Fertilisation in Flowering Plants

Fertilisation in Flowering Plants: Fertilisation is one of the essential characteristics of most organisms, irrespective of their size, shape, and level of organization. Fertilisation is very significant for the continuation of the generation of a particular species. Ralph B. Strasburger discovered fertilisation in flowering plants in 1884, and he also gave the idea of double fertilisation. Fertilisation in plants occurs after pollination, in which the fusion of the male and the female gamete takes place to form a diploid zygote. Read on to explore more about events leading to fertilisation in flowering plants and the fate of floral parts after fertilisation.

Fertilisation in Flowering Plants: Overview

In the fertilisation process, flowers play a significant role as they contain the male and female reproductive structures called stamen and pistil that produce the haploid male and female gamete. 

Fig: Structure of Flower

In angiosperms, fertilisation takes place after pollination, i.e., the transfer of pollen grains from anther to the stigma of the same or the different flowers but the same species. This transfer of pollen can take place naturally or with the help of pollinating agents like honey bees, butterflies, birds, wind, water, etc.

Fig: Flower

Events leading to Fertilisation in Angiosperms

The following events lead to the fertilisation in Angiosperms:

Development of Pollen grains and Male Gametes:

Each anther contains four pollen sacs in which the pollen grains are produced. The pollen grain contains the male gametes. So, to begin the fertilisation process, the pollen grains have to get transferred to the stigma of the same plant species. 

When the degenerate pollen nucleus divides into two nuclei: the smaller generative nucleus (male nucleus) and the larger tube nucleus (vegetative nucleus). In over 60percent of angiosperms, the pollen grains are transferred to the stigma at this two-celled stage.

Fig: Pollen Grain

Development of an Embryo Sac

An ovary may contain one or many ovules. Each ovule contains an embryo sac mother cell that is a large diploid cell. It undergoes meiotic division and gives rise to four haploid nuclei, out of which only one remains functional and the other three degenerate. The functional haploid nucleus grows and undergoes mitotic divisions to produce eight haploid nuclei. The structure formed is known as the embryo sac or female gametophyte.

An embryo sac contains seven celled or eight haploid nuclei, out of which three nuclei are present at the micropylar end called egg apparatus (One egg cell present at the middle and the other two cells present on either side of the egg are called synergids). The synergids have special thickenings at the micropylar tip called filiform apparatus. Another three nuclei are present at the chalazal end of the ovule (present opposite to the micropylar end), known as antipodal cells. The remaining two haploid nuclei are present at the centre of the embryo sac known as polar nuclei, which ultimately fuse at fertilisation to form a single diploid nucleus.

Fig: Development of an Embryosac

Pollen Germination

After the pollen grains land on the stigma, the events that lead to pollen germination are as follows:

• If stigma is receptive and the pollen lands on its compatible species, it begins to germinate.
• The intine (inner integument of a pollen grain) and its contents emerge from the germ pore as a short cytoplasmic outgrowth called germ tube and grew during degeneration of a pollen tube.
• The pollen tube is directed towards the ovary by chemicals, calcium-boron-inositol sugar complex and hence is called positively chemotropic and negatively aerotropic (grows away from the air).
• The sucrose solution secreted by the epidermal cells of the stigma acts as the stimulus for the formation of the pollen tube and provides nutrition.
• The pollen tube produces certain enzymes that digest the tissues of the stigma and style and enables the passage of the pollen tube to the ovule.
• The pollen tube has two nuclei – the vegetative or tube nucleus and the generative nucleus.
• The tube nucleus present at the tip of the pollen tube helps direct the pollen tube’s growth and ultimately disintegrates.
• The generative nucleus divides mitotically to produce two male nuclei called male gametes.
• The pollen tube grows and enters the ovule through a small opening called the micropyle.
• The pollen tube enters into one of the synergids through the filiform apparatus that guides the entry of the pollen tube.

Double Fertilisation in Angiosperms

• After entering the embryo sac, one of the synergids disintegrates.
• The pollen tube releases the two male gametes into the cytoplasm of the synergids.
• First fertilisation: One of the male gametes (n) fuses with the egg nucleus (n) to form the diploid zygote (2n). This process is the first fertilisation process or also known as syngamy.
• Second fertilisation: The other male gamete moves to the central cell. It fuses with the two polar nuclei or the secondary diploid nucleus (formed by the fusion of 2 polar nuclei). It forms a triploid nucleus called the primary endosperm nucleus (PEN). This process is known as the second fertilisation process or triple fusion.
• Since two sets of fertilisation occur, that is syngamy and triple fusion, and it is called double fertilisation. It is unique to flowering plants. 
• It was discovered by the Russian scientist S.G. Nawaschin in 1898.

Fig: Pollination and Fertilisation

Events after Double Fertilisation

• The diploid zygote (2n) divides mitotically to form the seed’s embryo.
• The triploid nucleus (3n) or primary endosperm nucleus (PEN) gives rise to a mass of tissue that develops into the seed’s endosperm. The endosperm provides nourishment to the growing embryo.

What is the Fate of Floral Parts After Fertilisation?

After the process of fertilisation is complete, the fate of sepals, petals, stamens and carpels are as follows:

• The petals, stamens, style, and stigma wither and generally fall off.
• The calyx may either fall off or remain intact in a dried and shrivelled form like tomato, guava, etc.
• The ovary enlarges to form fruit, and the ovary wall includes the fruit wall. The ovary wall may form a dry and hard fruit wall or a fleshy fruit wall.
• Ovules form the seeds.

Summary

 Fertilisation is very significant for the continuation of the generation of a particular species. In the fertilisation process, flowers play a significant role as they contain the male and female reproductive structures called stamen and pistil that produce the haploid male and female gamete. In flowering plants, fertilisation takes place twice. Firstly, one of the male gametes (n) fuses with the egg nucleus (n) to form the diploid zygote (2n). The other male gamete moves to the central cell. It fuses with the two polar nuclei or the secondary diploid nucleus (formed by the fusion of 2 polar nuclei). It forms a triploid nucleus called the primary endosperm nucleus (PEN). This process is known as the second fertilisation process or triple fusion. Since two sets of fertilisation occur, that is syngamy and triple fusion, and it is called double fertilisation. It is unique to flowering plants. 

FAQs

Q.1: What is double fertilisation?
Ans: In flowering plants, fertilisation takes place twice. Firstly, one sperm nucleus (n) fuses with the egg nucleus (n) to form the diploid zygote (2n). Secondly, the other sperm nucleus (n) fuses with the two polar nuclei (2n) present centrally in the embryo sac to produce the primary endosperm nucleus (PEN). So, this process is known as double fertilisation.

Q.2: What is triple fusion in plants?
Ans: Triple fusion is defined as the fusion of one haploid sperm nucleus (n) with the two polar nuclei (2n) present centrally in the embryo sac to produce the primary endosperm nucleus (3n). This process involves the fusion of three haploid nuclei, so it is known as triple fusion.

Q.3: Which regions of pistil form fruits and seeds?
Ans: The ovary develops into a fruit and the ovule develops into the seed after fertilisation.

Q.4: In which part of the flower does fertilisation occur?
Ans: The ovary is the part of the flower where fertilisation takes place.

Q. 5: Which part of gynoecium determines the compatible nature of pollen grain?
Ans: Stigma is the terminal receptive part of the pistil which functions as a landing platform for the pollen grains and determines the compatible nature of pollen grain.

Learn About Pollination Here

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