Reproduction in Flowering Plants: Gametogenesis, Pollination

Reproduction in Flowering Plants: Plants reproduce both asexually and sexually. In that, flowering plants, also known as angiosperms, use the sexual mode of reproduction. Reproduction in Flowering Plants, mainly occurs in the flower, as both the male and the female gametes are present in it. Different parts of a flower help in the process of reproduction in their own ways. So, to form a clear understanding of the process of reproduction in flowering plants, we need to look at the functions of each of these parts of the flower. Read on to learn interesting details about reproduction in flowering plants.

Reproduction in Flowering Plants

There are four main steps in the reproduction of flowering plants. They are:

• Pollination
• Gametogenesis
• Fertilization

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Reproduction In Flowering Plants/ Angiosperms

Angiosperms are the highest in the classification of kingdom Plantae. The angiosperms are the plants wherein, the seed is enclosed within the ovary which matures into a fruit. The flower is a specialized, reproductive part of the plant, containing the male and female reproductive organs to facilitate fertilization for the production of fruits and seeds.

A flower typically consists of two parts:

Essential Parts of a Flower

Androecium: It is the male reproductive organ containing anther and filament, collectively called Stamen. The anther is the region where pollen grains are produced for pollination and filament supports the anther. Stamen is the region in which the microsporangium or pollen sac is present.

Gynoecium: It is the female reproductive organ containing the pistil; which further includes stigma, style, and ovary. The stigma is the region of contact for pollen to enter the pistil, post which it goes through the pollen tube within the style, then fertilizes with egg in the ovary, to form seed and fruit.

Non-essential Parts of a Flower

Perianth: it is the non-reproductive region in the flower which contains calyx and corolla, hence preserving the reproductive region in a flower.

Sepals: It is the green part of the flower, which holds the petals and other parts of a flower. It acts as a support for the attachment of petals. The sepals are collectively called calyx. They protect the inner parts of a flower in the budding stage.

Petals: The petals are the colourful part of the flower which attracts bees and other insects for pollination and they also serve as protection to the reproductive area in the flower.

➤ Sexual reproduction is a process of fertilization of male and female gametes present in the flower to form a seed.

➤ Asexual reproduction is also known as apomixis, a process in which reproduction takes place without the fertilization of ovules. The seed is formed without undergoing cell division or fertilization with the sperm nucleus. This type of reproduction is found in certain plants such as strawberries and it helps in maintaining genetic features of the plant without undergoing variation.

Reproduction in Flowering Plants – Pollination

Pollination is a process in which the pollen grains from the anther is deposited on the receptive region of stigma in the pistil. Pollination is of two types:

Self-pollination

Self-pollination is a type of pollination where the pollen is transferred to the stigma of the same flower or another flower in the same plant. It is mostly observed in the plant where both the male and female reproductive parts are present. The genetic material is conserved, and pollen is not wasted but the unwanted traits will prevail in successive generations. It can take place by autogamy, as in rose (within the same flower) or geitonogamy, as in corn (between two flowers of the same plant).

Cross-pollination

In this type of pollination, the pollen of one plant is transferred to the stigma in another plant, consisting of different heritable configurations. This occurs in plants that are diecious or unisexual; the male and female reproductive organs are found in different plants. It is also called xenogamy. Cross-pollination usually occurs by the intervention of the pollinators like wind, insects, or water. E.g.: Jasmine, coriander, Calotropis.

Pollinators or pollinating agents:

Variation in characteristics and healthier progenies can be obtained. Adaptability is enhanced and more pollen grains are available but largely depends on pollinators. At certain times the reproducibility is reduced since the crossing over can cause a formation of non-fertile offspring. 

Reproduction in Flowering Plants – Gametogenesis

Gametogenesis is the production of haploid male and female gametes by diploid gametophyte, which take part in pollination and fertilization. The production of female gamete from megaspore mother cell is known as megasporogenesis. This process takes place in megasporangium. The production of male gamete from microspore mother cell is known as microsporogenesis. This process takes place in microsporangium.

Microsporogenesis/Formation of Pollen Grain

The pollen sacs or microsporangia is enclosed by tapetum, present in anther. The tapetal cells provide the pollen grain with a chemical called sporopollenin, a carotenoid, making it waterproof; thereby protecting it during transit. The mature pollen grains are developed within the pollen sacs and eventually rupture the pollen sacs to be released outside.

Mature pollen grain has 2 layers exine (outer layer) composed of sporopollenin, and intine (inner layer) consisting of cellulose or hemicellulose. The pollen grain undergoes mitosis to form two nuclei; the vegetative nucleus and generative nucleus. A thin wall differentiates them into large vegetative cell/tube cell and the small generative cell.

Upon pollination, the vegetative cell containing a tube nucleus forms the pollen tube. The generative cell enclosing the generative nucleus splits into two sperm nucleus/ male gametes by microgametogenesis.

Megasporogenesis/Formation of Egg Cell

Megasporangium contains the megaspores, which divides and differentiates to form egg cell and undergoes fertilization when fused with the sperm nucleus.

Megasporocyte or megaspore mother cell, a diploid cell in the megasporangium undergoes meiotic division to form four haploid daughter cells or megaspores. Only one of them survives, the rest have degenerated. This megaspore undergoes mitotic division in the process of megagametogenesis to form 8 nucleated, seven-celled megagametophyte or embryo sac.

The 3 cells (tripodal cells) move towards chalaza, opposite to micropyle, which later degenerates. Two polar nuclei fuse to form a central cell, which upon fusion with sperm nucleus forms endosperm. This is called a triple fusion. The egg cell is supported by two other cells called synergids (guides pollen tube during fertilization), present at micropyle, an opening for the entry of the sperm nucleus during fertilization.

The egg cell is fused with the sperm nucleus to form a zygote, and further into an embryo. The ovule encloses embryo which matures to form seeds. The bi-layered integuments later mature into seed coat.

Reproduction in Flowering Plants – Fertilization

Fertilization is the process of fusion of haploid male and female gametes to form a diploid zygote, giving rise to the embryo. In angiosperms, the fertilization takes place in the ovule, enclosed within the ovary of the flower, giving rise to seed and fruit respectively.

The mega gametogenesis gives rise to egg cell and micro gametogenesis gives rise to 2 sperm nuclei in the pollen grain, by mitosis. When the pollen grain encounters the sticky surface of the stigma, the pollen grain develops a pollen tube from the tube cell. There is a certain region called germ pore, on the pollen grain which enables the exit of the intine layer and the protrusion of pollen tube outside the pollen. The pollen tube traverses the style region and reaches the micropyle, an opening in the embryo sac.

The generative cell forms two male gametes, which travels through the pollen tube along with the vegetative nucleus. The pollen tube enters micropyle and synergids guide the pollen tube to the egg by releasing few chemotropic secretions for fusion or sometimes pollen tube enters synergids and ruptures to release the vegetative nucleus and 2 sperm nuclei.

The egg cell fuses with one of the sperm nuclei to form a zygote and another sperm nucleus fuses with polar nuclei to form endosperm. This process is known as double fertilization. And the fusion of polar nuclei with the sperm nucleus is called triple fusion. The ovule matures to form seed, the integuments from the seed coat. The pollen tube enters through chalaza (chalazogamy), through micropyle (porogamy) or through integuments (monogamy).

Embryo development (Post-fertilization Process)

The fertilization of haploid male sperm and haploid female egg cell results in a zygote, which later develops into an embryo. This process is known as embryogenesis.

The embryos in dicot and monocot plants are distinct from each other, structurally. In a dicot seed, the embryo has an embryonal axis aligned between the two cotyledons, laterally. The region of the embryonal axis above the attachment with cotyledons is called epicotyl, the tip of which is called plumule (embryonic shoot), and the region of the embryonal axis below the attachment with cotyledons is called hypocotyl, the tip is called radicle (embryonic root). In the monocot seeds, there is a single cotyledon.

Endosperm

The endosperm is obtained as a result of triple fusion in the embryo sac of the ovule. The endosperm serves as the nourishment tissue for the development of the seed. It often contains starch, proteins, oil, etc., essential for seed growth and for its unique characteristics. In monocotyledons, endosperm serves as the source of nutrition for the embryo during germination like in wheat, maize and in dicotyledons, the endosperm is absorbed, and cotyledons serve as a source of nutrition to the embryo during germination.

Seed Formation

The mature ovules develop into seeds post-fertilization. They enclose embryo, seed coats, and endosperm. The seeds may be monocot or dicot. Monocot refers to the seed which has only one embryonic leaf or cotyledon. The dicot refers to the seed which has 2 embryonic leaves or cotyledons.

Fruit Formation

Fruit is the product of fertilization, often eaten as a food and encloses the seed. The seeds in the fruit can germinate into new plant again. Fruits often attract animals, birds, and humans with their characteristic smell. This gives way for plant propagation through seed dispersal. The fruit often contains essential nutrients and a great source of nourishment.

The parts of fruit include the seed, which encloses endosperm within the embryo, and pericarp, which has exocarp (fruit coat), mesocarp (pulp), and endocarp (the inside lining which encloses seed).

FAQs on Reproduction in Flowering Plants

Here are some of the frequently asked questions on reproduction in flowers:

Q1: Define Morphogenesis.
Ans: Morphogenesis is defined as a biological process that controls the growth, development in size, shape and structure, and distribution of cells during the embryonic development of an organism.

Q2: What are the male and female reproductive parts of a flower?
Ans: A flower plays a vital role in the reproduction process of a plant. Therefore, it is called the reproductive organ of plants.
➤ The male reproductive parts of the flower include the stamen the filament and the anther, which are collectively termed the androecium.
➤ The female reproductive parts of the flower include carpels, pistils, stigma, style, and an ovary, which are collectively termed the gynoecium.

Q3: What is double fertilization?
Ans: In plants, double fertilization refers to the fusion of one female gametophyte with two male gametophytes. It is a complex process in all flowering plants.

Q4: What is pollination?
Ans: Pollination is a process of transferring pollen grains from the male anther of a flower to the flower’s female part called the stigma. Pollination is the sexual mode of reproducing, which is carried out by all flowering plants of a plant kingdom.

Q5: What are the different agents of pollination?
Ans: The pollinating agents are involved in transferring pollen grains from the male to the female part of the flower. Animals, birds, insects, wind, and other biotic and abiotic agents are all examples of pollinating agents.