DOUBLE FERTILISATION IN FLOWERING PLANTs

Definition

Double fertilisation is a unique phenomenon in flowering plants in which two fusions occur inside the embryo sac:

1. Syngamy

2. Triple fusion

Because two fertilisation events occur simultaneously, the process is called double fertilisation.

Step-wise Process of Double Fertilisation

1. Entry of Pollen Tube

• The pollen tube enters the embryo sac through the micropyle.

• It penetrates one of the synergids.

• The tip of the pollen tube bursts and releases two male gametes into the cytoplasm of the synergid.

2. First Fusion – Syngamy

• One male gamete moves towards the egg cell.

• The nucleus of the male gamete fuses with the nucleus of the egg.

Result

• A diploid zygote (2n) is formed.

Equation

[n + n = 2n]

3. Second Fusion – Triple Fusion

• The second male gamete moves towards the two polar nuclei present in the central cell.

• It fuses with the two polar nuclei.

Result

• A triploid Primary Endosperm Nucleus (PEN) (3n) is formed.

Equation

[n + n + n = 3n]

Why is it Called Double Fertilisation?

Two fusion events occur in the same embryo sac:

1. Syngamy

2. Triple fusion

Therefore, the phenomenon is called double fertilisation.

After Effects of Double Fertilisation

Formation of Embryo

• The zygote develops into the embryo.

Formation of Endosperm

• The central cell after triple fusion becomes the Primary Endosperm Cell (PEC).

• PEC develops into the endosperm.

• Endosperm provides nourishment to the developing embryo.

Important Terms

Term	Meaning
Syngamy	Fusion of male gamete with egg
Triple Fusion	Fusion of one male gamete with two polar nuclei
Zygote	Diploid cell formed after syngamy
PEN	Primary Endosperm Nucleus
PEC	Primary Endosperm Cell
Endosperm	Nutritive tissue for embryo

Diagrammatic Flow of Double Fertilization

Pollen tube enters synergid
            ↓
Two male gametes released
            ↓
 ┌───────────────────────┐
 │ First male gamete     │
 │ + Egg nucleus         │
 │ = Zygote (2n)         │
 └───────────────────────┘
            ↓
        Syngamy

 ┌───────────────────────┐
 │ Second male gamete    │
 │ + Two polar nuclei    │
 │ = PEN (3n)            │
 └───────────────────────┘
            ↓
      Triple Fusion
            ↓
      Double Fertilisation
            ↓
 Zygote → Embryo
 PEC → Endosperm

Significance of Double Fertilisation

• It is unique to angiosperms (flowering plants).

• Ensures formation of:

  • Embryo

  • Nutritive endosperm

• Helps in proper nourishment and development of the embryo.

Double fertilisation is an important reproductive event in flowering plants where one male gamete forms the zygote and the other forms the endosperm. This process ensures successful seed development and nourishment of the embryo.

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