Sexual Reproduction in Flowering Plants

The flower is the reproductive part of a plant. In papaya, the flower is unisexual and contains either stamen or pistil and it may be bisexual as in mustard, and hibiscus. Parts of flower – sepals, petals, stamens, and pistil.

  • Calyx – consisting of sepals,
  • Corolla – consisting of petals,
  • Androecium – consisting of stamens,
  • Gynoecium or pistil – consisting of carpels. (Stamens and pistils found in flowers are the reproductive parts that contain germ cells).

Stamen:  is the male reproductive part of the flower and it produces pollen grains that are yellowish in colour.

The pistil: It is present in the center of a flower and it is the female reproductive part.

  • The pistil is made up of three parts – first, the swollen bottom part is the ovary, second, the middle elongated part is the style, and third, the terminal part which may be sticky is the stigma.

Ovary and Ovules: the ovary contains an ovule and there is one egg in each ovule.

Click here to read more about flowers: https://pcsstudies.com/the-flower/

Processes of Reproduction in Flowering plants:

In flowering plants the whole process of Reproduction can be divided into three steps:

  • Pre fertilization structure and events
  • Double fertilization
  • Post fertilization structure and events

Pre – Fertilization: Structure and events

The pre-fertilization events can be categorized as

  • Pollen grain formation
  • Embrosac formation
  • Pollination
  • Pollen-pistil interaction
Pollen Grain formation:

The pollen grain formation involves two processes i.e. microsporogenesis and microgametogenesis.

1. Microsporogenesis:

Anther (Microsporangium) are formed inside anther and it is the fertile portion of stamen (Microsporophyll). A typical anther is bilobed and each lobe consists of two thecae. The anther shows the tetragonal structure in the cross-section.

  • The microsporangia develop to form pollen sacs that contain pollen grains. 
  • A microsporangium is surrounded by four wall layers, i.e. epidermis, endothecium, middle layer, and tapetum.
  • A group of compactly arranged homogenous cells called the sporogenous tissue occupies the center of each microsporangium in a young anther.
  • PMC: Each cell of the sporogenous tissue is a Potential Pollen Mother Cell (PMC) and can give rise to microspore tetrad.
  • Microsporogenesis is the formation of microspores from mother cells by reductional division.
Microsporangium in flowering plant

Microsporangium in a flowering plant

2. Microgametogenesis (Development of Pollen Grains)

Microgametogeneis consists of events that lead to the progressive development of a unicellular microspore into a mature microgametophyte containing gametes.

  • Through the mitotic division, each microspore is divided into two cells i.e. a large tube cell or vegetative cell and a small generative cell. The entire generative cell resides within the cytoplasm of the tube cell. These two cells form juvenile pollen grain i.e. the male gametophyte.
  • Later, the generative cells divide to form two sperm cells that remain in the tube cell cytoplasm. This three-celled structure is the mature male gametophyte (pollen grains).
Embryo Sac Formation/ Gametogenesis

Gametogenesis, the formation of female gamete in Angiosperms, includes two processes:

  • Megasporogenesis
  • Megagametogenesis
1. Megaspororangium (Ovule)

Ovule arises from the placenta. It is attached to the placenta by a stalk called a funicle.

Hilum: The hilum is a junction between the ovule and the funicle. 

Integuments:  Each ovule has one or two protectives envelop called integuments. These encircle the ovule.

Micropyle: It is an opening that is present at the tip where the integument is absent.

Chalaza: It is opposite the micropylar end representing the basal part of the ovule.

Nucellus: The integument encloses a mass of cells called the nucellus. It contains reserved food for the development of the embryo.

2. Megasporogenesis

Megasporogenesis is the process in which the Megaspore Mother Cell (MMC) undergoes meiosis to produce four haploid cells called ‘Megaspore’.

On the basis of the pattern of the cell plate and haploid cell formation, it can be monosporic, bisporic, and tetrasporic.

  • Most angiosperms are monosporic also known as polygonum type. In which out of four uninucleate megaspores, three degenerate resulting in a single functional megaspore.
  • Then the functional megaspore undergoes the further process to form the embryo sac.

 

Development of Embryo Sac: Megagametogenesis
  • The process of Megagametogenesis forms female gametophytes.
  • The functional gametophyte is the first cell of the gametophyte. It involves mitotic divisions of functional megaspores to produce eight haploid nuclei that are enclosed within a seven-celled embryo sac.
  • Three nuclei at the base of the embryo sac form antipodal cells.
  • There are two synergids and an egg cell at the opposite end. The egg cell fuses with a male gamete to form a zygote during fertilization.

Pollination

The transfer of pollen grains from the anther of stamen to the stigma of the carpel is called pollination.

Transfer of pollen grains from the anther to the stigma of a flower may follow various ways.

  • Examples- Pollination by wind, Pollination by insects (Entomophily), Pollination by Water (Hydrophily), and Pollination by Animals (Zoophile).

Pollination may be of two types:

  • Self-pollination
  • Cross-pollination

Reproduction in flowering plats

Self Pollination:

Self-Pollination– Transfer of the Pollen grains to the stigma of the same or from another flower on the same plant. Ex. pea and gram

The self-pollination can show the following adaptations:

  • Autogamy: In this type, the pollination within the same flower.
  • Cleistogamy: Cleistogamy is a type of automatic self-pollination of certain plants that can propagate by using non-opening, self-pollinating flowers. Example: Commelina sp.
  • Homogamy: in this type both Anther and stigma mature at the same time in the flower. Ex. Mirabilis.
  • Geitonogamy: the pollen of one flower is deposited on the stigma of another flower of the same plant.
Cross-Pollination

In cross-pollination the transfer of Pollen grains from a flower to the stigma of another flower of another plant of the same species. It is also known as allogamy or xenogamy.

Self – incompatibility

Self-incompatibility is a genetic mechanism to prevent self-pollination from fertilizing the ovule by inhibiting pollen germination or pollen tube growth in the pistil, through this self-pollination does not lead to seed formation.

Pollen-Pistil Interaction

The event from the deposition of pollen on the stigma till the entry of the pollen tube into the ovule is collectively called pollen-pistil interaction.

Fertilization:

In flowers, the process of fertilization initiates after the pollen comes on a suitable stigma, it has to reach the female germ cells which are in the ovary. For this, a tube grows out of the pollen grain and travels through the style to reach the ovary.

Important facts about fertilization in flowering plants:

  • Each pollen grain forms a small tube-like structure called a pollen tube which emerges through the germ pore.
  • The pollen tube grows through the tissues of the stigma and style and finally enters the ovule through the micropyle.
  • Here two types of fusion, syngamy, and triple fusion take place in an embryo sac, the process is termed double fertilization. After triple fusion, the triploid primary endosperm cell develops into an endosperm.
  • It Gives stimulus for the growth of the ovary, leading to fruit formation.
Post fertilization: Structures and Events

Events found are the development of endosperm and embryo and maturation of the ovule into seed and ovary into a fruit. i.e. ovule makes seed and ovary make fruits.

Click here to read about fruit

The Fruit – Morphology of Flowering Plants

Seed: The seed contains the future plant or embryo which develops as a seedling under appropriate conditions and the process is known as germination.

A seed is the final product of sexual reproduction, it consists of a seed coat, cotyledons, and an embryo sac. 

***Tissue culture– In tissue culture, new plants are grown by removing tissue or separating cells from the growing tip of a plant. These cells get placed in an artificial medium where they divide rapidly to form a small group of cells or callus. Then the callus is transferred to another medium containing hormones for growth and differentiation. The plantlets are then placed in the soil so that they can grow into mature plants.

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