Sexual Reproduction in Flowering Plants

Reproductive Structure of Flowering Plants

The flower is the reproductive structure of the plant, and includes the male and female reproductive organs. These are responsible for the plant's reproduction. The reproductive parts of the plant are given below-

1. Androecium (Male reproductive Organ): The stamen has the filament and anther, and helps in pollen production. The filament is the stalk-like structure supporting the anther. The anther manufactures and stores the pollen grains.

2. Gynoecium (Female Reproductive Organ): Carpel/Pistil is made up of the stigma, style, and ovary and produces the egg. The stigma is the top part and receives pollen during pollination. The style is the connection between the ovary and stigma and helps in the transfer of pollen. The ovary contains the ovules where eggs are made.

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• Difference between Somatic and Zygotic Embryogenesis

• Difference between Gametogenesis and Embryogenesis

• Dithecous anther

Megasporogenesis and Microsporogenesis

Megasporogenesis and Microsporogenesis are the processes in which female and male gametes are manufactured in the flowering plants. Megasporogenesis leads to the formation of the embryo sac, whereas microsporogenesis produces the pollen grains. The process of microsporogenesis and megasporogenesis is described below-

• Microsporogenesis is the process by which microspores, which are male gametophytes, are formed from the pollen mother cell inside the microsporangia of the anther.
• Each anther contains four microsporangia arranged in pairs, and each microsporangium has a well-organised structure with multiple layers, such as epidermis, endothecium, middle layers and tapetum inside the microspore.
• All the outer 3 layers help to protect the pollen grains and the inner parts, and the innermost layer, the tapetum, helps in the nourishment of pollen grains.
• The diploid pollen mother cells undergo meiosis to produce a tetrad of four haploid microspores. These microspores mature into pollen grains, which later take part in fertilisation.
• On the other hand, megasporogenesis occurs in the ovule within the megasporangium, also called the nucellus, which is surrounded by protective layers called integuments.
• A Single megaspore mother cell inside the nucellus undergoes meiosis to produce four haploid megaspores, of which generally only one remains functional
• The one functional megaspore gives rise to the female gametophyte. The ovule contains the funicle (stalk), hilum (attachment point), micropyle (opening for pollen entry), and chalaza (base of the ovule).
• All of them together, microsporogenesis and megasporogenesis, make sure the male and female gametes are produced for sexual reproduction in flowering plants.

Pollination In Flowering Plants

Pollination is an external process that involves the transfer of pollen grains from the anther to the stigma. There are two types of pollination - self-pollination and cross-pollination. There are certain differences between self-pollination and cross-pollination. Both of them are discussed below in the table:

Types of Pollination Based on Pollinating Agents

The abiotic agents of pollination are described below-

1. Entomophily (Pollination by Insects):

• Pollinating agents: Bees, butterflies, moths, beetles, etc.

• Flowers are usually brightly colored with fragrance and nectar to attract insects.

• In entomophily, pollen sticks to the insect’s body and is transferred from one flower to another.

• Example: Sunflower, Rose, Marigold

2. Chiropterophily (Pollination by Bats):

• Pollinating agents: Bats

• The flowers pollinated by the process of chiropterophily are large, dull-colored, and strongly scented, usually blooming at night.

• They produce lots of nectar and have sturdy structures to support bats.

• Example: Kigelia (Sausage tree), Baobab

3. Ornithophily (Pollination by Birds):

• In ornithophily, the pollinating agents are birds like hummingbirds, sunbirds, etc.

• Flowers are brightly colored, especially red or orange, and odourless.

• They have tubular shapes and produce copious nectar.

• Example: Hibiscus, Callistemon (Bottlebrush), Bignonia

Fertilisation in Flowering Plants

Fertilisation occurs when pollen-pistil interaction occurs, and finally when pollen grains reach the ovary. Here, the male gamete combines with the female gamete, the ovule, to form a zygote. The ovary turns into a fruit, and the fertile ovules turn into seeds. Reproduction can also be done separately. They undergo a different process called double fertilisation. Detailed discussion is mentioned below:

• Flowering plants have a different type of fertilisation known as double fertilisation, where there is the formation of a zygote after fertilisation of the egg and pollen.

• Two polar nuclei, when they fuse with the sperm, form an endosperm, which nourishes the embryo.

• One sperm is fertilised in the egg cell of the pollen tube, which results in the formation of a diploid zygote. This is further developed into an embryo.

• The second sperm cell gets fertilised at the central cell, containing two polar nuclei which form a triploid endosperm.

• After fertilisation, the ovule matures and the seed containing the embryo develops into a fruit.

Embryogeny and Post-Fertilisation Events in Flowering Plants

Embryogeny is the development of the embryo from the zygote after fertilisation. Post-fertilisation events also include the formation of endosperm, development of the ovule into a seed, and changes in the ovary into fruit.

• Post-fertilisation events begin right after fertilisation and include the development of the zygote, formation of the endosperm, embryogeny (development of the embryo), maturation of the seed, and transformation of the ovary into the fruit.

• The endosperm is formed before the embryo and serves as a nutritive tissue, providing nourishment to the developing embryo. It can be retained in mature seeds (as in cereals) or used up during development.

• Embryogeny is the process of development of the embryo from the zygote.

• Embryogeny in monocots differs slightly from that in dicots.

• In dicot seed embryos, the main parts formed are:

  • Radicle (future root)

  • Plumule (future shoot)

  • Cotyledons (two seed leaves that may store food)

  • Hypocotyl (part between radicle and cotyledons)

  • Epicotyl (part above the cotyledons)

• In monocots, such as grasses:

  • The embryo has only one cotyledon (called scutellum).

  • Other parts include coleoptile (covers the plumule), coleorhiza (covers the radicle), plumule, and radicle.

Seed and Fruit

Seeds and Fruits are the products of sexual reproduction in flowering plants. The seed contains the embryo and stores food for the germination process, and the fruit is developed from the ovary and protects the seed.

• The seed is the mature ovule, formed as a result of fertilisation and contains the embryo and seed coat. The various parts of a seed help in the protection and nourishment of the embryo.

• The seed is formed after fertilisation, and its dispersal is required for the spreading of the plants to new areas. The dispersal can occur by means of wind, water, animals, or mechanical force.

• The dicot seeds have 2 cotyledons, while Monocot seeds have only one cotyledon. The cotyledons help in the storage of food and support the embryo.

• The fruit is formed from the ovary after fertilisation. The fruit has a pericarp ,and the seeds are contained inside the fruit.

• There are mostly three kinds of fruits, which include simple fruits, aggregate fruits, and multiple fruits.

• Parthenocarpy is the process by which fruit and seed are formed without fertilisation, and the fruits develop without seeds.

• Polyembryony is characterised by the presence of more than one embryo in a seed. It happens naturally in plants and leads to formation of multiple seedlings from a single seed.

• The significance of seeds and fruit formation lies in helping plants reproduce, spread, survive unfavourable conditions, and provide food for animals and humans.

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