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Sexual Reproduction in Flowering Plants

Sexual Reproduction

Don’t beautiful and colourful flowers on plants and trees always please us? Did you know the flowers are the sexual organs of the body and help it to reproduce sexually? Plants can be unisexual and bisexual depending on the parts they contain. Let us understand these parts for plant reproduction in this section.

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Parts of a Flower in Plant Reproduction

A bisexual flower typically contains the male and female parts in it. There are other supporting structures as well apart from the reproductive parts for sexual reproduction.

Sexual Reproduction

(Source: Instructables)

There are four main layers of the parts of a flower:

  • Calyx
  • Corolla
  • Androecium
  • Gynoecium

Calyx

It is a collection of sepals. The sepals are the green coloured small florets that are considered the first layers of the flower from the base. In some cases, the sepals have colour. They are called petaloid. Their main function is to protect the flower while it is still in the bud stage.

Corolla

This layer is a collection of petals. It is the second layer of the flower, superior to the calyx layer. The petals are the colourful part of a flower that helps to attract insects and birds to the flower to facilitate pollination.

Androecium

It is the third layer of the flower superior to the Corolla. This is a term given to the male parts for sexual reproduction of a plant. The androecium is made up of a collection of stamens. Each stamen has the following parts:

  • Anther- It is present at the tip of the filament. It is internally lobed. Pollen grains are inside the Anther Lobe.
  • Filament- It is a thin stalk-like structure that holds the anther

Gynoecium

A Gynoecium is a collection of carpels. It is the fourth layer of a flower. It has three parts:

  • Stigma- It is a small and sticky landing structure. The pollen grain from the same or different flower stick to it. This structure acts as a landing for the insects or birds that act as pollinating agents.
  • Style- It is a thin stalk-like structure that holds the stigma.
  • Ovary- It is the base of the style and contains the ovules which contain the female gametes.

Pollination and Fertilization in Plant Reproduction

The transfer of pollen grains from the anther of one flower to the stigma of the same or another flower is known as pollination. It can be caused by insects, birds, wind, water and animals including man. These are together called as pollinating agents.

Sexual Reproduction

(Source: googlegalaxyscience)

Types of Pollination

  • Self-Pollination: Self-Pollination is when the pollen of one flower transfers to the stigma of the same flower. Many flowers that are hermaphrodite see this kind of pollination. However, there are many advantages and disadvantages of this type of pollination. Many flowers have various mechanisms to prevent self-pollination or promote cross-pollination.
  • Cross-Pollination: Cross-Pollination is when the pollen of one flower transfers to the stigma of another flower. This type of pollination helps brings about genetic variation in the species and allow the plant to withstand changes in the environment better. Once the pollen has landed on the stigma of a flower, the pollen tube develops to transfer the pollen to the ovules which contain the female gamete.

Microsporogenesis results in the formation of Male Gametes and Megasporogenesis results in the formation of Female Gametes.

Microsporogenesis

  • The anthers contain the pollen mother cell (2n-diploid) that undergoes meiosis to form microspores.
  • Tetrad is the result of the microspore mother cell diving and the formation of 4 microspores.
  • The Anther releases the microspores/pollen grains when it is mature.

Megasporogenesis

Megasporangium are the Ovules. They are in the ovary and contain the female gametes. Megasporogenesis is the formation of megaspores from the megaspore mother cell (diploid). The resultant of the meiosis fo the megaspore mother cell is 4 haploid megaspores. Of the four cells that form, only one is functional while the other degenerate.

Double Fertilization happens in angiosperms. This is because the male gamete that enters the ovule has two nuclei. One of the male gametes fuses with the female gamete to form a diploid zygote whereas the other one forms a triploid endosperm by fusing with the diploid polar nuclei. The zygote divides to form the future plant whereas the endosperm provides nutrition to the developing embryo.

Sexual Reproduction

(Source: jagranjosh)

Events after Fertilization in Plant Reproduction

After fertilization, the ovary becomes the fruit and the ovules become the seeds. The other structures like the calyx, corolla and the remaining parts of the androecium and gynoecium degenerate or fall off.

Solved Examples for You

Question: Which of the following is a triploid structure?

  1. Microspore
  2. Megaspore
  3. Polar Nuclei
  4. Endosperm

Solution: Endosperm. The male gamete that enters the ovule has two nuclei. One of the nuclei fuses with haploid male gamete to form a diploid zygote. The second male nucleus fuses with the diploid polar nuclei to form the triploid endosperm.

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