Biology · Sexual Reproduction in Flowering Plants · NEET
Double fertilisation means TWO fusion events happen inside one embryo sac. One male gamete fuses with the egg (syngamy) to make the diploid (2n) zygote. The second male gamete fuses with the two polar nuclei of the central cell (triple fusion) to make the triploid (3n) primary endosperm nucleus (PEN). Memory hook: "Two males, two jobs — one makes the baby (embryo), one makes the food (endosperm)." This event is unique to flowering plants (angiosperms), so NEET loves to test it.
Two male gametes enter the embryo sac: one fuses with the egg (syngamy -> 2n zygote -> embryo), the other fuses with the two polar nuclei (triple fusion -> 3n primary endosperm nucleus -> endosperm).
Your doubts, answered
Is syngamy the same as double fertilisation?
No. Syngamy is only ONE of the two fusions. Syngamy = one male gamete + egg cell -> diploid (2n) zygote. Double fertilisation = syngamy PLUS triple fusion happening together in the same embryo sac. So syngamy is a PART of double fertilisation, not the whole thing. Many students lose a mark by writing 'double fertilisation = syngamy' only.
Why is it called TRIPLE fusion if only two nuclei fuse there?
Because THREE haploid nuclei join together, not two. The second male gamete (1 nucleus) fuses with the TWO polar nuclei (2 nuclei) of the central cell. 1 + 2 = 3 haploid nuclei fuse, so the product is triploid (3n) and the event is named 'triple fusion'. The result is the triploid primary endosperm nucleus (PEN).
Which male gamete makes the embryo and which makes the endosperm?
One male gamete goes to the egg cell -> syngamy -> zygote -> later the EMBRYO (the future plant). The other male gamete goes to the central cell (two polar nuclei) -> triple fusion -> primary endosperm nucleus (PEN) -> the central cell becomes the primary endosperm cell (PEC) -> ENDOSPERM (the food tissue). So: egg = embryo, polar nuclei = endosperm.
Where exactly does double fertilisation happen?
Inside the embryo sac (female gametophyte) of the ovule. The pollen tube enters through the micropyle, pierces one synergid, and releases both male gametes into that synergid's cytoplasm. From there one gamete moves to the egg and the other to the central cell. The synergid is only the DELIVERY point; no gamete fuses with the synergid itself.
Why is double fertilisation unique to flowering plants?
Because only angiosperms have the special central cell with two polar nuclei that allows the SECOND fusion (triple fusion) to occur. Gymnosperms, algae and fungi show only one fusion (single fertilisation) and have no triploid endosperm formed this way. NCERT states double fertilisation is 'an event unique to flowering plants', so pick angiosperms in such NEET questions.
⚠️ The NEET trap ✗ Double fertilisation = fusion of two male gametes with one egg (or with two eggs). ✓ Double fertilisation = syngamy (one male gamete + egg -> 2n zygote) AND triple fusion (second male gamete + two polar nuclei -> 3n PEN). 🧠 NEET 2018 tested exactly this. The two male gametes do NOT both go to the egg. One goes to the egg, the other to the central cell. Never pick options that fuse both male gametes with eggs.
Real NEET questions
NEET 2017
Double fertilization is exhibited by
A · Gymnosperms
B · Algae
C · Fungi
D · Angiosperms ✓
Solution: Double fertilisation (syngamy + triple fusion in one embryo sac) is an event unique to flowering plants, i.e. angiosperms. Gymnosperms, algae and fungi do not show it. NCERT Ch 1: 'the phenomenon is termed double fertilisation, an event unique to flowering plants.'
NEET 2018
Double fertilization is
A · Fusion of two male gametes with one egg
B · Fusion of one male gamete with two polar nuclei
C · Fusion of two male gametes of a pollen tube with two different eggs
D · Syngamy and triple fusion ✓
Solution: Double fertilisation is the occurrence of TWO fusion events in one embryo sac: syngamy (one male gamete + egg -> diploid zygote) and triple fusion (second male gamete + two polar nuclei -> triploid PEN). Option 2 describes only triple fusion; options 1 and 3 wrongly send both male gametes to eggs. Correct = syngamy and triple fusion.
NEET 2023
In angiosperm, the haploid, diploid and triploid structures of a fertilized embryo sac sequentially are:
A · Synergids, Primary endosperm nucleus and zygote
B · Antipodals, synergids and primary endosperm nucleus
C · Synergids, Zygote and Primary endosperm nucleus ✓
D · Synergids, antipodals and Polar nuclei
Solution: After fertilisation, synergids remain haploid (n); syngamy gives the diploid (2n) zygote; triple fusion gives the triploid (3n) primary endosperm nucleus. So the haploid -> diploid -> triploid order is Synergids, Zygote, Primary endosperm nucleus.
Solved Sexual Reproduction in Flowering Plants NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Two fusions happen in one embryo sac. One male gamete joins the egg to form the zygote (future embryo). The second male gamete joins the two polar nuclei to form the primary endosperm nucleus (future food tissue). Both fusions together = double fertilisation.
What is the ploidy of the zygote and the PEN?
The zygote is diploid (2n) because it forms from egg (n) + one male gamete (n). The primary endosperm nucleus (PEN) is triploid (3n) because it forms from two polar nuclei (n + n) + one male gamete (n).
What is the difference between syngamy and triple fusion?
Syngamy joins TWO haploid nuclei (male gamete + egg) to give a diploid zygote. Triple fusion joins THREE haploid nuclei (male gamete + two polar nuclei) to give a triploid PEN. Both occur in the same embryo sac during double fertilisation.
Does the synergid fuse with a male gamete?
No. The pollen tube discharges both male gametes INTO a synergid, but the gametes then move out to the egg and the central cell. The synergid is only the site of delivery, so it stays haploid and does not fuse.
What forms after triple fusion?
The central cell becomes the primary endosperm cell (PEC) and develops into the endosperm, the food-storing tissue. The zygote separately develops into the embryo. Endosperm development usually starts before embryo development.
