BiologyICSE Class 9 · Chapter 5

Fertilisation ICSE Class 9: From Pollen Tube to Zygote, Double Fertilisation and Triple Fusion

Updated 15 October 2026 · Fact-checked

Fertilisation is the fusion of a male gamete with the egg to form a zygote. After pollination, the pollen grain grows a pollen tube down the style into the ovule and releases two male gametes. One fuses with the egg; the other fuses with two polar nuclei (triple fusion). This is double fertilisation.

What Fertilisation is about

Pollination only brings pollen to the stigma. The male gametes must still travel to the egg, deep inside the ovule. The fusion of male and female gametes is fertilisation, and its product, the zygote, grows into the embryo of the seed.

The CISCE 2028 syllabus (Class IX, Flowering Plants (iii)) covers: the events between pollination and fertilisation leading to the formation of the zygote in the embryo sac; a brief explanation of the terms double fertilisation and triple fusion; and the definition and significance of fruit and seed.

Weightage: CISCE publishes no chapter weightage for ICSE Biology. Fertilisation is topic 2, Flowering Plants (iii), of the Class IX part of the 2028 syllabus; the Class IX paper (80 marks, 2 hours) is set and marked by the school.Source: CISCE ICSE Biology (Science Paper 3) syllabus, examination year 2028 (Class IX)

Key concepts

Germination of pollen and growth of the pollen tube

1. A compatible pollen grain lands on the stigma, which is sticky and secretes a sugary fluid.
2. The pollen grain absorbs this fluid and germinates: its inner wall pushes out through a pore as a pollen tube.
3. The pollen grain contains two cells: a tube (vegetative) cell, whose nucleus moves to the tip of the tube and guides its growth, and a generative cell, which divides to form two male gametes.
4. The pollen tube grows down through the style, digesting its way with enzymes, and reaches the ovary.
5. It enters an ovule, usually through the micropyle (a small opening in the ovule's coverings), and reaches the embryo sac.
6. The tip of the tube bursts, releasing the two male gametes into the embryo sac. The tube nucleus degenerates.

Worked example

How many male gametes does one pollen grain give, and from which cell are they formed?

Try it first, then show the solutionSolution

The generative cell divides once.
It forms two male gametes.

Answer: Two, from the generative cell.

The ovule and the embryo sac

An ovule is attached to the placenta by a stalk (funicle). It has:

Integuments: one or two protective coverings, with a small opening, the micropyle.
Nucellus: tissue inside the integuments.
Embryo sac: the female gametophyte inside the nucellus.

A typical embryo sac has 7 cells and 8 nuclei:

At the micropylar end: the egg apparatus: one egg cell (female gamete) and two synergids (which help guide the pollen tube).
At the opposite (chalazal) end: three antipodal cells.
In the middle: a large central cell with two polar nuclei, which often fuse to form the secondary nucleus (diploid).

Worked example

Name the cells of the egg apparatus and state where it lies in the embryo sac.

Try it first, then show the solutionSolution

Egg cell and two synergids.
At the micropylar end of the embryo sac.

Answer: Egg + 2 synergids, at the micropylar end.

Double fertilisation and triple fusion

Inside the embryo sac, both male gametes fuse, so two fusions take place:

1. Syngamy (true fertilisation): male gamete (n) + egg (n) → zygote (2n). The zygote develops into the embryo.
2. Triple fusion: second male gamete (n) + two polar nuclei (n + n) (or the secondary nucleus, 2n) → primary endosperm nucleus (3n). This develops into the endosperm, a food-storing tissue that nourishes the developing embryo.

Double fertilisation is the occurrence of two fusions (syngamy and triple fusion) in the same embryo sac. It is found only in flowering plants (angiosperms).

After fertilisation the synergids and antipodals degenerate.

Worked example

Why is the endosperm called triploid?

Try it first, then show the solutionSolution

It forms from the fusion of one male gamete (n) with two polar nuclei (n each).
n + n + n = 3n.

Answer: It has three sets of chromosomes (3n).

Changes after fertilisation

Before fertilisation	After fertilisation
Ovary	Fruit (ovary wall → fruit wall, pericarp)
Ovule	Seed
Integuments	Seed coat (outer testa, inner tegmen)
Micropyle	Micropyle of the seed (lets in water at germination)
Zygote	Embryo (radicle, plumule, cotyledons)
Primary endosperm nucleus	Endosperm (may be used up, as in bean, or remain, as in maize)
Funicle	Stalk of the seed; leaves a scar, the hilum
Sepals, petals, stamens, style, stigma	usually wither and fall (sepals persist in brinjal and tomato)

Worked example

What do the following become after fertilisation: (a) integuments, (b) ovary wall, (c) zygote?

Try it first, then show the solutionSolution

(a) Seed coat.
(b) Fruit wall (pericarp).
(c) Embryo.

Answer: As above.

Fruit and seed: definition and significance

Fruit: a mature (ripened) ovary, developed after fertilisation, containing one or more seeds.

Significance of fruits:

Protect the developing seeds.
Help in dispersal of seeds: fleshy fruits are eaten by animals and seeds dropped far away; dry fruits may have wings, hairs, hooks or burst open.
Provide food for humans and animals (vitamins, minerals, sugars).

Seed: a mature, fertilised ovule containing an embryo (future plant), with stored food and a protective seed coat.

Significance of seeds:

Grow into a new plant: carry the next generation and its characters.
Store food for the embryo until it can make its own.
Can stay dormant and survive bad seasons (cold, drought).
Help dispersal to new places.
Provide food (cereals, pulses), oil and fibre for humans.

Worked example

Distinguish between a fruit and a seed in one line each and give one function of each.

Try it first, then show the solutionSolution

Fruit: a ripened ovary; protects seeds and helps dispersal.
Seed: a ripened fertilised ovule containing an embryo; grows into a new plant.

Answer: As above.

Fertilisation in real life

Each concept, met in an everyday situation. Read the situation, answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Seeds per fruit and pollen needed

Suppose a fruit farmer finds that a well-pollinated melon contains about 400 seeds, but melons from flowers visited by few bees are small and lopsided with only 150 seeds.

How many pollen grains and male gametes, at least, were used for the 400-seed melon? Why are poorly pollinated melons misshapen?

Try it first, then show the solutionSolution

Each seed comes from one fertilised ovule, which needs one pollen grain (one pollen tube).
400 seeds need at least 400 pollen grains, giving 400 × 2 = 800 male gametes (one for the egg, one for triple fusion in each ovule).
Where ovules are not fertilised, that part of the fruit does not swell, so the fruit is uneven.

Answer: At least 400 pollen grains and 800 male gametes; unfertilised regions do not grow.

Memory hook: One seed, one pollen grain, two male gametes.

Coconut water and endosperm

Suppose a student drinking tender coconut water is told that the water and the soft white flesh are both 'endosperm'.

Explain how the endosperm forms and its role.

Try it first, then show the solutionSolution

After pollination, one male gamete fuses with two polar nuclei (triple fusion).
The primary endosperm nucleus (3n) develops into endosperm: liquid at first (coconut water), later solid (the white kernel).
It stores food for the developing embryo.

Answer: Endosperm from triple fusion; food store for the embryo.

Memory hook: Coconut water is a drink made for the baby plant.

Seedless fruits

Suppose a shopper wonders how seedless bananas and seedless grapes can be fruits if a fruit is a ripened ovary with seeds.

Explain using the definition of fruit and seed.

Try it first, then show the solutionSolution

A fruit is a ripened ovary; a seed is a ripened fertilised ovule.
In these varieties the ovary develops into a fruit without fertilisation (or the seeds fail to develop).
So the fruit forms, but no proper seeds.

Answer: The ovary ripens without ovules becoming seeds.

Memory hook: Ovary can ripen alone; seeds need fertilisation.

Topics in this chapter

Pollen Tube, Double Fertilisation and Changes After Fertilisation

Fertilisation: key facts

Syngamy: $male gamete (n) + egg (n) \to zygote (2n)$; Zygote → embryo.
Triple fusion: $male gamete (n) + 2 polar nuclei (n + n) \to PEN (3n)$; PEN → endosperm.
Path of pollen tube: $stigma \to style \to ovary \to micropyle \to embryo sac$; Two male gametes released.
After fertilisation: $ovary \to fruit; ovule \to seed; integuments \to seed coat$; Zygote → embryo.

Fertilisation: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Pollen tube growth (L.S. of a carpel)

Draw a labelled diagram showing the growth of the pollen tube to the ovule.

Draw

A carpel: stigma at the top, long style, swollen ovary below.
Pollen grains on the stigma; one with a tube growing down the style.
Inside the ovary, an ovule with integuments and a micropyle facing the tube.
The tube entering the micropyle and reaching the embryo sac, with two male gametes at its tip.

Label

pollen grain
stigma
style
pollen tube
male gametes
ovary
ovule
micropyle
integuments
embryo sac
egg cell
polar nuclei

Watch out: Show the tube nucleus leading and two gametes behind it.

Ovule with embryo sac

Draw the structure of an ovule showing the embryo sac.

Draw

An oval ovule on a stalk (funicle).
Two integuments around it leaving a micropyle at one end.
Nucellus inside; a central embryo sac.
Egg + 2 synergids at the micropylar end, 3 antipodals at the other end, 2 polar nuclei in the middle.

Label

funicle
integuments
micropyle
nucellus
embryo sac
egg cell
synergids
polar nuclei
antipodal cells

Watch out: Keep the egg apparatus at the micropyle end.

How the school exam asks Fertilisation

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Sequence of events	Arrange events from pollination to zygote formation; describe the path of the pollen tube.	Land, germinate, grow, enter micropyle, burst, fuse.
Terms	Explain double fertilisation and triple fusion; define fruit, seed, syngamy.	Name what fuses with what and what it forms.
Diagram	Label an L.S. of a carpel showing pollen tube growth, or an ovule with embryo sac.	Use the checklist; show both male gametes at the tip.
Post-fertilisation changes	What does each part become after fertilisation?	Use the before-after table.
Chromosome numbers	If the leaf cell has 2n = x, find the number in egg, zygote, endosperm, pollen.	Gametes n, zygote 2n, endosperm 3n.

Tricks to remember Fertilisation

Short memory aids. Say each one aloud once, then use it on a question the same day.

1 + 1 = 2, 1 + 2 = 3
Male gamete + egg = zygote (2n); male gamete + two polar nuclei = endosperm (3n).
Use it when: Double fertilisation.
Embryo sac '3-2-3'
3 at the micropyle (egg + 2 synergids), 2 polar nuclei in the middle, 3 antipodals at the far end: 7 cells, 8 nuclei.
Use it when: Structure of embryo sac.
O-F, O-S
Ovary → Fruit; Ovule → Seed.
Use it when: Post-fertilisation changes.
Pollen tube route: 'Some Students Often Miss Exams'
Stigma, Style, Ovary, Micropyle, Embryo sac.
Use it when: Path of the pollen tube.

Fertilisation: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Triple fusion product · 1 mark · MCQExam level

The product of triple fusion develops into the:
(a) embryo
(b) seed coat
(c) endosperm
(d) fruit wall

Try it first, then show the solutionSolution

Primary endosperm nucleus (3n) → endosperm.

Answer: (c)

Example 2: Double fertilisation · 1 mark · Assertion-reasonHarder exam level

Assertion (A): Double fertilisation is so called because the egg is fertilised twice.
Reason (R): Each pollen tube releases two male gametes into the embryo sac.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

The egg fuses with only one male gamete; the other fuses with the polar nuclei: A false.
R is true.

Answer: (d)

Example 3: Events in order · 2 marks · Short answerExam level

Arrange in the correct order: (i) pollen tube enters micropyle, (ii) pollen lands on stigma, (iii) male gamete fuses with egg, (iv) pollen tube grows through style, (v) tip of pollen tube bursts, (vi) pollen germinates.

Try it first, then show the solutionSolution

Land → germinate → grow through style → enter micropyle → burst → fuse.

Answer: (ii), (vi), (iv), (i), (v), (iii)

Example 4: Terms explained · 3 marks · Short answerExam level

Explain briefly (a) syngamy, (b) triple fusion, (c) double fertilisation.

Try it first, then show the solutionSolution

(a) Fusion of a male gamete with the egg to form the diploid zygote.
(b) Fusion of the second male gamete with two polar nuclei to form the triploid primary endosperm nucleus.
(c) Occurrence of both syngamy and triple fusion in the same embryo sac.

Answer: As above.

Example 5: After fertilisation · 2 marks · Short answerHarder exam level

What happens after fertilisation to (a) the ovary, (b) the ovule, (c) the petals, (d) the micropyle?

Try it first, then show the solutionSolution

(a) Becomes the fruit.
(b) Becomes the seed.
(c) Wither and fall.
(d) Remains as a pore in the seed coat.

Answer: As above.

Example 6: Pollen tube path · 4 marks · Long answerHarder exam level

Describe the events from the landing of a pollen grain on the stigma until the release of the male gametes in the embryo sac.

Try it first, then show the solutionSolution

The pollen grain absorbs the sugary fluid on the stigma and germinates, putting out a pollen tube.
The generative cell divides into two male gametes; the tube nucleus leads.
The tube grows down the style into the ovary and enters an ovule through the micropyle.
It reaches the embryo sac, its tip bursts, and the two male gametes are released.

Answer: As above.

Example 7: Chromosome numbers · 3 marks · Short answerHOTS / competency

A plant's leaf cells have 24 chromosomes. How many chromosomes are there in (a) its egg cell, (b) its male gamete, (c) the zygote, (d) the endosperm, (e) a cell of the seed coat?

Try it first, then show the solutionSolution

Leaf cell is 2n = 24, so n = 12.
(a) Egg n = 12; (b) male gamete n = 12; (c) zygote 2n = 24.
(d) Endosperm 3n = 36; (e) seed coat comes from the integuments of the parent plant: 2n = 24.

Answer: 12, 12, 24, 36, 24.

Example 8: Experiment reasoning · 3 marks · Short answerHOTS / competency

A student removes the stigma and style of a flower just after pollen has landed on it. Another student removes them two days later. Predict which flower may develop a fruit and explain.

Try it first, then show the solutionSolution

Removing them immediately cuts off the pollen tube before it reaches the ovule: no fertilisation, so no fruit (usually).
After two days the tubes may already have reached the ovules and fertilisation taken place.
So the second flower may develop a fruit.

Answer: The second flower.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 5-mark question split into 1 + 2 + 2 marks.

Inside the ovule

Suppose a diagram shows an ovule with a pollen tube entering through a small opening at one end. Inside is a sac with three cells near the opening, three cells at the far end, and two nuclei in the middle.

(i) · 1 mark
Name the opening and the sac.
Answer
Answer: Micropyle; embryo sac.
(ii) · 2 marks
Name the three cells near the opening and the function of the middle one of them.
Answer
Answer: Egg cell and two synergids; the egg is the female gamete that fuses with a male gamete.
(iii) · 2 marks
What will the two nuclei in the middle form after fertilisation, and what is the process called?
Answer
Answer: With a male gamete they form the primary endosperm nucleus (3n) → endosperm; triple fusion.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Define fruit and seed. (2 marks) (2 marks)

Step	Marks
Fruit: mature (ripened) ovary after fertilisation	1
Seed: mature fertilised ovule containing an embryo	1

Explain double fertilisation. (3 marks) (3 marks)

Step	Marks
The pollen tube releases two male gametes into the embryo sac	1
One fuses with the egg → zygote (2n) (syngamy)	1
The other fuses with two polar nuclei → primary endosperm nucleus (3n) (triple fusion)	1

Where students lose marks in Fertilisation

The slips examiners see most often, and what to write instead.

Mistake: Pollination and fertilisation confused
Students describe pollen landing as fertilisation.
Wrong working
Fertilisation is when pollen reaches the stigma.
Right working
Fertilisation is the fusion of the male gamete with the egg inside the ovule.
Do this: Pollination is delivery; fertilisation is fusion.
Usually costs: 1 mark.
Mistake: Triple fusion described as three gametes
'Triple' is read as three gametes fusing.
Wrong working
Triple fusion: two male gametes and an egg fuse.
Right working
One male gamete + two polar nuclei (not gametes) → 3n nucleus.
Do this: Three nuclei: one male gamete, two polar nuclei.
Usually costs: 1 mark.
Mistake: Ovary and ovule mixed up after fertilisation
The words look alike.
Wrong working
Ovule becomes the fruit.
Right working
Ovary → fruit; ovule → seed.
Do this: Ovary is the larger box; ovules are inside it.
Usually costs: 1 mark.
Mistake: Pollen tube path missing the micropyle
Answers jump from style to egg.
Wrong working
The pollen tube grows through the style and fertilises the egg.
Right working
Through the style into the ovary, into an ovule through the micropyle, to the embryo sac where it bursts.
Do this: Name each stage of the path.
Usually costs: 1 mark.

Practice questions on Fertilisation

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQExam level
The seed coat develops from the:
(a) ovary wall
(b) integuments
(c) nucellus
(d) endosperm
Hint
Coverings of the ovule.
Answer
(b)
Q2 · 1 mark · MCQExam level
The number of nuclei taking part in triple fusion is:
(a) 2
(b) 3
(c) 4
(d) 5
Hint
One male gamete + two polar nuclei.
Answer
(b)
Q3 · 3 marks · Very short answerExam level
Name: (a) the cell of the pollen grain that forms male gametes, (b) the cells beside the egg, (c) the scar on a seed where it was attached.
Hint
Generative; synergids; hilum.
Answer
(a) Generative cell (b) Synergids (c) Hilum
Q4 · 2 marks · Short answerExam level
Give two points of significance of seeds.
Hint
New plant; survival.
Answer
They grow into new plants (next generation); they can stay dormant through unfavourable seasons (also store food, help dispersal, feed humans).
Q5 · 3 marks · Short answerHarder exam level
What is the role of (a) the sugary fluid on the stigma, (b) the tube nucleus, (c) the endosperm?
Hint
Germinate; guide; feed.
Answer
(a) Helps the pollen grain germinate. (b) Guides the growth of the pollen tube. (c) Stores food that nourishes the developing embryo.
Q6 · 2 marks · Short answerHarder exam level
Why is the zygote diploid but the endosperm triploid?
Hint
Count the sets.
Answer
Zygote = male gamete (n) + egg (n) = 2n. Endosperm nucleus = male gamete (n) + two polar nuclei (n + n) = 3n.
Q7 · 2 marks · Short answerHarder exam level
Give two differences between a fruit and a seed.
Hint
Origin; contents.
Answer
Fruit develops from the ovary, seed from the ovule. A fruit contains seeds and has a pericarp; a seed contains an embryo and has a seed coat with hilum and micropyle.
Q8 · 3 marks · Short answerHarder exam level
A flower is pollinated, but the pollen tube is blocked in the style. What will happen to (a) the ovary, (b) the ovules, (c) the petals? Give reasons.
Hint
No fertilisation.
Answer
(a) It usually does not develop into a fruit and drops off. (b) They are not fertilised and do not become seeds. (c) They wither anyway, as the flower ages.
Q9 · 3 marks · Short answerHarder exam level
Explain why double fertilisation is said to be unique to flowering plants and how it differs from fertilisation in animals.
Hint
Two fusions.
Answer
Only angiosperms have two fusions in one embryo sac: male gamete + egg (zygote) and male gamete + polar nuclei (endosperm). In animals, one sperm fuses with one egg to form a zygote; there is no second fusion and no endosperm.
Q10 · 3 marks · Short answerHOTS / competency
A pea pod has 6 seeds and 2 shrivelled ovules. (a) How many pollen grains at least reached the stigma and grew tubes successfully? (b) How many male gametes were used? (c) Suggest why 2 ovules shrivelled.
Hint
One tube per seed.
Answer
(a) At least 6. (b) 12 (two per fertilised ovule). (c) No pollen tube reached them, so they were not fertilised and did not develop.
Q11 · 3 marks · Short answerHOTS / competency
A plant has 2n = 14 in its root cells. Give the chromosome numbers of (a) the pollen's male gamete, (b) the polar nuclei together, (c) the endosperm, (d) the embryo, (e) the fruit wall.
Hint
n = 7.
Answer
(a) 7 (b) 14 (7 + 7) (c) 21 (d) 14 (e) 14 (it is ovary wall tissue of the parent).

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.How many male gametes does a pollen tube carry?Show answer
Two.
2.Through which opening does the pollen tube usually enter the ovule?Show answer
Micropyle.
3.Product of syngamy?Show answer
Zygote (2n).
4.Ploidy of endosperm?Show answer
Triploid (3n).
5.Cells of the egg apparatus?Show answer
Egg cell and two synergids.
6.What does the ovary become?Show answer
Fruit.
7.What do the integuments become?Show answer
Seed coat.
8.In which plants does double fertilisation occur?Show answer
Flowering plants (angiosperms).

Best order to study it

Learn the events from pollen landing to the bursting of the tube.
Learn the parts of an ovule and the 7-celled embryo sac.
Learn syngamy, triple fusion and double fertilisation with ploidy.
Learn the before-after table of post-fertilisation changes.
Learn definitions and significance of fruit and seed; practise the two diagrams.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you list the events between pollination and fertilisation in order?
Can you draw and label the embryo sac?
Can you explain double fertilisation and triple fusion in three lines?
Can you say what each floral part becomes after fertilisation?
Can you give two points of significance each for fruit and seed?

Fertilisation: frequently asked questions

Is a pollen grain itself a male gamete?

No. A pollen grain is a structure that produces and carries male gametes. Its generative cell divides to form two male gametes.

Why is double fertilisation useful to plants?

Endosperm (food) forms only when an ovule is fertilised, so the plant does not waste food on ovules that will not become seeds.

Do all seeds keep their endosperm?

No. In bean and pea the cotyledons absorb the endosperm as the seed matures (non-endospermic). In maize and castor the endosperm remains in the mature seed (endospermic).

How much detail is needed on the embryo sac?

The syllabus asks for events leading to the formation of the zygote in the embryo sac and a brief explanation of double fertilisation and triple fusion. Know the egg, synergids, polar nuclei and antipodals and what fuses with what.

Based on: CISCE ICSE 2028 Science (52) Biology (Science Paper 3) syllabus, Classes IX and X