Introduction
Like animals, in plants also the fusion of male and female gametes is needed to generate new offspring. And pollination is the external process that helps in the meeting of the two gametes inside the ovary. By the act of pollination, the pollen grains produced by the male part reach the stigma of the female reproductive part and germinate it to extend the pollen tube through the style up to the ovary. In this pollen tube, it reaches at the female gamete in the ovary and fertilizes it and completes the first step of sexual reproduction. Then this fertilized zygote grows into a seed inside the ovary which develops into the fruit of the plant. The seed is the medium of propagation for most of the plants in nature.
There are also some modes of propagation other than reproduction performed by plants such as budding cutting, vegetative propagation. Sexual reproduction in flowering plants requires both male and female gametes, and the process of such reproduction engages different parts of a flower. It also requires external pollinating vectors.
How Sexual reproduction takes place in flowering plants?
Flowers are the reproductive parts of a plant and contain both the male and female gametes. The male reproductive part has been termed as androecium in biology and the female reproductive part is termed as gynoecium. Both these parts are often found in a single flower and very rarely in two different flowers of a plant or two different plants together. The structure and various parts of both the reproductive parts is very crucial to understand and remember to understand the sexual reproduction in plants. The male organ consists of different parts such as filament, anther, and pollen grains. The male gametophytes are produced by the process of microsporogenesis. The female reproductive organ consists of stigma style and ovary. Ovary inside the flower is the container of ovum or eggs. And the ovum is produced by the process of megasporogenesis.
Reproductive organs of a plant
Androecium
The male reproductive organ in plants is the androecium. The main function of androecium is the production and storage of pollen.Unit of androecium is stamen. The androecium is made up of numerous stamens. A stamen has two parts – filament and another.
Filament- Filament resembles a stalk attached to a flower and provides support to another.
Anther- Anther is an oval-shaped pollen-producing part of a flower. Each anther has two chambers and they possess microsporangia. An anther consists of four microsporangia that lie at four corners.
Microsporogenesis
The cells of the microsporangium divided resulting in the growth of another. The formation of microspores or pollen grains inside the microsporangia by meiotic division is known as microsporogenesis. Microspores originate from microspore mother cells. Generally, the arrangement of microspores in a tetrad is tetrahedral or isobilateral. Microspores are responsible for the development of male gametophytes. The inner layer of the male gamete is made up of cellulose and pectose, and the outer layer is formed by sporopollenin.
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Gynoecium
The female reproductive organ in plants is the gynoecium. It undertakes the internal production of ovules which eventually produces an egg. The gynoecium has three parts – stigma, style and ovary.
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Stigma - Stigma is the anterior part of a carpel, where pollen germinates.
Style- Style is a tube-like filament connecting stigma and ovary. It helps in the passage of pollen from stigma to ovary.
Ovary- Ovary acts in a small chamber where ovules are produced. Gynoecium can be of various types-
Monocarpellary: It consists of one carpel.
Bicarpellary: It consists of two carpels.
Tricarpellary: It consists of three carpels.
Multicarpellary: It consists of many carpels.
The gynoecium can be either apocarpous or syncarpous. In apocarpous gynoecium, the carpels are free. Whereas, in syncarpous gynoecium, the carpels are fused together.
Megasporogenesis
Megasporogenesis means the formation of megaspores within the megasporangium. Megaspores originate from ovules by way of meiosis. It is these megaspores that lead to the formation of the female gametophyte. One megaspore converts into a gametophyte and the others eventually degenerate. The basal megaspore develops into the embryo sac. The male and female gametes developed from microsporogenesis and megasporogenesis respectively are the basic units of sexual reproduction in flowers.
Pollination
Pollination is an external process that includes the transfer of pollen grains from the anther to stigma. There are two types of pollination – self-pollination and cross-pollination.
1. Self-Pollination
Self-pollination occurs when the pollen grains transfer from the anther of a flower to the stigma of the same flower or genetically similar flower. It is mainly of two types-
Autogamy - In this type of pollination, the flower is pollinated by its own pollen. It can be occurred by three processes-
Homogamy- The flowers that exhibit this phenomenon expose their anthers and stigmas. It can be observed in Mirabilis.
Cleistogamy - The cleistogamous flowers are closed and thus their anthers and stigmas are not exposed. It can be observed in Balsam, Oxalis.
Geitonogamy- In this type of pollination, the pollen grains from one flower are transferred to the stigma of another flower that is either genetically similar or the same plant.
2. Cross-Pollination
Cross-pollination can be described as the pollination in which the pollen grains are transferred from the anther of one flower to the stigma of a genetically different plant. It generally takes place with the help of pollinating agents such as wind pollination (Anemophily), water pollination (Hydrophily), insect pollination (Entomophily), bird pollination (Ornithophily), bat pollination (Chiropterophily), etc.
Fertilization
takes place after pollination where the pollen grains reach the ovary. Here, the male gamete fuses with the female gamete, ovule, to form a zygote. The ovary turns into a fruit, and fertilized ovules convert to seed. In angiosperms, two acts of fertilization occur known as double fertilization.
Do You Know?
Reproduction can also take place by way of fragmentation. The fragmented segments of the plant body give rise to a newer organism. Plants are usually artificially harnessed in horticulture. Thereby, different parts of the plant such as branches, roots or stems are cut for reproduction to culture a desirable variety of plants. This process is significantly different from the natural flower reproductive system.
Test Yourself
i. The female gametophyte is represented by which of the following in angiosperms?
(a) Embryo sac
(b) Embryo
(c) Endosperm
(d) Synergid
ii. In absence of ovary fertilization, the development of fruit is known as -
(a) Parthenogenesis
(b) Agamospermy
(c) Parthenocarpy
(d) Apomixis
Solutions
i. (a) Embryo sac
ii. (c) Parthenocarpy
FAQs on Sexual Reproduction in Flowering Plants
1. Mention Various Stages of Plant Life-Cycle.
The different stages of plant life cycle are – seed stage, germination, growth, reproduction, pollination, dispersal of seeds. In addition, the reproductive cycle of flowering plants includes – pollen on stigma, growth of pollen tube and male gamete fusing with female gamete.
2. What are the Different Types of Pollination?
Pollination is the initial stage of flower reproduction. Pollination can be of two types – self-pollination and cross-pollination. Self-pollination involves a single flower, whereas cross-pollination requires a vector in the form of birds, bees, wind etc.
3. What are the main components of the reproductive structure of a plant?
The main components of the reproductive structure of a plant are – sepals, petals, carpels, and stamen. Stamens form male reproductive organs and carpels from the female reproductive organ are also essential.
4. State two differences between apocarpous and syncarpous gynoecium.
Apocarpous gynoecium | Syncarpous gynoecium |
i. In this type of gynoecium, the carpels are separate from each other. | i. In this type of gynoecium, carpels are fused. |
ii. The ovary is always unilocular. | ii. Ovary is either unilocular or multilocular. |
5. What are the different types of pollination observed in plants?
Pollination is the transfer of male gametes to reach the female gametes for fertilizing it. This is assisted by various natural factors such as wind and water that carry the pollen grains. Pollen grains are so small in size that they easily get lifted by the blowing wind or flowing water and travel long distances to reach the gametes. Sometimes they also stick to the body of insects that come near the flowers for nectar. If the pollen grains reach the stigma in the same flower or another flower on the same plant then it is known as self-pollination. contrarily if pollen grains reach the stigma in a flower of another plant then it is known as cross-pollination.
6. Is Fertilisation always required for the development of fruits?
In general terms, fruit is the protective organ of the seeds. In most cases, without seed formation, there is no development of fruit and without flower, there is no seed. But in some plants, it is known that some parts of the plant get modified by the deposition of starch or the food produced by the green parts of the plant. And we also use the storage parts of plants like fruits and vegetables. So it would not be wrong to say that fertilization is not the only method to develop fruits in plants.
7. What are all the other functions performed by plants?
It is very well known that plants are also living creatures inhabiting the earth like animals. Though they lack the ability to move around like animals but carry out all the other functions essential for sustaining their life. Their roots remaining penetrated inside the ground absorb water and nutrients required for their nutrition and produce glucose as their food. They also respite like animals and intake oxygen through the stomata of the leaves to break down the glucose and obtain energy.
8. Is it ok to complete only the previous year questions of this chapter if I am going to appear for the board exams?
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