Double fertilisation and post-fertilisation events (seed, fruit, apomixis)

Angiosperms show double fertilisation — a defining feature — after which the ovule becomes a seed and the ovary becomes a fruit.

Double fertilisation

The pollen tube enters the embryo sac through a synergid and releases two male gametes. One fuses with the egg (syngamy) to form the diploid zygote (2n). The other fuses with the two polar nuclei of the central cell (triple fusion) to form the triploid (3n) primary endosperm nucleus (PEN). Because two fusion events occur, the process is called double fertilisation — unique to flowering plants.

Post-fertilisation events

The PEN develops into the endosperm, which nourishes the embryo and usually forms before the embryo (coconut water is free-nuclear endosperm; the white kernel is cellular). In albuminous/endospermic seeds it persists (wheat, maize, castor, coconut); in non-albuminous/ex-albuminous seeds it is consumed (pea, groundnut, bean). The zygote develops into the embryo: a dicot embryo has two cotyledons; a monocot embryo has one cotyledon (scutellum) with a coleoptile and coleorhiza. The ovule becomes the seed (integuments → testa and tegmen) and the ovary becomes the fruit (ovary wall → pericarp). A true fruit develops only from the ovary; a false fruit (e.g. apple) also involves the thalamus. Parthenocarpic fruits (e.g. banana) form without fertilisation.

Special mechanisms

Apomixis is seed formation without fertilisation (asexual, but mimics sexual), seen in some grasses and Asteraceae. Polyembryony is the occurrence of more than one embryo in a seed, e.g. in citrus and mango.

Exam Tricks & Tips

• 🎯 Double fertilisation = syngamy (zygote, 2n) + triple fusion (PEN, 3n) — both happen; hence 'double'.
• 🎯 Endosperm is triploid (3n) because it forms from one male gamete + two polar nuclei.
• 🎯 Endosperm usually develops before the embryo — it nourishes the growing embryo.
• 🎯 Coconut water = free-nuclear endosperm; kernel = cellular endosperm.
• 🎯 Apomixis = seeds without fertilisation; polyembryony = many embryos per seed.
• ❌ Common mistake: calling the endosperm diploid. It is triploid (3n) — one sperm nucleus fuses with two polar nuclei in triple fusion.

Quick recap

Two male gametes: syngamy → zygote (2n); triple fusion → PEN (3n) → endosperm. Double fertilisation is unique to angiosperms. Endosperm forms first. Ovule → seed, ovary → fruit. Apomixis = seeds without fertilisation; polyembryony = >1 embryo/seed.

Cover the answer, recall, then check. 12 cards on double fertilisation and after.

Q1. What is syngamy?
A1. Fusion of one male gamete with the egg cell to form the diploid (2n) zygote.

Q2. What is triple fusion?
A2. Fusion of the second male gamete with the two polar nuclei of the central cell to form the triploid (3n) primary endosperm nucleus.

Q3. Why is it called double fertilisation?
A3. Because two fusion events occur — syngamy and triple fusion — a feature unique to angiosperms.

Q4. What is the ploidy of the endosperm and why?
A4. Triploid (3n), because it forms from one haploid male gamete and two haploid polar nuclei.

Q5. Which usually develops first, embryo or endosperm?
A5. The endosperm, because it nourishes the developing embryo.

Q6. Distinguish albuminous and ex-albuminous seeds with examples.
A6. Albuminous retain endosperm (wheat, maize, castor); ex-albuminous have it consumed (pea, groundnut, bean).

Q7. Name the parts of the embryonal axis in a dicot embryo.
A7. Epicotyl (bearing the plumule) and hypocotyl (bearing the radicle).

Q8. What is the single cotyledon of a monocot embryo called?
A8. The scutellum; the plumule and radicle are covered by coleoptile and coleorhiza.

Q9. What do the ovule and ovary become after fertilisation?
A9. The ovule becomes the seed and the ovary becomes the fruit.

Q10. Distinguish a true fruit from a false fruit.
A10. A true fruit develops only from the ovary; a false fruit (e.g. apple) also involves the thalamus.

Q11. What is apomixis?
A11. Formation of seeds without fertilisation — an asexual process that mimics sexual reproduction.

Q12. What is polyembryony? Give an example.
A12. The presence of more than one embryo in a single seed, e.g. in citrus and mango.

Angiosperms do something no other group does — they fertilise twice in one embryo sac. One fusion makes the embryo; the other makes its food. This 'double fertilisation' is a defining feature of flowering plants.

Core idea: After the pollen tube discharges two male gametes into the embryo sac, syngamy and triple fusion occur together — this is double fertilisation. It leads to the embryo, endosperm, seed and fruit.

Double fertilisation

The pollen tube enters one synergid and releases two male gametes into the embryo sac. Then:

• Syngamy: one male gamete (n) + egg (n) → zygote (2n) → embryo.
• Triple fusion: the other male gamete (n) + two polar nuclei (n + n) → primary endosperm nucleus, PEN (3n) → endosperm.

Because two fusion events involving both male gametes occur, the phenomenon is double fertilisation, and because three haploid nuclei fuse to give the endosperm it is also called triple fusion. Both are unique to angiosperms.

Post-fertilisation events

Endosperm

Develops from the PEN, is triploid (3n), and nourishes the embryo. Most common type is free-nuclear endosperm (nuclear divisions without wall formation first), e.g. coconut water (nuclear) and coconut 'meat' (cellular). Endosperm may be consumed by the embryo (non-endospermic/ex-albuminous seeds — pea, bean) or persist (endospermic/albuminous seeds — castor, maize, coconut).

Embryo

The zygote divides to form the embryo. Dicot embryo: two cotyledons, epicotyl (plumule), hypocotyl (radicle). Monocot embryo: one cotyledon (scutellum), with coleoptile and coleorhiza sheaths.

Seed and fruit

• Ovule → seed (integuments → seed coat: testa + tegmen).
• Ovary → fruit (ovary wall → pericarp).
• True fruit develops from ovary only; false fruit involves the thalamus too (e.g. apple).
• Parthenocarpic fruit = fruit without fertilisation (seedless, e.g. banana).

Apomixis and polyembryony

• Apomixis — formation of seeds without fertilisation; an asexual method producing seeds genetically identical to the parent (e.g. some Asteraceae, grasses). Valuable for fixing hybrid vigour.
• Polyembryony — more than one embryo in a seed (e.g. Citrus, mango).

Worked example

Q: State the ploidy of zygote, endosperm and nucellus.
A: Zygote = 2n (n egg + n male gamete); endosperm = 3n (n male gamete + 2×n polar nuclei); nucellus = 2n (maternal sporophytic tissue). This ploidy chain is a classic exam question.

Real-world / exam application

Apomixis is a major goal in agriculture: if hybrid crops could set apomictic seed, farmers could re-sow saved seed each year without losing hybrid vigour, cutting seed costs. Parthenocarpy gives seedless grapes and bananas.

Exam tricks & mnemonic

• Ovule→seed, Ovary→fruit (both start with 'O').
• Ploidy ladder: egg n, zygote 2n, endosperm 3n, nucellus/integument 2n, embryo 2n.
• Double fertilisation = syngamy + triple fusion (remember 'two events, three nuclei').

Worked Example

Problem: Explain the process of double fertilisation in angiosperms and its two products, and describe what the ovule and ovary become after fertilisation.

Solution:
Step 1 — Set the scene. After pollination, the pollen tube carries two male gametes into the embryo sac through a synergid.

Step 2 — Describe syngamy. One male gamete fuses with the egg cell (syngamy) to form the diploid (2n) zygote, which develops into the embryo.

Step 3 — Describe triple fusion. The second male gamete fuses with the two polar nuclei of the central cell (triple fusion) to form the triploid (3n) primary endosperm nucleus, which develops into the nutritive endosperm. Because two fusion events (syngamy + triple fusion) occur, the process is called double fertilisation.

Step 4 — Describe post-fertilisation changes. The ovule matures into the seed (with the embryo, endosperm, and a protective seed coat), and the ovary develops into the fruit. (In apomixis, seeds form without fertilisation.)

Answer: Double fertilisation = syngamy (male gamete + egg → 2n zygote) plus triple fusion (male gamete + 2 polar nuclei → 3n endosperm); afterwards the ovule becomes the seed and the ovary becomes the fruit.