Question
Explain why the endosperm in angiosperms is triploid (3n). Describe the process of double fertilisation that leads to endosperm formation. What is the role of endosperm in seed development?
(NEET 2023, similar pattern)
Solution — Step by Step
In angiosperms, double fertilisation involves two fusion events:
Fusion 1 (Syngamy): One male gamete (n) + egg cell (n) → Zygote (2n) → develops into the embryo
Fusion 2 (Triple fusion): The other male gamete (n) + two polar nuclei (n + n) → Primary Endosperm Nucleus (PEN) (3n)
This is why the endosperm is triploid — it forms from the fusion of three haploid nuclei (one sperm + two polar nuclei).
Inside the mature embryo sac (female gametophyte), there are 7 cells with 8 nuclei:
- 1 egg cell (n)
- 2 synergids (n)
- 3 antipodals (n)
- 1 central cell with 2 polar nuclei (each n)
The two polar nuclei are located in the large central cell. When the second sperm fuses with these polar nuclei (triple fusion), the resulting PEN is 3n.
The PEN divides repeatedly to form the endosperm — a nutritive tissue that provides food to the developing embryo.
Types of endosperm development:
- Nuclear type — PEN divides without cell wall formation initially (free nuclear divisions), walls form later. Most common (e.g., coconut water is free-nuclear endosperm)
- Cellular type — cell wall forms after each division from the start
- Helobial type — combination of both (first division is cellular, then nuclear)
Role of endosperm:
- Stores nutrients (starch, proteins, fats) for the embryo
- In many monocots (wheat, rice, maize), the endosperm persists in the mature seed — this is what we eat
- In many dicots (pea, bean), the endosperm is consumed by the cotyledons during seed development — the mature seed has no endosperm
Why This Works
Double fertilisation is unique to angiosperms — no other plant group has it. The evolutionary advantage is efficiency: the endosperm develops only when fertilisation is successful. Unlike gymnosperms (where the female gametophyte stores food before fertilisation, potentially wasting resources), angiosperms invest in endosperm only after the egg has been fertilised.
The triploid nature gives endosperm a higher dose of genes, which may help it grow rapidly and produce nutrients faster than a diploid tissue could.
Alternative Method — Endosperm vs Cotyledon Comparison
For NEET, know the key comparisons:
- Endospermic seeds (endosperm present in mature seed): wheat, rice, maize, castor, coconut
- Non-endospermic seeds (endosperm consumed, food stored in cotyledons): pea, bean, groundnut, gram
Also remember: coconut water = free-nuclear endosperm (liquid); coconut meat = cellular endosperm (solid). NEET has asked this specific example.
Common Mistake
Students often say “endosperm is diploid (2n) like other tissues.” The endosperm is triploid (3n) because it forms from triple fusion (one n sperm + two n polar nuclei). Also, don’t confuse the primary endosperm nucleus (PEN) with the primary endosperm cell — the PEN is the initial 3n nucleus that then divides to form the endosperm tissue. The term “primary endosperm cell” refers to the central cell after triple fusion.