BiologyISC Class 12 · Sexual Reproduction in Flowering Plants

Double Fertilisation and Post-fertilisation Events: ISC Class 12 Biology

Updated 15 October 2026 · Fact-checked

In double fertilisation one male gamete fuses with the egg to form a diploid zygote (syngamy) and the other fuses with the two polar nuclei to form a triploid primary endosperm nucleus (triple fusion). Then endosperm and embryo develop, ovules become seeds and the ovary becomes the fruit.

Understand Double Fertilisation and Post-fertilisation Events

Events leading to fertilisation. After compatible pollination the pollen grain germinates on the stigma and the pollen tube grows through the style into the ovary. If pollen was shed two-celled, the generative cell divides in the tube to give two male gametes. The tube enters the ovule through the micropyle and then enters one of the two synergids through the filiform apparatus, the thickened micropylar tip of the synergid that guides it. The tube releases both male gametes into the synergid's cytoplasm.

The two fusions.

Syngamy: one male gamete (n) fuses with the egg (n) → zygote (2n) → embryo.
Triple fusion: the other male gamete (n) fuses with the two polar nuclei (n + n) of the central cell, or with the secondary nucleus they form → primary endosperm nucleus, PEN (3n). The central cell becomes the primary endosperm cell and gives the endosperm.
Two fusions in one embryo sac = double fertilisation, unique to angiosperms (first described by Nawaschin).

Significance of double fertilisation. Endosperm forms only in ovules that are actually fertilised, so the plant does not spend food on ovules that will not become seeds; the endosperm feeds the embryo; and the endosperm carries genes of both parents.

Ploidy rule for crosses. Egg and polar nuclei come from the female parent's embryo sac; male gametes from the male parent. Each gamete carries half the parent's somatic set. Zygote = egg + male gamete; endosperm = 2 × polar nucleus + male gamete. A 4x female × 2x male gives zygote 3x and endosperm 5x.

Post-fertilisation events: endosperm and embryo development, ovule → seed and ovary → fruit.

Endosperm develops *before* the embryo, so that food is ready when the zygote starts dividing. Three types (ISC):
1. Nuclear: the PEN divides repeatedly without wall formation, giving many free nuclei; walls form later, usually from the periphery inwards. Commonest type, e.g. in polypetalous dicots. Coconut water is free-nuclear endosperm; the white kernel is cellular endosperm.
2. Cellular: every nuclear division is followed by wall formation from the start.
3. Helobial: the first division gives a large micropylar chamber and a small chalazal chamber; the micropylar chamber becomes free-nuclear (later cellular), the chalazal one stays small. Found mainly in some monocots (the name comes from the order Helobiales).

Embryo. The zygote lies at the micropylar end and usually divides only after some endosperm has formed. Stages: proembryo → globular → heart-shaped → mature. *Dicot (bean)*: embryonal axis with two cotyledons; above them the epicotyl ending in the plumule, below them the hypocotyl ending in the radicle with a root cap. *Monocot (maize, grass)*: one lateral cotyledon, the scutellum; the plumule is enclosed in the coleoptile, the radicle and root cap in the coleorhiza.

Seed (fertilised ovule). Integuments → seed coat (testa outer, tegmen inner); micropyle stays as a pore for water and oxygen at germination; hilum is the scar of the funicle. Water falls to about 10-15% and the embryo may become dormant.

Albuminous (endospermic): some endosperm remains (wheat, maize, barley, castor, coconut).
Non-albuminous (ex-albuminous): endosperm used up; food stored in cotyledons (pea, groundnut, bean).
Perisperm: remnant of the nucellus persisting in the seed (black pepper, beet). Do not confuse with pericarp, the fruit wall.

Fruit. The ovary wall becomes the pericarp: epicarp, mesocarp, endocarp. True fruit: from the ovary alone (mango, coconut). False fruit: other floral parts, mainly the thalamus, also form the fruit (apple, strawberry, cashew). *Mango (drupe)*: thin epicarp (skin), fleshy edible mesocarp, hard stony endocarp, one seed. *Coconut*: smooth epicarp, fibrous mesocarp (coir), hard endocarp (shell), seed with cellular endosperm (kernel) and liquid endosperm.

Significance. Fruits protect seeds and help their dispersal; seeds store food for the seedling, survive unfavourable periods (dormancy), carry new gene combinations and allow colonisation of new places.

Special modes (briefly). Parthenocarpy: fruit without fertilisation, seedless (banana; can be induced by growth hormones). Apomixis: seed without fertilisation (some grasses and Asteraceae), e.g. from a diploid egg formed without meiosis; offspring are clones. Polyembryony: more than one embryo in a seed, as in Citrus and mango, where nucellar cells also form embryos alongside the zygotic one.

Draw and label: fertilised embryo sac. Checklist: oval embryo sac inside nucellus and integuments; micropyle at one end, chalaza at the other; pollen tube entering through the micropyle into a degenerating synergid; zygote (2n) at the micropylar end; PEN (3n) in the central cell; antipodals at the chalazal end. L.S. of a monocot (maize) seed: pericarp fused with seed coat, endosperm, aleurone layer, scutellum, coleoptile, plumule, radicle, coleorhiza.

Double Fertilisation and Post-fertilisation Events: key facts

Syngamy and triple fusion: $egg (n) + male gamete (n) \to zygote (2 n); 2 polar nuclei (n + n) + male gamete (n) \to PEN (3 n)$; Two fusions in one embryo sac: double fertilisation.
Ploidy in a cross: $zygote = \frac{F}{2} + \frac{M}{2}; endosperm = 2 \cdot \frac{F}{2} + \frac{M}{2}$; F and M are the somatic ploidy of the female and male parents.
What becomes what: $ovary \to fruit; ovary wall \to pericarp; ovule \to seed; integuments \to seed coat; nucellus \to perisperm (if it persists)$; The zygote becomes the embryo; the primary endosperm cell becomes the endosperm.
Endosperm types: $nuclear (free nuclei first) ∣ cellular (walls at once) ∣ helobial (two chambers)$; Coconut water = free-nuclear; kernel = cellular.

How to answer Double Fertilisation and Post-fertilisation Events questions

ISC asks the sequence of fertilisation, identification of endosperm types or fruit parts from diagrams, and ploidy or chromosome counts. Keep the parts of the embryo sac and the parents straight.

1For 'explain fertilisation', write the events in order: germination, tube growth, entry by micropyle, entry into synergid, release of two male gametes, syngamy, triple fusion, products, the term double fertilisation.
2For ploidy questions, first write the ploidy of each gamete: female egg and polar nuclei from the female parent, male gametes from the male parent; then add.
3For chromosome counts, find n first (n = 2n ÷ 2, or endosperm ÷ 3); everything from the embryo sac or pollen is n, the zygote and all maternal tissues (nucellus, integuments, pericarp) are 2n, endosperm 3n.
4For diagrams of fruits, use epicarp, mesocarp, endocarp, not skin, flesh, stone.
5For seed comparisons, say where the food is stored: endosperm (albuminous) or cotyledons (non-albuminous).
6For special modes, link fruit without fertilisation to parthenocarpy and seed without fertilisation to apomixis.

Double Fertilisation and Post-fertilisation Events in real life

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

Free-nuclear and cellular endosperm in coconut

Suppose a street vendor offers two coconuts: a young green one full of sweet water with only a thin jelly-like lining, and an older brown one with less water and a thick white kernel.

Explain the difference between the two using endosperm development.

Try it first, then show the solutionSolution

Coconut endosperm is of the nuclear type: the primary endosperm nucleus first divides many times without forming walls.
In the young fruit most of the endosperm is still free-nuclear: thousands of nuclei suspended in fluid, which is the coconut water.
As the fruit ages, cell walls form from the periphery inwards, so a layer of cellular endosperm (the white kernel) thickens along the shell.
The older nut therefore has a thicker kernel and less liquid.

Answer: Both are endosperm: the water is free-nuclear endosperm and the kernel is cellular endosperm, which thickens as walls form with age.

Memory hook: Tender coconut drinks the nuclei; old coconut chews the cells.

Tricks to remember Double Fertilisation and Post-fertilisation Events

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

2 + 3 = double
Syngamy makes 2n, triple fusion makes 3n: two fusions, two products, one embryo sac.
Use it when: Writing the products of double fertilisation.
N-C-H endosperms
Nuclear = Nuclei free first; Cellular = Cells at once; Helobial = Halves (two chambers).
Use it when: Identifying endosperm types from diagrams.
Peri-SPERM stays in the seed, peri-CARP wraps the fruit
Sperm (seed) versus carp (karpos, fruit).
Use it when: Perisperm versus pericarp questions.

Double Fertilisation and Post-fertilisation Events: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Triple fusion · 2 marks · Short answerBoard level

What is triple fusion? Where does it occur and what does it produce?

Try it first, then show the solutionSolution

Fusion of one male gamete with the two polar nuclei (or secondary nucleus) of the central cell (1 mark).
It occurs in the central cell of the embryo sac and gives the triploid primary endosperm nucleus, which forms the endosperm (1 mark).

Answer: Male gamete + two polar nuclei in the central cell → triploid PEN.

Example 2: Three endosperm types · 3 marks · Short answerBoard level

Distinguish between nuclear, cellular and helobial endosperm. Which type makes up the liquid in a tender coconut?

Try it first, then show the solutionSolution

Nuclear: PEN divides repeatedly without wall formation, giving free nuclei; walls form later (1 mark).
Cellular: each division of the PEN is followed by wall formation (½ mark). Helobial: the first division forms a large micropylar and a small chalazal chamber; the micropylar chamber develops in the free-nuclear way (1 mark).
Coconut water is free-nuclear endosperm of the nuclear type (½ mark).

Answer: Nuclear: free nuclei first; cellular: walls at once; helobial: two chambers. Coconut water is free-nuclear endosperm.

Example 3: Ploidy in a hexaploid × tetraploid cross · 3 marks · Short answerHarder board level

Suppose flowers of a hexaploid (6x) plant receive pollen from a compatible tetraploid (4x) plant and set seed. Work out the ploidy of (a) the egg, (b) a male gamete, (c) the zygote, (d) the endosperm, (e) the seed coat.

Try it first, then show the solutionSolution

Gametes carry half the somatic set: egg and each polar nucleus 3x; male gamete 2x (1 mark).
Zygote = 3x + 2x = 5x; endosperm = 3x + 3x + 2x = 8x (1 mark).
Seed coat develops from the integuments, maternal tissue: 6x (1 mark).

Answer: (a) 3x (b) 2x (c) 5x (d) 8x (e) 6x.

Example 4: Fertilisation step by step · 5 marks · Long answerHarder board level

Describe the events from the arrival of the pollen tube in the ovary to the formation of the zygote and primary endosperm nucleus. Give the exact location and role of the filiform apparatus.

Try it first, then show the solutionSolution

The pollen tube, carrying two male gametes (formed from the generative cell if pollen was two-celled), reaches the ovule and enters through the micropyle (1 mark).
It enters one synergid, guided by the filiform apparatus, which lies at the micropylar tip of each synergid in the egg apparatus (1 mark).
The tube releases both male gametes into the synergid cytoplasm (½ mark).
One male gamete fuses with the egg nucleus: syngamy, giving the diploid zygote (1 mark).
The other fuses with the two polar nuclei in the central cell: triple fusion, giving the triploid PEN (1 mark).
Two fusions in one embryo sac: double fertilisation (½ mark).

Answer: Tube → micropyle → synergid (guided by filiform apparatus at its micropylar tip) → two male gametes released → syngamy (2n zygote) and triple fusion (3n PEN).

Example 5: Seeds on the dissecting table · 3 marks · Short answerHarder board level

A student soaks seeds of pea, castor and black pepper. (a) Which is non-albuminous? (b) In which is food stored outside the embryo in a tissue derived from the nucellus? (c) Name the tissue that stores food in castor and state its ploidy.

Try it first, then show the solutionSolution

(a) Pea: endosperm is used up and food lies in the cotyledons (1 mark).
(b) Black pepper: residual nucellus persists as perisperm (1 mark).
(c) Castor: endosperm, triploid (3n) (1 mark).

Answer: (a) Pea (b) black pepper (perisperm) (c) endosperm, 3n.

Example 6: Why endosperm first? · 3 marks · Short answerHOTS / competency

In most angiosperms the zygote remains undivided for a while after fertilisation, and the endosperm forms only after double fertilisation. Explain the advantage of each of these features to the plant.

Try it first, then show the solutionSolution

The zygote waits until some endosperm has formed, so the young embryo has an assured food supply from its first divisions (1 mark).
The endosperm forms from the primary endosperm cell only after triple fusion, so food tissue is built only in ovules that have actually been fertilised (1 mark).
Unfertilised ovules therefore waste no reserves; resources go only to seeds that will carry an embryo (1 mark).

Answer: Food is ready before the embryo needs it, and no food is wasted on ovules that are not fertilised.

Where students lose marks in Double Fertilisation and Post-fertilisation Events

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

Mistake: Using everyday words for fruit layers
In 2024 examiners noted labels such as skin, flesh and hard cover instead of epicarp, mesocarp and endocarp.
Wrong working
1: skin, 2: flesh, 3: hard cover.
Right working
1: epicarp, 2: mesocarp, 3: endocarp.
Do this: Learn the pericarp layers with the mango and coconut L.S.
Usually costs: 1 mark per label.
Mistake: Writing triploid by habit in a ploidy question
In 2025 many candidates wrote 3n for the endosperm of a tetraploid × diploid cross without working from the parents.
Wrong working
Endosperm is always 3n.
Right working
Endosperm = 2 × (female gamete ploidy) + male gamete ploidy; 4x × 2x gives 2x + 2x + 1x = 5x.
Do this: Write the ploidy of each gamete before adding.
Usually costs: The whole 1-mark item.
Mistake: Vague location of the filiform apparatus
Examiners (2024) received answers such as 'in the female part' or 'in the ovary'.
Do this: Write: at the micropylar tip of the synergids in the egg apparatus; it guides the pollen tube into the synergid.
Usually costs: 1 mark.
Mistake: Giving parthenocarpy for seeds without fertilisation
In 2024 many candidates wrote parthenocarpy for both fruit and seed formation without fertilisation.
Do this: Fruit without fertilisation: parthenocarpy. Seed without fertilisation: apomixis.
Usually costs: 1 mark.
Mistake: Misspelling helobial and nuclear
Examiners (2024) saw 'hellobial' and 'nucellar' for the endosperm types.
Do this: Nuclear (free nuclei), not nucellar (nucellus); helobial with one l.
Usually costs: ½ to 1 mark.

Exam tips

In a 5-mark fertilisation answer, name both fusions, both products with ploidy, and the term double fertilisation.
Central cell before fertilisation: one cell with two polar nuclei; mature embryo sac: 7 cells, 8 nuclei.
Nuclear endosperm is the commonest type and is found in polypetalous dicots.
In seed diagrams of maize, label scutellum, coleoptile, coleorhiza, endosperm and aleurone layer.
For significance questions, write what the structure does for the plant (protection, dispersal, nutrition), not how it forms.

Practice questions on Double Fertilisation and Post-fertilisation Events

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 · MCQBoard level
How many male gametes take part in double fertilisation in one embryo sac, and how many nuclei fuse in all?
(a) 1 and 3 (b) 2 and 5 (c) 2 and 4 (d) 2 and 3
Hint
Syngamy: 2 nuclei; triple fusion: 3 nuclei.
Answer
(b) 2 male gametes; 2 + 3 = 5 nuclei fuse.
Q2 · 2 marks · Short answerBoard level
Differentiate between a true fruit and a false fruit with one example of each.
Hint
Which floral parts form the fruit?
Answer
A true fruit develops from the ovary alone (mango, coconut). A false fruit develops from the ovary together with other floral parts, mainly the thalamus (apple, strawberry, cashew).
Q3 · 2 marks · Short answerHarder board level
A plant has 2n = 20. Give the number of chromosomes in (a) a synergid, (b) the primary endosperm nucleus, (c) a nucellus cell, (d) a cell of the pericarp, (e) the zygote.
Hint
n = 10; maternal tissues are 2n.
Answer
(a) 10 (b) 30 (c) 20 (d) 20 (e) 20.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): The white kernel and the water of a coconut are both endosperm.
Reason (R): In nuclear endosperm, free nuclear divisions are followed later by wall formation.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Liquid part and solid part of the same tissue.
Answer
(a). The water is the free-nuclear stage and the kernel the cellular stage of the same nuclear endosperm.
Q5 · 2 marks · Short answerHarder board level
Distinguish between (a) coleoptile and coleorhiza, (b) perisperm and pericarp.
Hint
Sheaths of a monocot embryo; seed tissue versus fruit wall.
Answer
(a) Coleoptile: hollow sheath enclosing the plumule (shoot apex and leaf primordia) of a monocot embryo; coleorhiza: sheath enclosing the radicle and root cap. (b) Perisperm: persistent remnant of the nucellus inside some seeds (black pepper, beet); pericarp: the wall of the fruit, formed from the ovary wall.
Q6 · 3 marks · Short answerHarder board level
Distinguish between apomixis and polyembryony (any two points), and name the process that gives seedless fruits without fertilisation.
Hint
One is about seeds without fertilisation, the other about the number of embryos.
Answer
Apomixis: seed formed without fertilisation; embryo genetically identical to the mother; usually one embryo. Polyembryony: more than one embryo in a seed; one may be zygotic (from fertilisation) and others nucellar, so the seedlings can differ. Seedless fruit without fertilisation: parthenocarpy (banana).
Q7 · 3 marks · Short answerHOTS / competency
Suppose the endosperm cells of a plant are found to have 27 chromosomes. (a) What is the diploid number of the plant? (b) How many chromosomes are in its pollen mother cell and in its male gamete? (c) If the plant were pollinated by a tetraploid relative with the same basic set, what would the endosperm chromosome number be?
Hint
Endosperm is 3n; a tetraploid's gamete is 2n.
Answer
(a) n = 27 ÷ 3 = 9, so 2n = 18. (b) Pollen mother cell 18; male gamete 9. (c) Polar nuclei 9 + 9 plus a male gamete of 18 (2n of the basic set) = 36.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose a fruit seller cuts a mango and a coconut lengthwise for a class.
(a) Name the layer of the mango that we eat and the layer of the coconut that gives coir. (1)
(b) Which layer forms the stone of a mango and the shell of a coconut? (1)
(c) Give two reasons why fruit formation is significant to the plant. (1)
Hint
Use the pericarp layers.
Answer
(a) Mango: fleshy mesocarp; coconut: fibrous mesocarp. (b) Endocarp in both. (c) The fruit protects the developing seeds and helps their dispersal (by animals that eat it, by floating for coconut); it may also supply nutrition.

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.What is formed by syngamy?Show answer
The diploid zygote.
2.What guides the pollen tube into the synergid?Show answer
The filiform apparatus.
3.Name the cotyledon of a grass embryo.Show answer
Scutellum.
4.Give two examples of albuminous seeds.Show answer
Wheat, maize, barley, castor or coconut (any two).
5.What is perisperm?Show answer
Remnant nucellus that persists in some seeds, such as black pepper and beet.
6.Which endosperm type has a micropylar and a chalazal chamber?Show answer
Helobial.

Double Fertilisation and Post-fertilisation Events: frequently asked questions

Why is it called double fertilisation?

Because two fusions happen in one embryo sac: syngamy (male gamete + egg) and triple fusion (male gamete + two polar nuclei).

Why is endosperm triploid?

It comes from the primary endosperm nucleus, formed by fusion of one haploid male gamete with two haploid polar nuclei.

Is coconut water endosperm?

Yes. It is free-nuclear endosperm; the white kernel around it is cellular endosperm.

What is the difference between albuminous and non-albuminous seeds?

Albuminous seeds keep part of the endosperm at maturity (wheat, castor); non-albuminous seeds use it up and store food in the cotyledons (pea, groundnut).

Why are apple and strawberry called false fruits?

Because the thalamus, not only the ovary, contributes to the fruit.