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  1. KS3
  2. Biology
  3. Reproduction
  4. Fertilisation, seeds and fruit

Reproduction · Process

Fertilisation, seeds and fruit

Pollen has landed. The two gametes are still several centimetres apart, and neither of them can move.

Start here

A pollen grain solves the problem by growing a tunnel.

There is no sperm here, nothing swims, and nothing is carried. The gametes are at opposite ends of a solid stalk of plant tissue, and the plant has to get one of them to the other.

So how does the male gamete reach the ovule?

Before and after · six parts

What each part turns into

0 of 6 opened

A fertilised flower does not build a fruit from nothing — it converts the parts it already has. Tap a row to see why.

Part of the flowerBeforeAfter fertilisation

The process, in order

Five steps, and only one of them is fertilisation.

  • Pollen lands · On the stigma — this is pollination

    A grain arrives, carried by an insect or the wind. If it is the wrong species the stigma usually does not respond at all; if it is the right one, the grain begins to grow.

  • The pollen tube grows · Down through the style

    The grain extends a single tube down through the tissue of the style towards the ovary, digesting its way through as it goes. The male gamete nucleus travels down inside it.

  • Fertilisation · Inside an ovule — and nowhere else

    The tube reaches an ovule and the male gamete nucleus fuses with the female gamete nucleus. That fusion, and only that fusion, is fertilisation. Every ovule that is going to become a seed needs its own pollen tube.

  • Seeds form · From the fertilised ovules

    Each fertilised ovule becomes a seed: an embryo plant, a store of food to start it off, and a tough coat around both. The seed can survive conditions that would kill the parent plant.

  • The fruit forms · From the ovary around them

    The ovary wall swells and becomes the fruit, and the rest of the flower withers away. Whether it ends up fleshy, dry, hooked or winged depends on how that species intends to move its seeds — which is the next lesson.

Key fact

Pollination is delivery to the stigma. Fertilisation is fusion at the ovule, later, at the bottom of a tube the pollen grain had to grow. A fertilised ovule becomes a seed and the ovary around it becomes the fruit.

Fertilisation, seeds and fruit: the carpel in section, before and afterTwo framed drawings above a key of five steps. The left frame is a carpel cut open from top to bottom. At the top is the stigma, a broad pad drawn with sticky dots, and on it sits a single pollen grain, spiky, numbered 01 and labelled pollination, the grain arrives. Below the stigma the style runs down as a tube drawn in section, its two walls visible. Inside the style an orange line, the pollen tube, grows down from the grain — numbered 02 and labelled the tube grows down through the style — carrying a dark dot, the male gamete nucleus, part of the way along it. At the foot of the style is the ovary, a chamber whose wall is drawn as a distinct thickness with the cavity shaded inside it. Three ovules sit within that cavity, each holding a small nucleus. The orange tube reaches the uppermost ovule and ends inside it, where two dots meet: numbered 03 and labelled fertilisation, inside an ovule. A bracket down the left of the chamber reads: the ovary, ovules inside it. The right frame is the same carpel after fertilisation. The ovary has swollen into the fruit, its wall much thicker, and inside it are three seeds in the places the three ovules occupied, each drawn with a tough double coat and a small embryo curled inside. Above the fruit the style and stigma are a shrivelled stub and two dried remains hang beside it, noted as the rest of the flower withered and fell. Beneath, two lines with drawn arrows: ovule becomes seed, one ovule becomes one seed; and ovary becomes fruit, the wall swells and the seeds stay in. The key reads: 01 pollen lands, on the stigma and this is pollination, a grain arrives carried by an insect or the wind. 02 the pollen tube grows, down through the style, the male gamete nucleus travels down inside it. 03 fertilisation, inside an ovule and nowhere else, the two gamete nuclei fuse and that fusion is fertilisation. 04 seeds form, from the fertilised ovules, an embryo plant, a food store and a tough coat. 05 the fruit forms, from the ovary around them, the ovary wall swells and the rest of the flower withers.THE CARPEL, IN SECTIONAFTER FERTILISATIONstigmastyleovaryovulethe ovaryovules inside it01pollination —the grain arrives02the tube grows downthrough the style03fertilisation —inside an ovuleThe pollen tube — one cell, growingThe gamete nuclei, male and femalethe rest of the flowerwithered and fellthe fruitseedovuleseedone ovule becomes one seedovaryfruitthe wall swells; seeds stay inFIVE STEPS, IN ORDER01Pollen landsOn the stigma — this is pollinationA grain arrives, carried by an insect or the wind02The pollen tube growsDown through the styleThe male gamete nucleus travels down inside it03FertilisationInside an ovule — and nowhere elseThe two gamete nuclei fuse. That fusion is fertilisation04Seeds formFrom the fertilised ovulesAn embryo plant, a food store, and a tough coat05The fruit formsFrom the ovary around themThe ovary wall swells; the rest of the flower withersPollination is 01. Fertilisation is 03. Between them the tube has to grow the whole length of the style.Counting the seeds in a fruit counts the ovules that were fertilised inside the ovary it grew from.
The grain lands at 01 and the two nuclei fuse at 03, and the drawing puts the whole length of the style between them: pollination is delivery, fertilisation is fusion, and they are not the same event. The three ovules are drawn inside the ovary wall; on the right the three seeds are inside the fruit wall, in the same three places. Ovule to seed and ovary to fruit are then one fact about containment rather than two labels to memorise. Nothing moves out of anything — check it: put one finger on the top ovule on the left and one on the top seed on the right, and nothing has travelled. It changed where it already was, and the ovary swelled around it. The tube has a distance to grow and takes time to grow it, which is why 01 and 03 are two events and not one.

Think again

“Pollination and fertilisation are the same thing.”

They are separated by a distance, a delay and a definition — and this is the single most common error in the whole of plant reproduction. Pollination is transport: a grain of pollen arrives on a stigma. Nothing has joined, nothing is fertilised, and the plant may still end up with no seeds at all. Fertilisation is the fusion of the pollen's gamete nucleus with the ovule's, which happens somewhere else entirely — down inside the ovary — after the grain has grown a tube the whole length of the style to get there. Hours later in some species; in an oak, months. It is the same distinction you drew in Gametes and fertilisation between a sperm reaching an egg and a sperm fusing with one. Arriving is not fusing, in an animal or in a plant.

“A tomato is a vegetable.”

In a kitchen, certainly. In biology the word fruit has a definition and it has nothing to do with sweetness: a fruit is the structure that develops from the ovary of a flower, and it contains the seeds. Cut anything open and the seeds settle it. Tomatoes, cucumbers, peppers, courgettes, pea pods, aubergines and — the one that surprises everybody — the tough case around a grain of wheat are all fruits. Carrots are roots, potatoes are underground stems, celery is a leaf stalk, and none of those has ever been near an ovary. Vegetable is a culinary word covering any plant part we happen to eat savoury; it draws no biological line at all. This is worth having straight for a reason beyond winning an argument: once you know a fruit is a converted ovary, you can look at any one and read backwards to the flower it used to be.

Mastery ladder

Not started yet.

Rungs 3 and 4 you mark yourself.

Rung 1 · Define it precisely

In a flowering plant, which of these is fertilisation?

Rung 2 · The one that catches people

Which of these is a fruit, biologically?

Rung 3 · Explain the sequence

Describe what happens from a pollen grain landing on a stigma to a fruit forming. Make clear which single step is fertilisation, and say what the ovule and the ovary each become.

Rung 4 · Take it somewhere new

A gardener pinches off every flower on a tomato plant as soon as it opens, to keep the plant tidy. Explain why they will get no tomatoes, using the words pollination, fertilisation, ovule and ovary.

Key note

After pollination, the pollen grain grows a pollen tube down through the style to an ovule. The male gamete nucleus travels down the tube and fuses with the female gamete nucleus inside the ovule: that fusion is fertilisation. The fertilised ovule becomes a seed containing an embryo plant and a food store, inside a tough coat. The ovary around it becomes the fruit. Petals, stamens and stigma have done their jobs and wither.

Going further

Flowering plants fertilise twice, with the same pollen tube. Two nuclei travel down it: one fuses with the egg nucleus and becomes the embryo, and the second fuses with other nuclei in the ovule to make a tissue called endosperm — the seed's food store. Nothing else on Earth does this, and it solves an accounting problem elegantly. A food store is expensive, so a plant that built one for every ovule would waste most of them; here the store is only started once fertilisation has actually happened, and an unfertilised ovule costs almost nothing. Then consider what that tissue is. A grain of wheat, rice, maize, barley or oats is mostly endosperm; white flour is very nearly pure endosperm with the embryo and the coat milled away. Most of the calories eaten by most people who have ever lived have been a plant's packed lunch for a seedling, redirected — and none of it would have been made at all if a pollen tube had not arrived.

Before this lesson

  • This follows on from Flowers and pollination.

Connects to

At GCSE this becomes

  • Seed structure and germination, plant hormones, and the genetics of self- and cross-fertilisation.

Where to next

Ask Mr Badmus AI

Want to check the difference between pollination and fertilisation?

Timings vary enormously between species: a pollen tube may reach the ovule in a few hours or, in some trees, after several months.

Lesson content © MrBadmusAI.

Mr. Badmus AI

KS3 Science Tutor

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