towards the ovary. The male gamete nucleus travels down inside the pollen tube. The tube enters the ovule through a small opening. The male gamete nucleus then fuses with the female gamete nucleus to form a zygote, which is fertilisation.

Model answer [4 marks]

Explain two ways in which a wind-pollinated flower is adapted for pollination by wind. [4]

The anthers hang outside the flower on long filaments, so that moving air can catch the pollen and carry it away from the plant. The stigma is also outside the flower and is large and feathery, which gives a big surface area to catch pollen grains being carried past in the air. The plant also produces very large amounts of small, light, smooth pollen, because most grains will never reach a stigma.

Model answer [3 marks]

Explain why many flowers have features that prevent self-pollination. [3]

Self-pollination combines genetic material from only one plant, so the offspring show little variation. Cross-pollination brings together genetic material from two different plants, so the offspring are more varied. Variation means some offspring are more likely to survive if the environment changes, and it is what natural selection acts on.

Mark-losing trap. Pollination is TRANSFER; fertilisation is FUSION. Swapping them loses the mark in either direction.

Mark-losing trap. Ovule → seed, ovary → fruit. Learn the pair together; one without the other is half an answer.

Mark-losing trap. The pollen tube grows down through the STYLE. It does not travel, drop or get carried — it grows.

Mark-losing trap. Wind-pollinated flowers are not 'badly adapted' or 'simple'. Every feature they lack is a feature they do not need.

Mark-losing trap. Say WHY an adaptation works, not just what it is. 'Feathery stigma' scores nothing; 'feathery stigma gives a large surface area to catch pollen from the air' scores.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — What is pollination?
2. Grade 7 · Identify [2 marks] — A flower has small green petals, no scent, anthers hanging outside on long filaments, and a large feathery stigma. How is it pollinated, and which feature is the clearest evidence?
3. Grade 8 · Describe [5 marks] — A pollen grain lands on the stigma of a flower. Describe what happens next, up to the formation of a seed. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Calculate [3 marks] — A wind-pollinated grass plant produces 4.2 × 10⁶ pollen grains, of which 630 reach a stigma of the same species. Calculate the percentage of its pollen grains that reach a stigma.
5. 9+ · Analyse [5 marks] — A plant species has flowers whose anthers ripen and release all their pollen several days before the stigmas of the same flower become sticky and receptive. A student says this must be a fault, since it wastes pollen. Select every statement that belongs in a full-mark evaluation.

The people behind this science

Two ways into the same idea — the one who asked what all this machinery is avoiding, and the one who took control of pollination to make it tell him something. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Charles Darwin — the one who asked what all this machinery is avoiding

Darwin spent years on flowers — a whole book on how orchids are fertilised by insects, another on the different forms of primrose — and he was not cataloguing. He was pursuing one question: why do so many flowers have elaborate arrangements that make self-pollination difficult, when self-pollination would be so much easier? His answer is the last section of this page. He also ran the experiment: over a decade he raised plants from self-pollination and from cross-pollination side by side and measured them, and the cross-pollinated offspring were consistently taller and stronger.

• “Why do so many flowers make it hard to pollinate themselves?”
• “What did you find when you grew self-pollinated and cross-pollinated plants side by side?”
• “How does an orchid make sure an insect leaves carrying its pollen?”
• “Is a flower's shape designed for its insect, or the insect for the flower?”
• “What would happen to a plant that only ever pollinated itself?”

Gregor Mendel — the one who took control of pollination to make it tell him something

Mendel's entire result depends on the machinery described on this page, used deliberately. Pea flowers normally self-pollinate, which is exactly why he chose them — he could rely on a plant breeding true unless he intervened. To make a cross he opened the flower before it was ready, cut out the anthers so it could not pollinate itself, and brushed pollen from a chosen plant onto the stigma by hand. Every conclusion he reached about inheritance rests on knowing precisely which two plants the gametes came from, and this is how he knew.

• “Why did you remove the anthers before making a cross?”
• “Why were peas a better subject than a flower that cross-pollinates freely?”
• “How did you stop an insect from ruining a cross you had made?”
• “What would have gone wrong if a plant had pollinated itself without your noticing?”

Carry on through the course

• Previous lesson: Sexual and Asexual Reproduction
• Next lesson: Seeds and Germination
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• The Placenta and the Amniotic Fluid — An exchange surface with a constraint no other one has: two bloodstreams that must trade materials without ever mixing.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification