Sexual Reproduction In Flowering Plants Notes

Introduction

Sexual reproduction in flowering plants, also known as angiosperms, is a complex and fascinating process that involves the formation, growth, and fusion of gametes to produce seeds. This process ensures genetic diversity and the continuation of plant species. In this study note, we will explore the various stages of sexual reproduction in flowering plants, from the development of reproductive structures to fertilization and seed formation.

Structure of a Flower

A flower is the reproductive organ of angiosperms and consists of several parts:

1. Sepals: These are the outermost parts that protect the flower bud.
2. Petals: Brightly colored parts that attract pollinators.
3. Stamens: The male reproductive organs, consisting of:
   • Anther: Produces pollen grains.
   • Filament: Supports the anther.
4. Carpels/Pistils: The female reproductive organs, consisting of:
   • Stigma: Receives pollen.
   • Style: Connects the stigma to the ovary.
   • Ovary: Contains ovules, which develop into seeds after fertilization.

Development of Male Gametophyte

Microsporogenesis

Microsporogenesis is the process of formation of microspores (pollen grains) from microspore mother cells (MMC) in the anther.

1. Microspore Mother Cells (MMC): Diploid cells in the anther undergo meiosis to produce four haploid microspores.
2. Tetrad Formation: The four microspores remain attached in a tetrad.
3. Pollen Grain Formation: Each microspore develops into a pollen grain.

Pollen Grain Structure

1. Exine: The outer hard layer, resistant to environmental conditions.
2. Intine: The inner layer, made of cellulose and pectin.
3. Generative Cell: Divides to form two male gametes.
4. Vegetative Cell: Forms the pollen tube.

Development of Female Gametophyte

Megasporogenesis

Megasporogenesis is the process of formation of megaspores from megaspore mother cells (MMC) in the ovule.

1. Megaspore Mother Cell (MMC): Diploid cell in the ovule undergoes meiosis to produce four haploid megaspores.
2. Functional Megaspore: Usually, only one of the four megaspores survives, and the others degenerate.

Embryo Sac Formation

The functional megaspore undergoes three mitotic divisions to form an eight-nucleate, seven-celled structure called the embryo sac.

1. Three Antipodal Cells: Located at the chalazal end.
2. Two Polar Nuclei: Located in the center, eventually fuse to form the secondary nucleus.
3. Egg Apparatus: Consists of one egg cell and two synergids at the micropylar end.

Pollination

Pollination is the transfer of pollen grains from the anther to the stigma. It can occur via various agents:

1. Self-Pollination: Transfer of pollen within the same flower or between flowers of the same plant.
2. Cross-Pollination: Transfer of pollen between flowers of different plants.

Agents of Pollination

• Wind (Anemophily): Pollen is carried by the wind.
• Water (Hydrophily): Pollen is carried by water.
• Animals (Zoophily): Pollen is carried by animals, such as insects (entomophily), birds (ornithophily), and bats (chiropterophily).

Fertilization

Fertilization involves the fusion of male and female gametes to form a zygote. In flowering plants, this process is called double fertilization.

Steps in Fertilization

1. Pollen Germination: Pollen grain lands on the stigma and germinates, forming a pollen tube.
2. Pollen Tube Growth: The pollen tube grows through the style towards the ovule.
3. Entry into Ovule: The pollen tube enters the ovule through the micropyle.
4. Double Fertilization:
   • One sperm cell fuses with the egg cell to form a diploid zygote.
   • The other sperm cell fuses with the two polar nuclei to form a triploid endosperm.

$$ \text{Sperm} + \text{Egg} \rightarrow \text{Zygote} (2n) $$

$$ \text{Sperm} + \text{Polar Nuclei} \rightarrow \text{Endosperm} (3n) $$

Seed and Fruit Formation

Seed Development

After fertilization, the ovule develops into a seed. The zygote develops into an embryo, and the endosperm provides nourishment.

1. Embryo: Consists of the radicle, plumule, and cotyledons.
2. Seed Coat: Develops from the integuments of the ovule.
3. Endosperm: Provides nourishment to the developing embryo.

Fruit Development

The ovary develops into a fruit, which encloses the seeds. Fruits can be classified based on their development:

1. Simple Fruits: Develop from a single ovary (e.g., mango).
2. Aggregate Fruits: Develop from multiple ovaries of a single flower (e.g., strawberry).
3. Multiple Fruits: Develop from the ovaries of multiple flowers (e.g., pineapple).

Conclusion

Sexual reproduction in flowering plants is a highly coordinated process involving the development of male and female gametophytes, pollination, fertilization, and seed and fruit formation. Understanding these processes is crucial for the study of plant biology and for improving agricultural practices.

By breaking down each stage and exploring the intricate details, we gain a deeper appreciation for the complexity and beauty of plant reproduction.