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Pre-fertilization is the process that happens before real fertilization, i.e. the process that occurs before gamete fusion. Plants are capable of both sexual and asexual reproduction. Almost all blooming plants reproduce sexually. Flowers are important because they include both male and female reproductive components, known as androecium and gynoecium. Before fertilization, two events happen namely Gametogenesis, i.e, the formation of male and female gametes and Gamete transfer, which is bringing the male and female gametes together.
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Key Terms: Pre-Fertilization, Pollination, Pollen Grains, Gametogenesis, Gamete Transfer, Anther, Ovule, Zygote, Flowers, Gametes
Fertilization in Plants
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The process of amphimixis that occurs as a result of pollination and germination is known as fertilization in plants. Following carpal pollination, male gametes (pollen grains) are infused with feminine gametes (ovum) to produce a diploid zygote by a physicochemical process. The whole thing happens in a zygote, which eventually turns into a seed.
Pollinators transport male gametes into female reproductive organs during the procedure (butterflies, honey bees, birds, flower beetles, and bats,). As a result, an embryo forms throughout the seed's growth. Angiosperm flowers are also reproductive organs, and they reproduce uniquely.
Fertilization in Plants
Read More: Morphology of Flowering Plants
Gametogenesis
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It is the process of gamete formation. Inside the anther, male gametes are formed. Microsporogenesis and microgametogenesis are two of the phases. Megasporogenesis and megagametogenesis are the two processes involved in the formation of female gametes inside the ovules. The structures and procedures are described here.
Structures Involved in Gametogenesis
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The structures involved in the gametogenesis process are elaborated in detail below.
- Microsporangium: The Microsporangium is surrounded by the epidermis, middle layers, endothecium, and tapetum (Pollen sac). Pollen grains are formed in this area. On the plant, there were four pollen sacs. A part of each sac is made up of sporogenous tissues, which are a collection of compactly packed homogenous tissues.
- Ovules: Ovules are the sites where female gametes develop. These are located at the bottom of a carpel, which is a vase-like structure with a style neck and a stigma opening at the top.
- Pollen Grains: Pollen Grains are available in a range of sizes. It has two walls: a cellulose-based inner wall called endospore and a sporopollenin-based outer wall.
- Anther: The anther is a two-lobed structure. The strip of the sterile section is connected to the two lobes.
- Stamen: The stamen's filament is the stalk. The thalamus, which is a flower petal, is linked to the filament's proximal end.
Structure of Flower in Sexual Reproduction
Read More: Flower Structure
Formation of Male Gametes
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The process of formation of Male Gametes is as follows:
Microsporogenesis
- Microsporogenesis is the first step in the production of microspores or pollen grains in the microsporangium, the pollen sac of the anther.
- Meiosis produces four haploid microspores from the diploid pollen mother cell.
- A pollen grain develops from each microspore.
Read More: Difference between Haploid and Diploid
Microgametogenesis
- The creation of a male gametophyte from a pollen grain is known as micro gametogenesis.
- Pollen grains undergo mitosis, which produces two types of cells: smaller reproductive cells and larger vegetative cells. Pollination takes happens in this area.
- The generative cells create two male gametes, whilst the vegetative cells produce pollen tubes.
Formation of Female Gametes
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The process of formation of Female Gametes is as follows:
Megasporogenesis
- The megaspore forms in the megasporangium. This process is known as megasporogenesis.
- The process takes place in the nucellus, a structure within the ovule.
- The diploid megasporocyte within the ovule undergoes meiosis, culminating in the creation of four haploid megaspores.
- Only one megaspore grows into a megagametophyte, while the other three dissolve.
Mega-gametogenesis
- The female gamete is produced by a fully functional megaspore. This process is known as megagametogenesis.
- The megaspore undergoes three rounds of mitosis before forming an embryo sac with eight nuclei.
- Each chalaza, micropyle, and the end had four nuclei at first. During fertilization, one of them from each pole merges in the centre to form a diploid nucleus.
- The three cells eventually dissolve at the chalaza end chalaza.
- The egg cell develops from one of the three cells at the micropyle. The last two secrete chemicals that guide the pollen tube.
Read More: Difference Between Pollination and Fertilization
Gamete Transfer
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Pollination is the transfer of pollen grains from one strong bloom to the stigma of another, and it is the same as reproduction. It's also possible that it'll show up in the same blossom. This method is the initial step in sexual reproduction in flowering plants. Pollen grains contain the male gamete, which is located in the anthers of the floral part.
Pollination
Read More: Difference Between Cross-Pollination and Self Pollination
Things to Remember
- Pre-fertilisation can be defined as the process that occurs before the fusion of gametes or actual fertilization.
- Two events take place before fertilization namely Gametogenesis (Formation of male and female gametes) and Gamete transfer (Bringing the male and female gametes together).
- Gametogenesis is the process of formation of male and female gametes.
- The male gametes are formed inside the anther and involve two major steps namely microsporogenesis and microgametogenesis in their formation.
- Similarly, the female gametes are formed inside the ovules and involve two steps namely megasporogenesis and megagametogenesis.
- Pollination is essential for the formation of seeds. In pollination, the pollen grains are transferred to the stigma from anther. They germinate on the stigma and reach the ovule through the pollen tube, where the male gametes are then released near the egg.
Sample Questions
Ques. What events lead to fertilization in a flower? (3 Marks)
Ans. A pollen tube extends from the pollen grain, through the style, and into the ovary when pollen grains settle on the stigma. The pollen grain's sperm cells go down the pollen tube and into the ovary, which houses the ovules. When one sperm cell fuses with the egg within an ovule, the process of fertilization occurs.
Ques. What is the difference between spermatogenesis and oogenesis? (3 Marks)
Ans. Male and female gametes are formed through the processes of spermatogenesis and oogenesis. Oogenesis is helpful in the creation of ova, whereas, on the other hand, spermatogenesis aids in the formation of sperm. When sperm and ova fuse with each other, a zygote is formed, which then develops into an embryo.
Ques. What is meant by syngamy and triple fusion? (3 Marks)
Ans. Syngamy is the fusion of male and female gametes during fertilization. Triple fusion refers to the sperm cell's fusion with the two polar nuclei during double fertilization and it is called triple fusion. It creates a zygote, which develops into an embryo. It generates endosperm cells, which mature into the endosperm.
Ques. Define Double Fertilisation. (3 Marks)
Ans. Only flowering plants undergo double fertilization, which is a one-of-a-kind process. One of the male gametes combines the nucleus of an egg cell to form the zygote after entering the synergid. When another male gamete interacts with the polar nuclei, the main endosperm nuclei are created. Because it requires both triple fusion and syngamy, this process is referred to as double fertilization.
Ques. What are Apomixis and Polyembryony? (3 Marks)
Ans. Apomixis is a sort of asexual reproduction in which seeds are produced without being fertilised. Asexual reproduction is used to simulate sexual reproduction in this case. Polyembryony is the process of producing many embryos from a single fertilised egg. It's a sexual reproduction method.
Ques. What is endosperm function? (3 Marks)
Ans. During seed development and germination, the endosperm plays a crucial role in promoting embryonic growth by delivering nutrients, protecting the embryo, and limiting embryo expansion by functioning as a mechanical barrier.
Ques. What are the steps of spermatogenesis? (3 Marks)
Ans. Spermatogenesis is divided into three stages:
- Meiosis, in which the number of chromosomes in each cell is decreased to half, or 23;
- Meiosis II, in which each haploid cell produces spermatids; and
- Spermiogenesis, in which each spermatid develops into a sperm cell with a head and tail.
Ques. How endosperm is formed? (3 Marks)
Ans. The zygote is formed when one sperm nucleus fertilises the egg cell, while the main endosperm cell is formed when the other sperm nucleus fuses with the binucleate central cell (the nucleus is often referred to as the triple fusion nucleus). The endosperm develops from the cell formed during double fertilisation.
Ques. What is meant by Polyembryony? (3 Marks)
Ans. Polyembryony is a situation in which a single fertilised egg produces two or more embryos, resulting in identical twins in humans. Polyembryony is a frequently occurring phenomenon in many plants and animal species. It happens in the nine-banded armadillo, which normally has four identical offspring.
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