Types and Processes of Fertilisation in Plants
Plants substantiate their name as primary or universal producers. Providing us humans with life essentials such as clothing, shelter, food, and oxygen, they are indeed nature’s gift. As in the case of all other living beings, plants respire, vegetate, germinate, defecate and produce. In 1884, it was Ralph B. Strassburger who first discovered the fertilisation process in flowering plants. Here, let us talk in detail regarding the fertilisation process in plants.
What is Fertilisation?
Fertilization in plants is a process in which they undergo sexual reproduction in consequence of pollination and germination. In a physicochemical process, the male gametes (known as pollen) fuse with the female gametes (known as ovum) form diploid zygote after carpal pollinates. The entire process happens in the zygote which later germinates into a seed. During the process, what happens is male gametes are transferred into female reproductive organs through pollinators (butterflies, birds, honey bees, bats, and flower beetles). This results in an embryo being formed in a seed. Flowers are the reproductive organs of angiosperms and reproduce in an entirely different method.
The process of fertilisation in plants can be categorised into four major stages.
Pollination: In this, the male gametes covered in pollen are carried away by water, wind, or through other agents such as animals and insects. Then, the pollen is made to land on the stigma of the plant. This process is called pollination.
Germination: This is a process in which the pollen tube matures in a matter of minutes and enters the egg cell. The tube serves as a passage for the pollen grain to reach the egg.
Ovule Penetration: The pollen tube which contains the sperm penetrates into the embryo sag or ovule.
Fertilisation: After successfully penetrating into the ovule, the sperms travel through the tube to fertilise the egg. Most flower-bearing plants undergo double fertilisation in which the polar nuclei and egg in the ovule are fertilised.
Post fertilisation, the zygote cells divide and develop into an embryo which can also be termed as a developing plant. The embryo enveloped in a seed capsule lay dormant till favourable environmental conditions prevail to help it develop into a plant.
Fertilisation in Seed Plants
During the process of fertilisation in gymnosperms or seed-bearing plants, the seeds grow on both sides of the leaves or scales or develop cones or prevail at the end of short stalks. The seed remains confined in the ovary.
Fertilisation in Flower Bearing Plants
In flower-bearing plants or angiosperms, an additional fertilisation process occurs in the central cell. Two sperm cells get released from either one of the pollen grains. The first cell fuses with the zygote for egg fertilisation and the second fuses with the twin polar nuclei to produce endosperm.
Process of Fertilisation
As we said above, fertilisation happens as a result of the fusion of female and male gametes. For fusion to take place, both the female and male pollen grains reach the stigma's surface. This process is termed pollination. Once the pollen is transferred to the stigma, it undergoes germination, passing through the style to enter the ovule. The pollen or the microspores has two cells namely generative cell and pollen tube cell. This pollen tube cell develops a pollen tube by which generative cells navigate.
For pollen tubes to germinate, it requires oxygen, water, and several other chemical signals. While pollen passes through style to reach the ovule or embryo sac, it is the tissue of the style which supports the growth of the pollen tube. During the entire process, in case the generative cell has not yet ruptured into two cells, it breaks down into two sperm cells. The pollen is aided by the chemical secretion of synergids in the embryo sac as it penetrates into the ovule through the micropyle.
One sperm cell from the sperm cells germinates the egg cell to form a diploid zygote. The remaining sperm cells undergo fusion with the polar nuclei to form a triploid cell which produces an endosperm. These twin fertilisation events occurring in angiosperms or flowering plants are termed double fertilisation. Once the fertilisation process is over, it is not possible for another sperm to enter. The fertilised embryo sac produces the seed while the ovary’s tissues become the fruit that envelops the seed.
Self-Fertilisation
The process in which gametes of two different species – male and female cross-fertilise is known as allogamy. There is also autogamy or self-fertilisation when dual gametes from a single individual fuse. This is commonly seen in hermaphrodites such as certain plants and flatworms.
Types of Fertilisation
There are three types of fertilisation, categorised mainly on the way in which pollen tubes enter the ovule or embryo sac.
Porogamy
The most common form of fertilisation happens in flowering plants or angiosperms. In this form, a pollen tube reaches the ovule through a micropyle. A common example is lilies.
Chalazogamy
This form of fertilisation happens in a species of plants namely casuarina. Here, the pollen tube permeates into the ovule through chalaza rather than entering through micropyle. This is not very common. Examples would be Casuarina, Betula and Juglans.
Mesogamy
This form of fertilisation happens in cucurbit plants including bitter gourd, ridge gourd, various other gourd plants and pumpkin. In this form, a pollen tube passes through integuments or its middle part to penetrate into the ovule.
Double Fertilisation
Double Fertilisation happens when a single female gamete undergoes fusion with two male gametes. Here, one of the sperm cells undergo fusion with the zygote while the other fuse with the two nuclei to produce endosperm. The entire angiosperm plants go through the process of double fertilisation.
What Makes Double Fertilisation Significant is
In double fertilisation, two products are formed
Increases the feasibility of the seeds of the angiosperm
The plant will have a greater chance to survive and polyembryony (the chance to form more than a single embryo from one seed or a single fertile ovum)
Double fertilisation results in an endosperm which nourishes the growing embryo
The two male gametes which are produced by pollen grains into effective use.
These are the different types of fertilization in plants and their processes. Understand the concept of fertilization to know the difference between these types of fertilization.
FAQs on Fertilisation in Plants
1. What is fertilisation in the context of flowering plants?
In flowering plants (angiosperms), fertilisation is a core process of sexual reproduction that occurs after successful pollination. It involves the fusion of male and female gametes to form a diploid zygote. Specifically, a male gamete carried within the pollen tube fuses with the female gamete, or the egg cell, which is located inside the ovule.
2. What are the main steps in the process of plant fertilisation?
The process of fertilisation in plants unfolds in a series of sequential steps following pollination:
- Pollen Germination: After landing on a compatible stigma, the pollen grain absorbs moisture and nutrients, germinating to form a pollen tube.
- Pollen Tube Growth: The pollen tube grows down through the style, guided by chemical signals from the ovule, creating a pathway for the male gametes.
- Entry into Ovule: The pollen tube enters the ovule, typically through an opening called the micropyle.
- Gamete Discharge and Fusion: The pollen tube releases two male gametes into the embryo sac. One gamete fuses with the egg cell (syngamy), and the other fuses with the central cell, completing the process of double fertilisation.
3. What is double fertilisation and which plants exhibit it?
Double fertilisation is a complex mechanism unique to all flowering plants (angiosperms). It involves two separate fusion events happening simultaneously within the embryo sac. These are:
- Syngamy: The fusion of the first male gamete (n) with the egg cell (n) to form the diploid zygote (2n), which develops into the embryo.
- Triple Fusion: The fusion of the second male gamete (n) with the two polar nuclei (n+n) in the central cell to form a triploid Primary Endosperm Nucleus (PEN, 3n), which develops into the nutritive endosperm.
4. Why is the endosperm resulting from double fertilisation triploid?
The endosperm is triploid because it is formed from the fusion of three haploid nuclei. During triple fusion, one haploid (n) male gamete fuses with the central cell of the embryo sac, which already contains two haploid (n) polar nuclei. The resulting Primary Endosperm Nucleus (PEN) is therefore triploid (n + n + n = 3n), giving it a unique genetic makeup that is distinct from the embryo (2n).
5. What is the main purpose or significance of double fertilisation?
The primary significance of double fertilisation is that it ensures the developing embryo has a dedicated food supply. The endosperm, which provides nourishment, only begins to develop after the egg is successfully fertilised. This is a highly efficient mechanism that prevents the plant from wasting resources on nourishing an ovule that has not been fertilised. It also increases the viability of the seed and restores the diploid condition in the zygote.
6. What are the different ways a pollen tube can enter an ovule?
Based on the point of entry into the ovule, fertilisation can be of three types:
- Porogamy: This is the most common method, where the pollen tube enters the ovule through the micropyle (a small opening in the ovule).
- Chalazogamy: In this type, the pollen tube enters through the basal, or chalazal, end of the ovule. This is seen in plants like Casuarina.
- Mesogamy: Here, the pollen tube penetrates the ovule by piercing through its integuments (outer layers). This is observed in plants like Cucurbita.
7. What are the major changes that occur in a flower after fertilisation is complete?
After successful fertilisation, the flower undergoes significant transformations known as post-fertilisation changes to facilitate seed and fruit development. Key changes include:
- The zygote develops into an embryo.
- The ovules mature into seeds.
- The ovary develops and ripens into a fruit, which encloses the seed(s).
- The petals, sepals, stamens, style, and stigma typically wither and fall off as they are no longer needed.
8. How does a flower ensure it is fertilised by pollen from the correct species?
Plants have a sophisticated recognition system called pollen-pistil interaction. This is a chemical dialogue between the pollen grain and the surface of the pistil (the stigma). The pistil has the ability to recognise and accept compatible pollen from the same species and reject incompatible pollen, whether from a different species or, in some cases, from the same plant to prevent self-fertilisation. If the pollen is recognised as compatible, the pistil promotes its germination and the growth of the pollen tube.
