Q.(i) Explain the process of double fertilization in angiosperms.
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Triple Fusion Process – A First Look
Imagine you are baking a cake. You have three separate ingredients: flour, eggs, and sugar. On their own, each is useful but incomplete. When you mix them together and bake, they fuse into something entirely new — a cake that has properties none of the individual ingredients had. That is the spirit of triple fusion, though in biology the ingredients are far more specific.
What Triple Fusion Actually Means
In the context of plant reproduction — specifically in flowering plants — triple fusion is a fertilisation event that happens inside the ovule (the part that becomes the seed). It involves the fusion of three nuclei: one sperm cell from the pollen grain fuses with two polar nuclei present in the central cell of the embryo sac. The result is a triploid (3n) nucleus, which then develops into the endosperm — the nutritive tissue that feeds the developing embryo.
This is not a random event. It is a carefully orchestrated step that occurs simultaneously with the fusion of another sperm cell with the egg cell (which forms the zygote). Because two fertilisation events happen together — one forming the zygote, the other forming the endosperm — the entire process is called double fertilisation. Triple fusion is the name given specifically to the second of these two events.
Triple fusion is not a separate process from double fertilisation. It is the second half of it. Double fertilisation = fusion of sperm with egg (syngamy) + fusion of sperm with two polar nuclei (triple fusion). The "triple" refers to the three nuclei that come together.
Why Does Triple Fusion Matter?
The endosperm produced by triple fusion is triploid — a condition unique to flowering plants. This triploid tissue is rich in nutrients (starch, proteins, oils) and serves as the food supply for the developing embryo. In many seeds we eat — like wheat, rice, maize, and coconut — the endosperm is the edible part.
Without triple fusion, the embryo would have no built-in food source. The seed would either fail to develop or would be too small to survive. This is why triple fusion is considered a key evolutionary innovation of angiosperms (flowering plants). It gave them a reproductive advantage over gymnosperms (like pines and cycads), whose endosperm is haploid and formed before fertilisation.
What the NCERT Textbook Says
The NCERT Class 12 Biology textbook (Chapter 2: Sexual Reproduction in Flowering Plants) describes triple fusion as follows:
- One of the two male gametes (sperm cells) moves towards the egg cell and fuses with it to form the zygote (2n).
- The other male gamete moves towards the central cell of the embryo sac, where it fuses with two polar nuclei (both haploid, 1n each). This fusion of three haploid nuclei produces a triploid primary endosperm nucleus (PEN).
- The PEN then divides repeatedly to form the endosperm, which nourishes the embryo.
The textbook emphasises that this is a unique feature of angiosperms — no other plant group does this.
Triple fusion is not the fusion of three cells. It is the fusion of three nuclei — one sperm nucleus + two polar nuclei. The result is a triploid nucleus, not a triploid cell. The cell that contains this nucleus is called the central cell, and it becomes the endosperm mother cell.
A Simple Way to Remember
Think of it as a three-way handshake inside the ovule:
- Player 1: Sperm nucleus (from pollen)
- Player 2: First polar nucleus (in the central cell)
- Player 3: Second polar nucleus (also in the central cell) …
Double fertilization delivers the pollen tube's two male gametes to two different targets inside the embryo sac in one coordinated event, and each part of the resulting fertilized ovule then transforms in its own specific way as the seed develops. …
In angiosperms, one pollen tube delivers two male gametes into the embryo sac; one fuses with the egg (syngamy) and the other fuses with the two polar nuclei (triple fusion) — together called double fertilization — after which the fertilized ovule's parts transform into the seed.
(i) Double fertilization:
After pollination, the pollen tube grows through the style and enters the ovule (usually through the micropyle), releasing its two non-motile male gametes into the cytoplasm of the embryo sac (which contains an egg cell, two synergids, a central cell with two polar nuclei, and three antipodal cells).
- One male gamete fuses with the egg cell (syngamy) to form the diploid (2n) zygote.
- The other male gamete migrates to the central cell and fuses with its two polar nuclei (or the secondary nucleus formed by their earlier fusion) — this is called triple fusion, producing the triploid (3n) primary endosperm nucleus (PEN).
Since two separate fusion events occur using both male gametes, this unique process (found only in flowering plants, first described by S.G. Nawaschin) is called double fertilization.
(ii) Post-fertilization changes in the ovule:
- Zygote → develops (by repeated mitotic divisions) into the embryo. …
Showing the 12 most recent of 21 on this concept.
- CBSE 2026Set A1 markMCQQ.Which cell of the embryo sac becomes primary endosperm cell after triple fusion?(a) Central cell(b) Synergids(c) Antipodals(d) Egg cell
›Reveal solutionSolution
One male gamete fuses with the two polar nuclei of the central cell (triple fusion), turning the central cell into the primary endosperm cell.
In double fertilisation, one male gamete fuses with the egg (syngamy) to form the zygote. The second male gamete fuses with the two polar nuclei present in the central cell — this is triple fusion, producing a triploid primary endosperm nucleus (PEN). Because this fusion occurs in the central cel …
- CBSE 2026Set ANNUAL1 markMCQQ.The cells of endosperm of a plant have 27 chromosomes. What will be the number of chromosomes in its gametes?(a) 18(b) 9(c) 36(d) 27
›Reveal solutionSolution
Endosperm in angiosperms is triploid (3n); dividing its chromosome number by 3 gives the haploid number found in the plant's gametes.
In flowering plants, double fertilisation produces two products: the diploid (2n) zygote from fusion of the egg and one male gamete, and the triploid (3n) primary endosperm nucleus from fusion of the two polar nuclei (which together are 2n) with the second male gamete (n).
Given the endosperm has 27 chromosomes: 3n = 27, so n = 27 / 3 = 9.
…
- CBSE 2025Set ANNUAL1 markMCQQ.What is the ploidy of endosperm in flowering plants ?(a) 2n(b) 3n(c) 4n(d) 6n
›Reveal solutionSolution
Triple fusion during double fertilisation makes the endosperm 3n.
In flowering plants, double fertilisation involves two fusion events inside the embryo sac. One male gamete (n) fuses with the egg cell (n) to form the diploid (2n) zygote (syngamy). The second male gamete (n) fuses with the diploid (2n) secondary nucleus — itself formed by fusion of the two polar nuclei — to form the primary endosperm nucleus (PEN). Since this event involves the fusion of three haploid nuclei (one male gamete + two …
- CBSE 2024Set D1 markMCQQ.How many chromosomes are present in endosperm (Angiosperm)?(a) n (Haploid)(b) 2n (Diploid)(c) 3n (Triploid)(d) 4n (Tetraploid)
›Reveal solutionSolution
The endosperm of angiosperms is triploid (3n), formed by the fusion of one male gamete with the two haploid polar nuclei of the central cell.
In angiosperms, double fertilisation occurs inside the embryo sac. One of the two male gametes (n) delivered by the pollen tube fuses with the egg cell (n) to form the diploid zygote (2n), which develops into the embryo. This is called syngamy.
…
- CBSE 2024Set BOTANY1 markQ.Correct the statement, if necessary, by changing the underlined word(s) only: Primary endosperm nucleus is produced by double fertilization.
›Reveal solutionSolution
Strictly, the PEN arises from triple fusion, not from double fertilization as a whole.
In angiosperms, one pollen tube delivers two male gametes into the embryo sac. One male gamete fuses with the egg cell (syngamy) to form the diploid zygote. The other male gamete fuses with the two polar nuclei (or the secondary nucleus formed by their fusion) located in the central cell — this second event is called triple fusion because three haploid nuclei (one male gamete + two polar nuclei) are involved, and it produces the triploid (3n) Primary Endosperm Nucleus (PEN), which develops into the endosperm. Since both syngamy and …
- CBSE 2024Set ANNUAL1 markQ.The central cell after triple fusion becomes the ________.
›Reveal solutionSolution
The central cell after triple fusion becomes the primary endosperm cell (PEC); the triploid nucleus inside it is the primary endosperm nucleus (PEN).
During double fertilisation in flowering plants, one male gamete fuses with the egg to form the diploid zygote (syngamy), while the second male gamete fuses with the two polar nuclei of the central cell — this is triple fusion, involving three haploid nuclei (one male gamete + two polar nuclei). The triploid (3n) nucleus produced is the primary endosperm nucleus (PEN). The central cell that contained the polar nuclei is now called the **primary endosperm cell ( …
- CBSE 2023Set ANNUAL1 markMCQQ.Double fertilization occurs in -(a) Algae(b) Bryophytes(c) Gymnosperms(d) Angiosperms
›Reveal solutionSolution
Double fertilization — a defining feature of flowering plants — occurs only in angiosperms.
Double fertilization involves two fusion events inside the embryo sac: syngamy, in which one male gamete fuses with the egg cell to form the diploid zygote, and triple fusion, in which the second male gamete fuses with the two polar nuclei to form the triploid primary endosperm nucleus (PEN). Both …
- CBSE 2023Set ANNUAL1 markMCQQ.The primary endosperm nucleus formed in angiosperms is(a) haploid(b) diploid(c) triploid(d) tetraploid
›Reveal solutionSolution
Triple fusion (one sperm + two polar nuclei) produces the primary endosperm nucleus, which is triploid — this is unique to angiosperms.
In angiosperms, double fertilisation occurs inside the embryo sac: one male gamete (n) fuses with the egg cell (n) to form the diploid (2n) zygote (syngamy), and the second male gamete (n) fuses with the two polar nuclei (each n) located in the central cell, in a process called triple fusion. This produces the …
- CBSE 2023Set ANNUAL1 markMCQQ.In angiosperms, triple fusion is required for the formation of(a) embryo(b) endosperm(c) fruit(d) seed
›Reveal solutionSolution
Triple fusion, in which one male gamete fuses with the two polar nuclei of the central cell, produces the triploid primary endosperm nucleus that develops into the nutritive endosperm tissue.
In double fertilization, one of the two male gametes delivered by the pollen tube fuses with the two polar nuclei located in the central cell of the embryo sac — since three haploid nuclei are involved in this single fusion event, it is called triple fusion. The resulting triploid (3n) nucleus is the primary endosperm nucleus (PEN), which divides repe …
- CBSE 2023Set ANNUAL1 markMCQQ.Which of the following is an example of free nuclear endosperm?(a) Coconut water(b) Castor(c) Sugarcane juice(d) Groundnut
›Reveal solutionSolution
Free-nuclear endosperm forms many nuclei without immediate cell-wall formation; coconut "water" is liquid free-nuclear endosperm.
Endosperm development begins with repeated divisions of the primary endosperm nucleus (PEN, formed by triple fusion). In the more common type, free nuclear endosperm, the nucleus divides repeatedly, but no cell wall is laid down around the resulting nuclei for some time, so numerous free (naked) nuclei remain suspended in the common cytoplasm; only later does cellularisation usually occur towards the periphery. The liquid endosperm of a tender coconut (the "coconut water") is a well-known example of persistent free-nuc …
- CBSE 2023Set ANNUAL1 markMCQQ.What is the ploidy of the chromosomes in the endosperm of angiosperm plants?(a) n(b) 3n(c) 2n(d) both(a) and (b)
›Reveal solutionSolution
Angiosperm endosperm is triploid (3n).
In double fertilisation of angiosperms, one male gamete (n) fuses with the egg to form the diploid (2n) zygote, and the second male gamete (n) fuses with the two polar nuclei (n + n) of the central cell. This triple fusion produces t …
- CBSE 2022Set ANNUAL1 markMCQQ.Double fertilization occurs in -(a) Algae(b) Bryophyta(c) Gymnospermic Plants(d) Angiospermic Plants
›Reveal solutionSolution
Double fertilization is unique to flowering plants (angiosperms).
In double fertilization, one of the two male gametes delivered by the pollen tube fuses with the egg cell (syngamy) to form the diploid zygote, while the second male gamete fuses with the two polar nuclei (or the secondary nucleus) of the central cell to form the triploid primary endosperm nucleus (triple fusion). Because it involves both syngamy and tr …
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