BiologyCBSE Class 12 · Sexual Reproduction in Flowering Plants

Double Fertilisation and Post-fertilisation Events

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). Afterwards the endosperm forms first, the zygote becomes the embryo, ovules become seeds and the ovary becomes the fruit.

Understand Double Fertilisation and Post-fertilisation Events

Two fusions in one embryo sac. The pollen tube enters one of the synergids and releases two male gametes into its cytoplasm.

One male gamete moves to the egg cell and fuses with its nucleus: syngamy. Result: a diploid zygote (2n).
The other moves to the two polar nuclei in the central cell and fuses with them: triple fusion (three haploid nuclei). Result: a triploid primary endosperm nucleus (PEN, 3n); the central cell becomes the primary endosperm cell (PEC).

Because two kinds of fusion (syngamy and triple fusion) happen in one embryo sac, the event is called double fertilisation, a feature unique to flowering plants. The zygote develops into the embryo; the PEC develops into the endosperm.

Flow chart:
male gamete 1 (n) + egg (n) → zygote (2n) → embryo
male gamete 2 (n) + 2 polar nuclei (n + n) → PEN (3n) → endosperm (3n)

Post-fertilisation events are endosperm development, embryo development, maturation of ovules into seeds and of the ovary into a fruit.

Endosperm comes first. The PEC divides repeatedly to form triploid endosperm tissue filled with reserve food, which nourishes the developing embryo. In the most common type, the PEN undergoes successive nuclear divisions *without* cell walls, giving free nuclei (free-nuclear endosperm); walls form later and the endosperm becomes cellular. The number of free nuclei before walls form varies a lot. In a tender coconut, the coconut water is free-nuclear endosperm (thousands of nuclei) and the surrounding white kernel is cellular endosperm. The endosperm may be used up completely by the embryo before the seed matures (pea, groundnut, beans) or may persist in the mature seed (castor, coconut) and be used during germination.

Embryo development. The embryo develops at the micropylar end of the embryo sac, where the zygote lies. Most zygotes divide only after some endosperm has formed: an adaptation that assures nutrition to the young embryo. Early stages are similar in monocots and dicots:
zygote → proembryo → globular embryo → heart-shaped embryo → mature embryo.

Dicot embryo: an embryonal axis and two cotyledons. Above the level of the cotyledons is the epicotyl, ending in the plumule (stem tip). Below is the hypocotyl, ending in the radicle (root tip), covered by a root cap.
Monocot (grass) embryo: one cotyledon, called the scutellum, attached to one side of the embryonal axis. The radicle and root cap are enclosed in a sheath, the coleorrhiza. Above the scutellum, the epicotyl carries the shoot apex and a few leaf primordia, enclosed in a hollow foliar sheath, the coleoptile.

Seed. The fertilised ovule is the seed: seed coat(s), cotyledon(s) and embryonal axis. Cotyledons are often thick and swollen with stored food (legumes).

Non-albuminous (ex-albuminous) seeds have no residual endosperm: pea, groundnut.
Albuminous seeds keep part of the endosperm: wheat, maize, barley, castor.
In some seeds (black pepper, beet) remnants of the nucellus persist as perisperm.

The integuments harden into seed coats; the micropyle stays as a small pore through which oxygen and water enter during germination. As the seed matures, its water content falls to about 10-15 per cent of its mass, metabolism slows and the embryo may enter dormancy; it germinates when moisture, oxygen and a suitable temperature are available.

Fruit. The ovary becomes the fruit; its wall becomes the pericarp. Fruits may be fleshy (guava, orange, mango) or dry (groundnut, mustard). In apple, strawberry and cashew the thalamus also contributes: these are false fruits; fruits from the ovary alone are true fruits. Some fruits, such as banana, develop without fertilisation: parthenocarpic (seedless) fruits, which can also be induced with growth hormones.

Seed facts examiners like. Seeds can stay viable for a very long time: a seed of the lupine *Lupinus arcticus* from the Arctic tundra germinated after an estimated 10,000 years, and a roughly 2000-year-old date palm seed (*Phoenix dactylifera*) from near the Dead Sea also germinated. Seeds allow dispersal to new habitats, carry stored food for the seedling, and, being products of sexual reproduction, bring new genetic combinations.

Draw and label 1: stages of a dicot embryo. Checklist: four sketches left to right: zygote at the micropylar end; globular proembryo with a suspensor; heart-shaped embryo with two cotyledon lobes; mature embryo. Labels: suspensor, radicle, hypocotyl, cotyledons, plumule.

Draw and label 2: L.S. of a grass (monocot) embryo. Checklist: axis with a large shield-shaped cotyledon on one side. Labels: scutellum, coleoptile, shoot apex (plumule), epicotyl, radicle, root cap, coleorrhiza.

Double Fertilisation and Post-fertilisation Events: key facts

Syngamy: $male gamete (n) + egg (n) \to zygote (2 n)$; The zygote develops into the embryo.
Triple fusion: $male gamete (n) + 2 polar nuclei (n + n) \to PEN (3 n)$; PEN develops into the triploid endosperm.
Ploidy in a seed and fruit: $embryo 2 n, endosperm 3 n, seed coat 2 n, pericarp 2 n, perisperm 2 n$; Seed coat, perisperm and pericarp are maternal tissues (integuments, nucellus, ovary wall).
Gametes per seed: $1 seed = 1 ovule + 1 pollen grain = 2 male gametes$; Both male gametes of one pollen tube are used: one in syngamy, one in triple fusion.
Which part becomes what: $ovule \to seed; integuments \to seed coats; ovary \to fruit; ovary wall \to pericarp$; Thalamus also forms part of false fruits (apple, strawberry, cashew).

How to answer Double Fertilisation and Post-fertilisation Events questions

Questions here are about naming the fusion, giving ploidy, sequencing development, or identifying seed and embryo parts.

1For 'explain double fertilisation', write both fusions with their products and ploidy, then the reason for the name (two kinds of fusion in one embryo sac).
2For chromosome counts, find n from the given cell: leaf or root cells are 2n. Then embryo/zygote = 2n, endosperm = 3n, male gamete = n.
3For endosperm questions, state the type (free-nuclear first, cellular later), the coconut example and whether it is consumed or persists.
4For embryo stages, give the sequence zygote → proembryo → globular → heart-shaped → mature, and say it happens at the micropylar end.
5For dicot vs monocot embryo, compare in pairs: two cotyledons vs scutellum; no sheaths vs coleoptile and coleorrhiza.
6For 'differentiate' items (perisperm vs pericarp, albuminous vs non-albuminous), give the origin and one example of each.

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.

Ploidy and origin of tissues in a seed

Suppose a student opens a tender coconut and a maize grain on the same day for a project, and her teacher tells her that the coconut palm's leaf cells have 32 chromosomes.

Which parts of the coconut are free-nuclear and cellular endosperm, how many chromosomes do their nuclei have, and why does a maize grain keep some endosperm while a pea seed does not?

Try it first, then show the solutionSolution

Coconut water is free-nuclear endosperm (nuclei without cell walls); the white kernel is cellular endosperm.
Leaf cells are 2n = 32, so n = 16; endosperm is 3n = 48 chromosomes.
Maize is albuminous: part of the endosperm persists in the mature grain as food for germination; pea is non-albuminous: the embryo uses up the endosperm before the seed matures and food is stored in the cotyledons.

Answer: Water = free-nuclear, kernel = cellular endosperm; both 48 chromosomes (3n). Maize retains endosperm (albuminous); pea stores food in cotyledons (non-albuminous).

Memory hook: Coconut: water first, walls later; both are 3n.

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.

1 + 1 and 1 + 2
Syngamy fuses 1 + 1 nuclei (2n); triple fusion fuses 1 + 2 nuclei (3n).
Use it when: Getting ploidy right instantly.
Food before family
Endosperm (food) forms before the embryo (child) starts dividing.
Use it when: Justifying why endosperm development precedes embryo development.
C-ROOT, C-SHOOT
ColeoRhiza covers the Root; ColeoPtile protects the shoot (Plumule).
Use it when: Labelling a grass embryo.

Double Fertilisation and Post-fertilisation Events: solved examples

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

Example 1: Naming the fusions · 2 marks · Very short answerBoard level

What is triple fusion? Where does it occur and what is its product?

Try it first, then show the solutionSolution

Triple fusion is the fusion of one male gamete with the two polar nuclei, i.e. three haploid nuclei (1 mark).
It occurs in the central cell of the embryo sac; the product is 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: Chromosomes in maize tissues · 3 marks · Short answerHarder board level

The root tip cells of a plant contain 14 chromosomes. Write the number of chromosomes in its (a) zygote (b) endosperm (c) male gamete. Name and explain the process that forms (a) and (b). (A question of this kind was asked in 2024.)

Try it first, then show the solutionSolution

Root cells are 2n = 14, so n = 7: (a) zygote 14, (b) endosperm 21, (c) male gamete 7 (1 mark).
Process: double fertilisation. The pollen tube releases two male gametes into a synergid (1 mark).
One fuses with the egg (syngamy) to give the 2n zygote; the other fuses with the two polar nuclei (triple fusion) to give the 3n primary endosperm nucleus (1 mark).

Answer: (a) 14 (b) 21 (c) 7; formed by double fertilisation: syngamy and triple fusion.

Example 3: Endosperm before embryo · 3 marks · Short answerHarder board level

Endosperm development precedes embryo development. Justify, and describe the most common way the endosperm develops using coconut as an example.

Try it first, then show the solutionSolution

The zygote usually divides only after some endosperm has formed; the endosperm stores food, so the young embryo is assured of nutrition (1 mark).
In free-nuclear development, the PEN divides repeatedly without wall formation to give many free nuclei; later cell walls form and the endosperm becomes cellular (1 mark).
In a tender coconut, the coconut water is free-nuclear endosperm and the white kernel around it is cellular endosperm (1 mark).

Answer: Endosperm first to assure nutrition; free-nuclear then cellular; coconut water and kernel.

Example 4: Grass embryo parts · 2 marks · Very short answerBoard level

Give the function or identity of: (a) scutellum (b) coleoptile (c) coleorrhiza (d) tassels of a corn cob.

Try it first, then show the solutionSolution

(a) Scutellum: the single cotyledon of the grass family, on one side of the embryonal axis. (b) Coleoptile: hollow foliar sheath enclosing the shoot apex and leaf primordia (1 mark).
(c) Coleorrhiza: undifferentiated sheath enclosing the radicle and root cap. (d) Tassels: the stigmas and styles that trap wind-borne pollen (1 mark).

Answer: Scutellum = cotyledon; coleoptile covers the shoot; coleorrhiza covers the root; tassels = stigma and style.

Example 5: Who owns the endosperm genes · 5 marks · Long answerHOTS / competency

A plant with genotype aa (female parent) is pollinated by a plant with genotype AA. Write the genotypes of (a) the egg (b) each polar nucleus (c) the male gametes (d) the embryo (e) the endosperm (f) the seed coat. Explain why the endosperm and the seed coat differ in their parental contribution.

Try it first, then show the solutionSolution

Female gametophyte comes from the aa plant: egg a, each polar nucleus a (1 mark).
Male gametes come from the AA pollen: A and A (1 mark).
Embryo = egg + male gamete = Aa (1 mark).
Endosperm = two polar nuclei + one male gamete = aaA: two maternal doses and one paternal dose (1 mark).
Seed coat develops from the integuments of the ovule, which are tissues of the mother plant, so it is aa, purely maternal; the endosperm is a product of fertilisation, so it carries paternal genes too (1 mark).

Answer: (a) a (b) a, a (c) A, A (d) Aa (e) aaA (f) aa. Endosperm is a fertilisation product (2 maternal + 1 paternal); seed coat is maternal tissue.

Example 6: Perisperm and pericarp · 3 marks · Short answerHarder board level

A black-pepper seed and a mango fruit are cut open. Name the tissue left from the nucellus in the black-pepper seed and the tissue forming the wall of the mango, giving the origin of each. Then classify castor and pea seeds as albuminous or non-albuminous, with the reason.

Try it first, then show the solutionSolution

Perisperm: persistent remains of the nucellus in a seed (black pepper, beet). Pericarp: wall of the fruit, formed from the ovary wall (any fruit, e.g. mango) (1½ marks).
Albuminous: part of the endosperm persists in the mature seed (wheat, maize, castor). Non-albuminous: endosperm fully consumed by the embryo (pea, groundnut) (1½ marks).

Answer: Perisperm (nucellus) vs pericarp (ovary wall); albuminous (castor) vs non-albuminous (pea).

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

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

Mistake: Writing that both male gametes fuse with the egg
Students confuse double fertilisation with two gametes entering the egg.
Wrong working
Double fertilisation: two male gametes fuse with the egg.
Right working
One male gamete + egg (syngamy); the other + two polar nuclei (triple fusion).
Do this: Write both fusions on separate lines with products and ploidy.
Usually costs: 1-2 marks.
Mistake: Giving the endosperm as diploid
Students count the polar nuclei as one.
Wrong working
Endosperm of a plant with 2n = 14 has 14 chromosomes.
Right working
Endosperm is 3n = 21.
Do this: Find n first, then multiply by 3.
Usually costs: 1 mark.
Mistake: Calling castor non-albuminous
Examples are memorised loosely.
Wrong working
Castor, pea and beans are ex-albuminous seeds.
Right working
Castor is albuminous; pea, groundnut and beans are non-albuminous.
Do this: Castor and coconut keep their endosperm.
Usually costs: 1-mark AR or MCQ.
Mistake: Confusing perisperm with pericarp
Similar names.
Wrong working
Pericarp is the remains of the nucellus.
Right working
Perisperm = nucellus remains (in seed); pericarp = fruit wall (from ovary wall).
Do this: PERI-SPERM is in the seed (sperma = seed); PERI-CARP is the fruit wall (karpos = fruit).
Usually costs: 1 mark.

Exam tips

In 5-mark answers on embryo development, a clean diagram of the four stages earns most of the marks; label the suspensor.
Always say where the embryo develops: at the micropylar end of the embryo sac.
Use 'syngamy' and 'triple fusion' by name; examiners award marks for the terms.
When asked for the fate of male gametes, give both fates and both products.
Learn one example for each category: free-nuclear (coconut water), perisperm (black pepper, beet), false fruit (apple, strawberry, cashew), parthenocarpy (banana).

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
The primary endosperm nucleus is formed by the fusion of:
(a) a male gamete with the egg (b) a male gamete with two polar nuclei (c) two synergids with a male gamete (d) two male gametes with the egg
Hint
Triple fusion.
Answer
(b) a male gamete with two polar nuclei.
Q2 · 1 mark · Assertion-reasonBoard level
Assertion (A): The primary endosperm nucleus is triploid.
Reason (R): It is formed by the fusion of a male gamete with the egg cell.
(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
Which nuclei form the PEN?
Answer
(c). PEN is triploid, but it is formed from a male gamete and two polar nuclei, not the egg.
Q3 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Pea seeds have no endosperm at maturity.
Reason (R): In pea, the endosperm is completely consumed by the developing embryo.
(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
Non-albuminous seed.
Answer
(a). Pea is non-albuminous because the embryo uses up the endosperm.
Q4 · 2 marks · Very short answerHarder board level
How many pollen grains and how many male gametes are needed for 240 ovules to form 240 seeds? How many meiotic divisions are needed in all?
Hint
One pollen grain per ovule; one meiosis per ovule; one meiosis per four pollen grains.
Answer
240 pollen grains, carrying 480 male gametes (one for syngamy and one for triple fusion in each ovule). Meioses: 240 (one per megaspore mother cell) + 240 ÷ 4 = 60 (male side) = 300.
Q5 · 3 marks · Short answerHOTS / competency
Suppose a fruit seller advertises 'seedless bananas grown with no pollination'. A customer says this is impossible because all fruits need fertilisation. (a) Who is right? (b) Name the phenomenon and say how it can also be brought about artificially. (c) Is the banana fruit true or false? Explain.
Hint
Fruit without fertilisation.
Answer
(a) The seller: some fruits develop without fertilisation. (b) Parthenocarpy; it can be induced by applying growth hormones. (c) True fruit: it develops from the ovary; parthenocarpy means no fertilisation, not a different origin, and it is seedless because no ovules are fertilised.
Q6 · 4 marks · Case-basedHarder board level
Read and answer.
Suppose a botanist sections an embryo sac shortly after fertilisation and sees: a cell P near the micropylar end formed by fusion of one male gamete with the egg, and a large central structure Q formed by fusion of the other male gamete with two nuclei. A few days later, Q has many nuclei but no cell walls, and P has not divided yet.
(a) Name P and Q and give their ploidy. (1)
(b) Name the type of development seen in Q. (1)
(c) Why has P not divided yet? Explain the advantage. (2)
Hint
Endosperm precedes embryo.
Answer
(a) P = zygote (2n); Q = primary endosperm cell/nucleus (3n). (b) Free-nuclear endosperm development. (c) The zygote usually divides only after some endosperm has formed; this ensures the developing embryo has a ready food supply.
Q7 · 2 marks · Very short answerBoard level
Name the parts of a mature dicot embryo that (a) form the root, (b) form the shoot, (c) store food in a bean seed, and state where the epicotyl and hypocotyl lie.
Hint
Use the level of the cotyledons as reference.
Answer
(a) Radicle (b) plumule (c) cotyledons. Epicotyl: the part of the embryonal axis above the level of the cotyledons, ending in the plumule; hypocotyl: below them, ending in the radicle.

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.Ploidy of the endosperm?Show answer
Triploid (3n).
3.Coconut water is which type of endosperm?Show answer
Free-nuclear endosperm.
4.What is the scutellum?Show answer
The single cotyledon of a grass embryo.
5.Name two seeds with perisperm.Show answer
Black pepper and beet.
6.What does the ovary wall become?Show answer
The pericarp.

Double Fertilisation and Post-fertilisation Events: frequently asked questions

Why is it called double fertilisation?

Because two fusions take place in the same embryo sac: syngamy (male gamete with egg) and triple fusion (male gamete with two polar nuclei).

Why is the endosperm triploid?

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

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

Albuminous seeds (wheat, maize, castor) keep part of the endosperm at maturity; non-albuminous seeds (pea, groundnut) have none because the embryo used it up.

What are false fruits?

Fruits in which the thalamus also contributes to the fruit, as in apple, strawberry and cashew. True fruits develop from the ovary only.

Why does the endosperm develop before the embryo?

The endosperm stores food. Forming it first assures nutrition to the embryo when it starts to grow.