How Does Double Fertilization Work in Angiosperms?
Double fertilisation is a special form of fertilisation that occurs in flowering plants (angiosperms). It involves the fusion of two male gametes (sperm cells) with different female reproductive components. This process ensures that both a zygote and an endosperm are formed, which are essential for the development of seeds.
In flowering plants, also known as angiosperms, reproduction involves a fascinating process called double fertilisation. This unique and crucial event helps plants develop seeds and fruits, ensuring the next generation of plants. In this article, we will break down the process of double fertilisation, its significance in plants, and why it is essential for the growth and development of seeds and fruits.
Key Steps in Double Fertilisation:
Pollination: Pollen grains carrying male gametes land on the stigma of the female flower.
Pollen Tube Formation: The male gametes travel through the style and enter the ovary.
Fertilisation: One sperm fertilises the egg cell, forming a zygote (2n). The second sperm fuses with two polar nuclei, forming the endosperm (3n).
This two-step fertilisation process is what we call double fertilisation in angiosperms. The zygote develops into an embryo, and the endosperm serves as a food reserve for the developing embryo.
Also Read: Pollination
The Process of Double Fertilisation in Angiosperms
Let's look at the step-by-step process of double fertilisation in plants:
Pollination: The first step in fertilisation is pollination, where pollen grains from the male part (anthers) are transferred to the female part (stigma) of the flower.
Germination of Pollen Grain: After pollination, the pollen grain germinates, forming a pollen tube that grows down the style toward the ovary. The pollen tube carries two sperm cells.
Fertilisation:
Syngamy: One of the sperm cells fuses with the egg cell in the ovule, forming a zygote.
Triple Fusion: The second sperm cell fuses with two polar nuclei in the central cell, forming the primary endosperm nucleus (PEN), a triploid (3n) structure.
Zygote and Endosperm Development: The zygote will develop into the embryo, while the PEN becomes the endosperm, nourishing the growing embryo inside the seed.
Significance of Double Fertilisation in Angiosperms
Double fertilisation plays a vital role in the reproductive success of flowering plants. Here are the key reasons why it is significant:
Efficient Resource Allocation: Double fertilisation ensures that the resources are used optimally for seed development. The formation of both a zygote and an endosperm ensures the embryo gets nourishment as it grows.
Seed Viability: The endosperm created during the process provides essential nutrients to the developing embryo, increasing seed viability and the plant's chance of survival.
Development of Fruits and Seeds: The fertilised ovule develops into a seed, and the ovary forms a fruit, both crucial for the plant's reproduction and dispersal.
Polyembryony: In some cases, double fertilisation may result in polyembryony, where multiple embryos are formed, increasing the plant's chances of successful reproduction.
Double Fertilisation Diagram
Here’s a simple double fertilisation diagram to help you visualise the process:
Pollen grain lands on the stigma.
The pollen tube grows through the style and into the ovary.
One sperm cell fertilises the egg cell, forming the zygote.
The second sperm cell fuses with the polar nuclei, forming the endosperm.
Unique Aspects of Double Fertilisation
Besides fertilising the egg to form the zygote and the polar nuclei to form the endosperm, double fertilisation has some unique aspects that are not seen in many other plant species, such as gymnosperms (conifers). The fusion of two male gametes ensures that both the embryo and the nutritive tissue (endosperm) are formed in the seed, which is vital for the proper growth of the seedlings after germination.
Conclusion
Double fertilisation in angiosperms is a fascinating process that ensures the development of viable seeds and fruits, ultimately leading to the successful reproduction of plants. Understanding its process, significance, and the formation of both zygote and endosperm helps us appreciate the intricacies of plant biology.
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FAQs on Double Fertilization in Angiosperms: Complete Guide for Students
1. What is meant by double fertilisation in angiosperms as per the CBSE Class 12 syllabus?
Double fertilisation is a complex reproductive process unique to flowering plants (angiosperms). It involves two distinct fusion events occurring within the ovule. One male gamete fuses with the egg cell to form the zygote (2n), while the second male gamete fuses with the two polar nuclei in the central cell to form the primary endosperm nucleus (PEN) (3n). This dual fusion ensures the simultaneous development of both the embryo and its nutritive tissue.
2. What are the main steps involved in the process of double fertilisation?
The process of double fertilisation follows the germination of the pollen grain on the stigma. The key steps are:
- Pollen Tube Growth: A pollen tube grows down through the style, carrying two male gametes, and enters the ovule through the micropyle.
- Syngamy: The first fusion event, where one male gamete (n) fuses with the egg cell (n) to form a diploid zygote (2n), which develops into the embryo.
- Triple Fusion: The second fusion event, where the other male gamete (n) fuses with the two polar nuclei (n+n) located in the central cell. This results in the formation of a triploid (3n) primary endosperm nucleus (PEN).
3. What is the biological significance or importance of double fertilisation in flowering plants?
Double fertilisation is highly significant for the survival and success of angiosperms for several reasons:
- It ensures the formation of nutritive tissue (endosperm) only after fertilisation is successful, preventing the plant from wasting resources on unfertilised ovules.
- The triploid endosperm provides rich nourishment to the developing embryo, increasing the viability of the seed and the chances of successful germination.
- This process is a key evolutionary adaptation that has contributed to the dominance of angiosperms in terrestrial ecosystems.
4. What are the final products formed as a result of double fertilisation?
The two primary end products of double fertilisation are the diploid zygote (2n) and the triploid primary endosperm nucleus (PEN) (3n). The zygote subsequently develops into the embryo, which is the future plant. The PEN develops into the endosperm, a food-storing tissue that nourishes the embryo during its development and germination.
5. Why is the process in angiosperms specifically called 'double' fertilisation?
It is called 'double' fertilisation because two separate fusion events, involving both male gametes from a single pollen tube, occur almost simultaneously inside the embryo sac. The first event is syngamy (fertilisation of the egg), and the second is triple fusion (fertilisation of the central cell). Since two fertilisation events happen, the process is aptly named double fertilisation.
6. How does double fertilisation in angiosperms differ from fertilisation in gymnosperms?
The key difference is that gymnosperms do not undergo double fertilisation. In gymnosperms, only one male gamete fuses with the egg cell to form a zygote. There is no second fusion event to form an endosperm. The nutritive tissue in gymnosperm seeds, known as the female gametophyte, is haploid (n) and develops before fertilisation, whereas the endosperm in angiosperms is triploid (3n) and develops only after fertilisation.
7. What would be the consequence if triple fusion failed to occur in an angiosperm, but syngamy was successful?
If syngamy occurred but triple fusion failed, a viable zygote (2n) would still form. However, the primary endosperm nucleus (PEN) would not be formed. Without the development of the endosperm, the embryo would lack the necessary nourishment to grow and mature. This would ultimately lead to the formation of a non-viable, empty seed that would fail to germinate, even though fertilisation of the egg was successful.
8. What is the advantage of the endosperm being triploid (3n) instead of diploid (2n)?
The triploid (3n) nature of the endosperm is believed to provide an enhanced level of genetic information and metabolic potential, making it a highly efficient and rich source of nutrients for the developing embryo. This phenomenon, known as the hybrid vigour effect at a cellular level, contributes to increased seed viability and robust seedling growth. The combination of genetic material from two maternal polar nuclei and one paternal gamete creates a highly nutritive tissue perfectly suited for the embryo.
