What is The Sexual Reproduction in Flowering Plants?
Sexual reproduction in flowering plants involves the formation and fusion of male and female gametes. This process is unique to angiosperms (flowering plants), where the flower contains both reproductive organs. Below, we explore the structure of a flower, key stages like pollination and fertilisation, and additional insights that will help you understand this essential biological process.
Parts of a Flower
A typical flower is made up of four main whorls – calyx, corolla, androecium, and gynoecium. Some of these parts are reproductive, while others are protective or attractive structures.
Calyx (Sepals)
Outermost whorl, usually green and leaf-like.
Protects the flower when it is in the bud stage.
Sometimes, sepals can be coloured (petaloid).
Corolla (Petals)
Second, whorl petals are often brightly coloured and may produce fragrance.
Helps attract insects and other agents of pollination.
Androecium (Male Reproductive Part)
Third whorl, composed of stamens.
Each stamen has an anther (produces pollen) and a filament (a stalk that holds the anther).
Gynoecium (Female Reproductive Part)
Innermost whorl, made up of one or more pistils (also known as carpels).
Each pistil consists of a stigma (pollen receptor), style (tube-like stalk), and ovary (contains ovules).
Anther (microsporogenesis) → Pollen Formation
Ovary (megasporogenesis) → Egg Cell Formation
Pollination → Transfer of Pollen to Stigma
Pollen Tube Growth → Delivery of Male Gametes
Fertilisation → Zygote and Endosperm Formation
Seed and Fruit Formation
Using a labelled diagram can help visualise how these parts fit together in the flower.
Androecium and Microsporogenesis
The androecium comprises all the stamens in a flower. Within each anther, microsporogenesis takes place, leading to the production of pollen grains (male gametes).
Structure of the Anther
Each anther typically has two lobes, and each lobe usually contains two pollen sacs (microsporangia).
Inside each microsporangium, there are layers of cells that help in pollen development:
Epidermis: Outermost protective layer.
Endothecium: Lies just beneath the epidermis; often develops fibrous thickenings.
Middle Layer: Consists of thin-walled cells, usually three to four layers.
Tapetum: Innermost layer, provides nutrients to developing pollen.
Formation of Pollen Grains
The microspore mother cells (microsporocytes) undergo meiosis to form haploid microspores.
Each microspore develops into a pollen grain, which has two layers:
The outer layer (exine) contains a tough substance called sporopollenin.
The inner layer (intine) is made of cellulose and pectin.
Development of the Male Gametophyte
Within the pollen grain, one vegetative (tube) cell and one generative cell are formed after the first division.
The generative cell later divides to form two male gametes (sperm cells).
When the pollen grain lands on a stigma, the vegetative cell produces the pollen tube that carries the male gametes towards the ovule.
Gynoecium and Megasporogenesis
The gynoecium is the female reproductive organ comprising one or more pistils. Each pistil has:
Stigma: The top surface where pollen grains land.
Style: A tube-like structure that connects the stigma to the ovary.
Ovary: Enlarged basal part containing one or more ovules.
Formation of Ovules (Megasporogenesis)
Inside each ovule lies a megaspore mother cell (megasporocyte).
Through meiosis, this cell produces four haploid megaspores. Usually, only one megaspore remains functional while the others degenerate.
The functional megaspore undergoes divisions to form the female gametophyte (embryo sac), which contains the egg cell and other nuclei (including polar nuclei).
Pollination
Pollination is the process of transferring pollen from the anther to the stigma, which is essential for the sexual reproduction of flowering plants.
Self-Pollination: The pollen reaches the stigma of either the same flower or another flower on the same plant.
Cross-Pollination: The pollen moves from the anther of one plant to the stigma of a different plant of the same species.
Factors like wind, water, insects, and animals play a significant role in pollinating different plant species. Cross-pollination enhances genetic diversity within a population.
Fertilisation
After successful pollination, a pollen tube emerges from the pollen grain on the stigma and grows through the style to reach the ovary. This tube carries the two male gametes:
One male gamete fuses with the egg cell in the embryo sac to form the zygote.
The other male gamete fuses with the polar nuclei, forming the endosperm (a nutrient-rich tissue that supports the developing embryo).
This process, called double fertilisation, is characteristic of angiosperms.
After fertilisation, the ovule transforms into the seed, and the ovary usually develops into the fruit.
Functions of a Flower
Ensures sexual reproduction by producing and protecting male and female gametes.
Facilitates pollination through attractive petals, scents, and nectar.
Provides a site for fertilisation and seed (ovule) formation.
In many species, the fruit formed from the ovary helps in seed dispersal.
Additional Insights
Monocarpic Plants
Some plants (e.g., bamboo) flower only once in their lifetime, produce seeds, and then die. These are known as monocarpic plants.
Gregarious Flowering
In certain species of bamboo, all individuals of the same species flower at the same time, regardless of location, usually after intervals of several decades.
Quiz Time
Test your knowledge with these quick questions!
Which part of the flower holds the anther?
A. Style
B. Filament
C. Stigma
D. Ovary
Answer: B. Filament
Where does double fertilisation occur in a flowering plant?
A. Ovary
B. Anther
C. Petal
D. Sepal
Answer: A. Ovary
Which one is the male gamete-bearing structure in the pollen grain?
A. Vegetative cell
B. Generative cell
C. Endosperm
D. Tapetum
Answer: B. Generative cell
What type of pollination involves pollen transfer between different plants of the same species?
A. Self-pollination
B. Cross-pollination
C. Natural pollination
D. Synthetic pollination
Answer: B. Cross-pollination
Related Topics
FAQs on Sexual Reproduction in Flowering Plants: Parts, Diagram, and Steps
1. What is meant by sexual reproduction in flowering plants?
Sexual reproduction in flowering plants, or angiosperms, is the process involving the formation and fusion of male and female gametes. This process occurs within the flower's reproductive organs and results in the formation of a zygote, which eventually develops into an embryo contained within a seed.
2. What are the key parts of a flower involved in sexual reproduction?
The key reproductive parts of a flower are organised into two main whorls:
- Androecium: The male reproductive part, composed of stamens. Each stamen consists of an anther (which produces pollen grains) and a filament.
- Gynoecium: The female reproductive part, composed of one or more pistils (or carpels). Each pistil has a stigma (receives pollen), a style (connects stigma to ovary), and an ovary (contains ovules).
The calyx (sepals) and corolla (petals) are non-reproductive whorls that primarily offer protection and attract pollinators.
3. What are the five main steps in the sexual reproduction process of a flowering plant?
The five primary steps are:
- Gametogenesis: The formation of male gametes (pollen grains) in the anther and the female gamete (egg cell) in the ovule.
- Pollination: The transfer of pollen grains from the anther to the stigma of a pistil.
- Pollen Tube Growth: The germination of the pollen grain on the stigma and the growth of a pollen tube down the style to the ovule.
- Fertilisation: The fusion of one male gamete with the egg cell to form a zygote, and the other with the polar nuclei to form the endosperm.
- Seed and Fruit Formation: The development of the ovule into a seed and the ovary into a fruit following fertilisation.
4. What is the difference between microsporogenesis and megasporogenesis?
Microsporogenesis is the process of forming male gametes. It occurs inside the anther's microsporangia, where microspore mother cells undergo meiosis to produce haploid microspores, which mature into pollen grains. In contrast, megasporogenesis is the formation of the female gamete. It takes place inside the ovule, where a megaspore mother cell undergoes meiosis to form a functional haploid megaspore, which develops into the embryo sac containing the egg cell.
5. Why is double fertilisation considered a unique and significant event in angiosperms?
Double fertilisation is significant because it involves two separate fusion events, ensuring efficient resource allocation. One male gamete fuses with the egg cell to form the diploid zygote (embryo). The second male gamete fuses with the two polar nuclei to form the triploid primary endosperm nucleus (PEN). This PEN develops into the endosperm, a nutritive tissue that supports the embryo. This process is unique to flowering plants and prevents the plant from investing energy in creating nutritive tissue unless fertilisation has successfully occurred.
6. How does cross-pollination provide a genetic advantage over self-pollination?
Cross-pollination involves the transfer of pollen between two different plants of the same species. This process introduces new gene combinations, leading to increased genetic variation in the offspring. This variation enhances the population's ability to adapt to changing environmental conditions, resist diseases, and evolve. Self-pollination, which involves pollen and ovules from the same plant, limits genetic diversity, potentially making the offspring more vulnerable.
7. What happens to the different parts of a flower after fertilisation?
After successful fertilisation, the flower undergoes significant changes. The zygote develops into an embryo, the ovule matures into a seed, and the ovary develops into a fruit. Other parts, such as the petals, sepals, stamens, stigma, and style, typically wither and fall off as they are no longer needed. The fruit's primary function is to protect the seeds and aid in their dispersal.
8. What is the importance of sporopollenin in a pollen grain?
Sporopollenin is a highly resistant organic material that makes up the exine, or the outer layer, of a pollen grain. Its primary importance lies in its durability; it protects the male gametes from harsh environmental conditions such as high temperatures, strong acids, and enzymatic degradation during their journey from the anther to the stigma. This resilience also allows pollen to be preserved in fossils for millions of years.
9. Why do some plants, like bamboo, flower only once and then die?
Plants that flower, set seed, and then die are called monocarpic plants. This life strategy, seen in bamboo and agave, is an evolutionary adaptation. By accumulating resources over a long period (often decades) and expending all energy in one massive reproductive event, the plant maximises its chances of producing a large number of viable seeds. This synchronous flowering and seeding can overwhelm seed predators, ensuring that more offspring survive.
