Process of Double Fertilisation — Revision Notes

⚡ 30-Second Revision

Double Fertilisation: — Two fusions in angiosperms.
Syngamy: — Male gamete ($n$) + Egg cell ($n$) $\rightarrow$ Zygote ($2n$). Forms embryo.
Triple Fusion: — Male gamete ($n$) + Secondary nucleus ($2n$) $\rightarrow$ Primary Endosperm Nucleus (PEN) ($3n$). Forms endosperm.
Ploidy: — Male gamete ($n$), Egg cell ($n$), Zygote ($2n$), Polar nuclei ($n+n$), Secondary nucleus ($2n$), PEN ($3n$), Endosperm ($3n$), Embryo ($2n$).
Pollen Tube: — Carries male gametes, guided by synergids.
Synergids: — Guide pollen tube via filiform apparatus; degenerate post-entry.
Antipodals: — Degenerate post-fertilisation.
Post-Fertilisation: — Ovule $\rightarrow$ Seed; Ovary $\rightarrow$ Fruit.

2-Minute Revision

Double fertilisation is the defining feature of sexual reproduction in flowering plants, involving two distinct fusion events. It begins with a compatible pollen grain germinating on the stigma, forming a pollen tube that grows through the style and enters the ovule, usually via the micropyle, guided by synergids.

The pollen tube then releases two non-motile male gametes into the embryo sac. The first fusion, called syngamy, occurs when one male gamete (n) fuses with the egg cell (n) to form a diploid zygote (2n), which will develop into the embryo.

The second fusion, known as triple fusion, involves the other male gamete (n) fusing with the diploid secondary nucleus (2n) of the central cell (formed by the fusion of two polar nuclei). This results in a triploid primary endosperm nucleus (PEN) (3n), which develops into the endosperm (3n), providing nourishment to the developing embryo.

Post-fertilisation, the ovule matures into a seed, and the ovary develops into a fruit. Key structures to remember are the synergids with their filiform apparatus for guidance, and the distinct ploidy levels of the zygote (2n) and endosperm (3n).

5-Minute Revision

Double fertilisation is a complex yet highly efficient reproductive strategy unique to angiosperms. The process initiates with pollination, where a compatible pollen grain lands on the stigma. The pollen grain then germinates, forming a pollen tube from its vegetative cell.

This tube grows through the stigma and style, guided by chemical signals, eventually reaching the ovule. The pollen tube typically enters the ovule through the micropyle (porogamy) and is specifically guided into the embryo sac by the synergids, which possess a specialized structure called the filiform apparatus.

Upon entering a synergid, the pollen tube ruptures, releasing its two male gametes (n) into the embryo sac.

Now, the 'double' part occurs:

1
Syngamy (Generative Fertilisation): — One male gamete ($n$) fuses with the **egg cell ($n$) to form a diploid zygote ($2n$). This zygote is the precursor to the new plant embryo**.
2
Triple Fusion (Vegetative Fertilisation): — The second male gamete ($n$) fuses with the **secondary nucleus ($2n$) of the central cell. The secondary nucleus itself is formed by the prior fusion of two polar nuclei ($n+n$). This fusion results in a triploid primary endosperm nucleus (PEN) ($3n$). The PEN then undergoes numerous divisions to form the endosperm ($3n$)**, a nutritive tissue that provides food for the developing embryo.

After these fusions, significant post-fertilisation changes occur: the zygote develops into the embryo, the PEN develops into the endosperm, the ovule matures into a seed, and the ovary develops into a fruit.

The synergids and antipodal cells typically degenerate. Understanding the ploidy levels (n, 2n, 3n) of each structure (gametes, egg, zygote, polar nuclei, central cell, PEN, endosperm, embryo) is paramount for NEET.

For example, if a plant's somatic cells are $2n=20$, then its endosperm cells will be $3n=30$.

Prelims Revision Notes

Double fertilisation is an angiosperm-specific process involving two fusions. Pollen-pistil interaction leads to pollen germination on the stigma, forming a pollen tube. The pollen tube grows through the style, guided by chemotropism (signals from synergids), and enters the ovule, usually through the micropyle (porogamy). Inside the embryo sac, the filiform apparatus of the synergids directs the pollen tube and facilitates the release of two male gametes (n).

Fusion Events:

1
Syngamy: — One male gamete ($n$) + Egg cell ($n$) $\rightarrow$ **Zygote ($2n$). The zygote develops into the embryo**.
2
Triple Fusion: — Second male gamete ($n$) + Secondary nucleus ($2n$) $\rightarrow$ **Primary Endosperm Nucleus (PEN) ($3n$). The PEN develops into the endosperm**.

Ploidy Levels (Crucial for NEET):

Male gamete: $n$
Egg cell: $n$
Zygote: $2n$
Polar nuclei: $n$ each (fuse to form $2n$ secondary nucleus)
Central cell (secondary nucleus): $2n$
Primary Endosperm Nucleus (PEN): $3n$
Endosperm: $3n$
Embryo: $2n$
Synergids/Antipodals: $n$

Post-Fertilisation Changes:

Zygote $\rightarrow$ Embryo
PEN $\rightarrow$ Endosperm
Ovule $\rightarrow$ Seed
Ovary $\rightarrow$ Fruit
Integuments $\rightarrow$ Seed coat
Synergids and Antipodals degenerate.

Types of Seeds:

Endospermic/Albuminous: — Endosperm persists in mature seed (e.g., castor, maize).
Non-endospermic/Exalbuminous: — Endosperm consumed by embryo; food stored in cotyledons (e.g., pea, bean).

Remember the functions: pollen tube for gamete delivery, synergids for guidance, endosperm for nourishment.

Vyyuha Quick Recall

Double Fertilisation: Synergy Triple Fusion.

Synergy: Single male gamete + Egg $\rightarrow$ Zygote (2n) $\rightarrow$ Embryo. (S.E.Z.E.)

Triple Fusion: Two polar nuclei + Second male gamete $\rightarrow$ Primary Endosperm Nucleus (3n) $\rightarrow$ Endosperm. (T.S.P.E.N.E.)

Think of it as 'SEZE' for the embryo and 'TSPENE' for the endosperm. The 'T' in TSPENE reminds you it's Triploid.