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Sexual Reproduction in Flowering Plants

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  • Last Updated : 28 Nov, 2022
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Flowering plants reproduce by sexual and asexual means. The process of sexual reproduction is as follows: Pre-fertilization-which consists of Gametogenesis and Gamete transfer or pollination, Fertilisation- Syngamy + triple fusion, and Post fertilisation-Which consists of endosperm and embryo development. Maturation of ovule into seed

 

Pre-fertilization 

Structures and events: Flowers are the sites of sexual reproduction. In a flower, the androecium represents the male reproductive system and the Gynoecium represents the female reproductive part.

Androecium

  • The androecium consists of whorls of stamens. A stamen consists of the filament (long and slender stalk) and anther (the terminal bilobed structure). Each lobe is dithecous (2 thecae). Theca is separated by a longitudinal groove running lengthwise.
  • The anther is a tetragonal structure consisting of 4 microsporangia/microsporangium located at the corners 2 in each lobe.
  • The microsporangia develop further and become pollen sacs that are packed with pollen grain.

Structure of Microsporangium

  • The Microsporangium is surrounded by four wall layers, which are the epidermis, endothecium, middle layers, and inner tapetum.
  • In the young anther, the centers of each microsporangium contain homogeneous cells called sporogenous tissue.

Microsporogenesis

  • It is the process of formation of microspores from a pollen mother cell through meiosis called microsporogenesis.
  • As development occurs in another, the sporogenous tissue in a microsporangium is differentiated into microspores. Microspore undergoes meiosis tetrad.
  • As the other mature, the microspores get detached from each other and develop into a pollen grain. In 60% of angiosperms, pollen grain are shed at two-celled stage while in other generative cell undergoes mitosis to form two male gamete (three celled stages)
  • The viability of pollen grains after shed/release depends upon temperature and humidity its ranges from 30 minutes (eg: wheat, rice) to a few months (Rosaceae pollens).

Pollen grains

It represents the male gamete and is spherical having a two-layered wall.

  • Outer exine: Hard layer made of sporopollenin which is extremely resistant and can withstand high temperature, acidic, alkaline conditions, and enzymes.
  • Inner intine: It is a thin and continuous layer made up of cellulose and pectin.
  • A mature pollen grain contains two cells.
    • Vegetative cell: It is bigger in size and reserves food and a large nucleus.
    • Generative cell: It is small in size and floats in the cytoplasm of the vegetative cell. It has a nucleus and it mitotically divides to form 2 male gametes.

Gynoecium

Gynoecium represents the female reproductive part of a flower. It consists of whorls of pistils. Each pistil has three parts.

  • Stigma-It is a landing platform for pollen grains
  • Style- It is a long slender part below the stigma
  • Ovary- It is a basal swollen part of the pistil. Inside the ovary, there is a tissue called the placenta upon which ovules are developing. A number of ovules in the ovary may be one or many. The ovary has one or more cavities called locules.

Megasporagium/Ovule

  • The ovule is attached to the placenta by a stalk (Funicle). The junction where the body of the ovule and funicle fuse is called the hilum.
  • Each ovule has one or two protective envelopes called integuments which cover the rest of the ovule except for a small opening called the micropyle.
  • Opposite the micropyle end, is the Chalaza which represents the basal part of the ovule.
  • The mass of cells that are enclosed within the integuments is called the nucleus. It contains reserved food materials. The embryo sac or female gametophyte is located within the nucleus.

Megasporogenesis

The process of formation of megaspore form. The megaspore mother cell (microspore) is called megasporogenesis.
Microspore is large and contains a dense cytoplasm. It undergoes meiosis to produce four megaspores. The single functional megaspore develops into the female gametophyte. This kind of development is called monosporic development.

Formation of Embryosac

The nucleus of the functional megaspore divides mitotically to form two nuclei which move toward the opposite pole and form 2 nucleate embryosac. Two more sequential mitotic nuclear divisions ensure leading to the 8 nuclei and 8 nucleate embryo sac each opposite end has 4 nucleus group.

Distribution of the cell within embryo sac

  • 6 of the 8 nuclei get surrounded by the cell wall and the remaining two called polar nuclei are situated at the center.
  • 3 of the 6 cells are grouped together at the micropylar pole and constitute the egg apparatus (it consists of 2 synergids and one egg cell). The synergids have Filiform apparatus which guides the pollen tubes into the synergid.
  • The other 3 cells at the chalazal end are called the antipodals.
  • Thus, a typical mature angiosperm embryo is 8 nucleates and 7 cells.

Pollination

Pollination is a course of moving the dust/pollen grains across the anther and the stigma of a similar flower of a plant or to various plants for the process of fertilization and the creation of seeds. The specialists engaged in moving the pollen grains are birds, wind, creatures, and water.

Autogamy (self-pollination)- It is the transfer of pollen grain from another to the stigma of the same flower. It requires the anther and stigma lie closely. Plants like viola, oxalis, and commelina produce 2 types of flowers.

  • Chasmogamous (Open flower): Flowers with exposed anthers and stigma.
  • Cleistogamous (close flower)-Flowers that do not open at all.
  • Geitonogamy (Cross pollination) – It is the process of transfer of pollen grain from the anther of one flower to the stigma of another flower in the same plant. It requires a pollination agent.
  • Xenogamy (cross-pollination)- It is the process of the transfer of pollen grain from anther to the stigma of a different plant. This brings a genetic difference.

Agents of pollination

Plants use air, water, and animals for pollination. There are two types of pollination agents, which are abiotic and biotic. Abiotic agents include wind and water whereas biotic agents include animals.

  • Pollen-Pistil interactions: It is a dynamic process involving pollen recognition followed by promotion or inhibition of the pollen.
  • If the pollen is compatible (right type) the pistil accepts pollen germination.
  • If the pollen is incompatible (wrong type) the pistil prevents pollen germination.
  • The pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.
  • The 2 male gametes of the pollen grain move into the pollen tube.
  • Pollen tube grows through the tissues of the stigma and style.
  • Reaches the ovary and enters the ovule through the filiform apparatus.

Fertilization

After entering into synergids, the pollen tube releases 2 male gametes into the cytoplasm of the synergies.

  • Syngamy: One of the male gametes fused with the egg cell forms the zygote (2n) and develops into an embryo.

Male gamete (n) + Egg cell (n) —- Zygote (2n) —Embryo—Plant (2n)

  • Triple fusion– The other male gametes fuses with 2 polar nuclei to produce a triploid primary endosperm nucleus and develops into the endosperm.

Male gamete (n) + 2 polar nuclei (n+n) —primary endosperm nucleus (3n) –Endosperm (3n)

  • The fusion of syngamy and triple fusion that occur in the embryo sac is termed double fertilization.

Post-fertilization

Structure and events

Following double fertilization, the event of endosperm and embryo development, maturation of ovule into seed, and ovary into fruit are collectively termed post-fertilization events.

Endosperm– Endosperm cells are filled with reserved food materials. They are used for the nutrition of the developing embryo.
Steps of endosperm development

  • Step-1 Primary endosperm nucleus undergoes successive nuclear divisions to give rise to free nuclei. This is called a free nuclear endosperm.
  • Step-2 Subsequently cell wall formation occurs and the endosperm becomes cellular. The tender coconut water is a free nuclear endosperm and the surrounding white kernel is the cellular endosperm.

Development of Embryo

  • The embryo develops at the micropylar end of the embryo sac where the zygote is situated.
  • The zygote gives rise to the pro embryo and subsequently to the globular, heart-shaped, and mature embryo. 

Conceptual Questions

Question 1: What are the agents of pollination?

Answer:

Plants use air, water, and animals for pollination. There are two types of pollination agents, which are abiotic and biotic. Abiotic agents include wind and water whereas biotic agents include animals.

Question 2: What are the methods of sexual reproduction in plants?

Answer:

Sexual reproductive system involves the fusion of gametes thereby produces offspring flowers which are genetically different from the parent. The male and the female reproductive structures in flowers are androecium and gynoecium. This can be divided into three stages;

  • Pollination: Pollination is a course of moving the dust/pollen grains across the anther and the stigma of a similar flower of a plant or to a various plant for the process of fertilization and the creation of seeds. The specialists engaged with moving the pollen grains are birds, wind, creatures, and water.
  • Development of a Zygote: During the pollination process the deposited pollen grains over the stigma will be converted into pollen tubes, then the pollen tubes will move towards the over through stigma and style. During relocation two gametes are shaped inside the dust tube. At the point when the pollen tube arrives at the ovary, it bursts delivering two male gametes in the embryo sac. The male gametes combine with the ovum to frame a zygote in the twofold treatment process. One of the sperm cores wires with the egg core to shape a 2n zygote. The following core meld with the 2n core to shape a 3n core which by mitotic division brings about endosperm development.
  • Formation of Fruits and seeds: After the arrangement of a zygote, creating in an embryo is permitted. The ovules form into seeds and ovary forms into an organic product. A definitive result of sexual multiplication is seeds in angiosperms.

Question 3: Define syngamy and triple fusion?

Answer:

Syngamy: One of the male gametes fused with the egg cell forms the zygote (2n) and develops into an embryo.
Male gamete (n) + Egg cell (n) —- Zygote (2n) —Embryo—Plant (2n)
Triple fusion- The other male gametes fuses with 2 polar nuclei to produce a triploid primary endosperm nucleus and develops into the endosperm.
Male gamete (n) + 2 polar nuclei (n+n) —primary endosperm nucleus (3n) –Endosperm (3n)

Question 4: Why vegetative reproduction is also considered as a type of asexual reproduction?

Answer:

Vegetative reproduction is a cycle during which new plants are obtained without the get together of seeds or spores. It includes the engendering of plants through specific vegetative parts like the rhizome, sucker, tuber, bulb, and so on. It doesn’t include the combination of the male and the female gamete and requires only one parent. Consequently, vegetative proliferation is considered as a sort of asexual reproduction.

Question 5: Difference between sexual and asexual reproduction?

Answer:

Sexual reproduction

Asexual reproduction

It involves two different individuals It involves only one individual
It involves the fusion of the male and female gamete It does not require the fusion of the male and the female gamete
Offspring produced in sexual reproduction are different from their parents. Offspring produced in asexual reproduction is exactly similar to their parent.
It is a slow process. It is a fast process
Occurs in higher plants and animals. occurs in lower invertebrates and lower chordates

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