Double Fertilisation
இருமுறை கருவுறுதல்
Double fertilisation is the hallmark of Angiosperms. It is not just a process; it is an evolutionary masterstroke that prevents wastage of energy. If the endosperm formed before fertilisation, the plant would starve. Remember this: Syngamy and Triple Fusion occur simultaneously.
The idea
Pollen tube releases two male gametes into the embryo sac. One male gamete fuses with the egg cell to form a diploid zygote, termed syngamy. The second male gamete fuses with two polar nuclei to form a triploid Primary Endosperm Nucleus (PEN), known as triple fusion. This combined process is double fertilisation.
Why it matters
It ensures that the endosperm, which provides nutrition to the embryo, is formed only when fertilization of the egg has successfully occurred, optimizing resource allocation.
NEET lens
NEET focuses on the ploidy levels: Zygote is 2n, PEN is 3n. Examiners frequently swap these in MCQs to trap you. Memorise the location of these events within the embryo sac. Expect at least one question on the products of syngamy and triple fusion.
Clinical significance
In ecosystems, this ensures the endosperm provides nutrition only after successful fertilisation, preventing resource depletion. Failure of this mechanism leads to seed abortion. It is the biological equivalent of 'just-in-time' nutrient delivery, ensuring the survival of the next generation.
Introduction
Double fertilization is a unique feature of angiosperms (flowering plants) in which both male gametes from the pollen tube participate in fusion: one fuses with the egg (syngamy → embryo) and the other fuses with the central cell (triple fusion → endosperm).
Core concept
Process: Pollen tube enters ovule via micropyle → enters synergid → discharges 2 male gametes (n). First fusion (Syngamy): male gamete (n) + egg cell (n) → zygote (2n) → develops into embryo. Second fusion (Triple fusion): male gamete (n) + secondary nucleus (2n, two fused polar nuclei) → primary endosperm nucleus (3n) → endosperm. Both fusions together = double fertilization. Result: Seed with 2n embryo + 3n endosperm. Discovered by: S.G. Nawaschin (1898) in Lilium and Fritillaria.
Go deeper
Why endosperm is 3n: both polar nuclei (each n) fuse → 2n secondary nucleus → fuses with male gamete (n) → 3n. Endosperm nourishes embryo. In coconut: liquid endosperm = coconut water (free nuclear endosperm). Solid = coconut meat (cellular endosperm). In cereal grains (wheat, rice, maize): endosperm = bulk of seed = food for humans.
From Class 10
In animals, fertilization involves one sperm fusing with one egg. In flowering plants, something unique happens — two fusions occur simultaneously. This is double fertilization.