What is Fertilisation?
Fertilisation in plants is the joining (fusion) of male and female gametes to form a diploid zygote. This process is a crucial part of sexual reproduction and usually occurs after pollination has successfully transferred pollen grains from the anther (male part) to the stigma (female part). In flowering plants (angiosperms), fertilisation was first discovered by Ralph B. Strasburger in 1884.
Fertilisation Process in Plants
Pollination: In many flowering species, pollination in plants is carried out by animals such as honeybees, birds, bats, and butterflies. Wind and water can also serve as natural pollinating agents. Once the pollen grain lands on the stigma, it starts to germinate.
Pollen Tube Formation: After germination, the pollen grain develops a pollen tube, which grows through the style to reach the ovary.
Entry into the Ovule: The pollen tube enters the ovule through a small opening called the micropyle. Upon reaching the embryo sac, the pollen tube bursts, allowing the male nucleus to be released.
Fusion of Gametes: One male gamete fuses with the egg (female gamete), forming the diploid zygote. This zygote later divides and develops into an embryo.
Seed Formation: After fertilisation, the ovule matures into a seed, which protects the growing embryo. Eventually, seed dispersal happens through various agents like wind, water, or animals, ensuring that plants can colonise new habitats.
Types of Fertilisation
Depending on how the pollen tube enters the ovule, fertilisation can be divided into three types:
Porogamy:
The most common type in angiosperms.
The pollen tube enters the ovule directly through the micropyle.
Chalazogamy:
Observed in plants belonging to the genus Casuarina.
The pollen tube enters the ovule through the chalaza (the region opposite the micropyle).
Mesogamy:
Commonly seen in certain Cucurbit plants (such as pumpkins, ridge gourds, and bitter gourds).
The pollen tube enters the ovule through its middle region or through the integuments.
Double Fertilisation
Double fertilisation is unique to angiosperms. In this mechanism:
One male gamete fuses with the egg cell to form the zygote.
The other male gamete unites with two polar nuclei in the embryo sac to form the triploid endosperm.
The endosperm provides essential nutrients to the developing embryo, making double fertilisation highly efficient for plant growth and reproduction.
Significance of Fertilisation
Genetic Variation: Since fertilisation involves the fusion of male and female gametes, it introduces genetic diversity in the offspring.
Seed and Fruit Formation: Successful fertilisation leads to the formation of seeds and often the development of fruit around the seed.
Continuation of the Species: By producing new seeds that undergo seed dispersal, plants can spread to different regions and maintain their populations over generations.
Efficient Nutrient Use: Double fertilisation ensures that endosperm (the food source) is only created after the egg is fertilised, preventing wastage of resources.
Quick Quiz (With Answers)
Question: Which scientist discovered fertilisation in flowering plants in 1884?
Answer: Ralph B. Strasburger
Question: In double fertilisation, which cell fuses with the two polar nuclei?
Answer: The second male gamete (sperm cell)
Question: What is the role of endosperm in seed development?
Answer: It provides essential nutrients to the developing embryo.
Related Topics
FAQs on Fertilisation in Plants
1. What is fertilisation in plants?
Fertilisation in plants is the biological process involving the fusion of male and female gametes to form a diploid zygote. This crucial event in sexual reproduction occurs after pollination, where pollen grains are transferred to the stigma. The zygote then develops into an embryo, which is contained within a seed.
2. What are the main steps in the process of plant fertilisation?
The fertilisation process in flowering plants follows a clear sequence:
- Pollination: The transfer of pollen from the anther to the stigma.
- Pollen Germination: The pollen grain forms a pollen tube that grows down through the style towards the ovary.
- Entry into Ovule: The pollen tube enters the ovule, typically through an opening called the micropyle.
- Fusion of Gametes: One male gamete fuses with the egg cell (syngamy) to form the zygote, while the other male gamete fuses with the polar nuclei to form the endosperm.
3. What is the importance of double fertilisation in flowering plants (angiosperms)?
Double fertilisation is a unique and highly efficient process in angiosperms. Its primary importance lies in the formation of two key structures: the diploid zygote (which develops into the embryo) and the triploid endosperm (a nutrient-rich tissue). This ensures that the food supply for the embryo is created only after the egg has been successfully fertilised, preventing the plant from wasting valuable resources.
4. How is fertilisation in plants different from fertilisation in animals?
There are several key differences. Plant fertilisation requires a preliminary step called pollination, often relying on external agents like wind or insects, whereas animal fertilisation usually involves direct physical contact or mating. Furthermore, flowering plants undergo double fertilisation to form both an embryo and a nutritive endosperm, a process not found in the animal kingdom.
5. What are the different types of fertilisation based on the pollen tube's entry into the ovule?
Based on the path of the pollen tube, fertilisation is classified into three types:
- Porogamy: The pollen tube enters the ovule through the micropyle. This is the most common type.
- Chalazogamy: The pollen tube enters through the chalaza, the basal part of the ovule.
- Mesogamy: The pollen tube penetrates the ovule through its middle part by piercing the integuments.
6. Is there a difference between pollination and fertilisation?
Yes, they are two distinct but related processes. Pollination is the physical transfer of pollen grains from the male anther to the female stigma. Fertilisation is the subsequent biological event where the male gamete from the pollen grain fuses with the female egg cell inside the ovule. Therefore, pollination is a necessary prerequisite for fertilisation to occur in most plants.
7. Why is double fertilisation considered an evolutionary advantage for angiosperms?
Double fertilisation is a major evolutionary advantage because it is extremely resource-efficient. By forming the nutritive tissue (endosperm) only after the egg is successfully fertilised, the plant avoids investing energy and nutrients into an ovule that will not develop into a viable seed. This conservation of resources allows the plant to support the development of more successful seeds, enhancing its reproductive success.
8. How does the pollen tube 'know' where to grow to reach the ovule?
The growth of the pollen tube is not random; it is a guided process known as chemotropism. The synergid cells, located near the egg cell within the ovule, secrete specific chemical signals. The pollen tube detects these chemicals and grows directionally towards them, ensuring it reaches the micropyle to deliver the male gametes for fertilisation.
9. What happens to the different parts of a flower after fertilisation is complete?
After successful fertilisation, significant changes occur in the flower as it transitions to producing a fruit and seed. The ovule develops into the seed, and the ovary wall matures into the fruit (pericarp) that encloses the seed(s). Other floral parts, such as the petals, sepals, stamens, and style, typically wither and fall off as they are no longer needed.
