Sexual Reproduction | Sexual Reproduction In Plants [Latest]

Sexual Reproduction In Plants: There are two modes of plant reproduction: asexual reproduction and sexual reproduction. Sexual reproduction in plants consists of alternating, multicellular haploid and diploid generations. In angiosperms, the female gametophyte is the embryo sac and the male gametophyte is the pollen. The haploid egg and sperm fuse to form diploid zygotes, from which new sporophytes develop.

What Is Sexual Reproduction?

It is an advance mode of reproduction, which is exhibited by majority of the animals. These animals show differentiation of sex. In sexual reproduction, the DNA of two different sources get mixed with each other, to produce a new organism. This mixing results in the development of some variations.

Due to these reasons, the offspring (children) of sexually reproducing organisms are not the carbon copy of their parents. These variations are very important for the improvement of a species. These variations are the raw material for the process of evolution.

Since the amount of DNA is fixed in an organism, the mixing causes doubling of the DNA amount. To solve this problem DNA amount is reduced to half before mixing. This is acheived with the help of a process called meiosis. The cells which are produced by meiosis, contain half DNA amount (or chromosomes) and are known as gametes. So, gamete formation and their fusion (mixing or fertilisation) are the important events in sexual reproduction.

Important Features of Sexual Reproduction

In sexual reproduction one/two individual are used which have male and female reproductive organs. If only one individual is used, then it must possess both male and female reproductive organs. Such animals are called bisexual or hermaphrodite. (Example : Earthworm, Leech etc.).

· But in most of the animals, male and female individuals are separate (i.e, they are unisexual).

· The most striking feature of sexual reproduction is the production of specialised haploid cells called gametes. One gamete, which is immotile and contains stored food (for the future development of the zygote) is the female gamete (egg or ovum), and the other which is motile and contain less or no reserve food is called male gamete (sperm).

· During reproductive process, male and female gametes fuse to form zygote. This fusion is known as fertilisation.

· In case of sexual reproduction, the offspring (new individuals) are not the carbon copy of their parents. Hence, they are not the clones. They can better be called as hybrids.

Sexual Reproduction In Flowering Plants

Sexual Reproduction In Plants With Example:

· In angiosperms, the main plant body is diploid in nature and is known as sporophyte.

· The haploid stage in angiosperms is highly reduced and is known as gametophyte.

· Flower is the reproductive organ of

· It contains 4 whorls:

· Outermost : Calyx (Its units are called sepals).

· Inner to calyx : Corolla (Its units are called petals).

· Inner to corolla : Androecium (It is male sex organ and its units are called stamen)

· Innermost : Gynoecium (It is female sex organ and its units are called pistils or carpels).

· The flower may be unisexual, when it contains either male sex organs (stamens) or female sex organs (carpels). Examples of plants with unisexual flower are watermelon.

· In most of the cases, flower is bisexual (or hermaphrodite) and it contains both types of sex Organs. Examples of bisexual flowers are- Hibiscus (China rose) and mustard.

Male Sex Organ

· The male sex organ or reproductive part is called stamen. A flower may contain many stamens.

· These are usually yellowish in colour. Each stamen consists of a filament and anther.

· Anther is a swollen structure and it produces male units, called pollen grains.

· The pollen grains are produced as a result of meiosis and therefore these are haploid structures.

· On further development, pollen grains give rise to two male gametes or sperms (male germ cell.

Female Sex Organ

· The female sex organ is called carpel (pistil). A flower may contain one or more carpels. These are present in the centre of the flower.

· Each carpel consists of a terminal part, called stigma, a middle elongated part, called style and swollen bottom part, called ovary.

· Stigma is sticky in nature (this helps stigma to capture pollen grains during pollination).

· Ovary contains many ovules. Ovule produces female gamete (egg).

Important to Note!

· Gynoecium and androecium are called essential whorls. A flower must contain at least one of the two whorls. When only Androecium is present, flower is called staminate, and when only gynoecium is present the flower is called pistillate. When both essential whorls are present in the same flower the flower is called bisexual or hermaphrodite.

· The calyx and the corolla are referred to as the non-essential whorls, since they are not responsible for the formation of gametes and seeds.

Various steps involved in sexual reproduction

1. Pollen grains and formation of male germ cell (sperms)

· Stamens contain anthers or microsporangia. These structures produce specialised haploid cell known as pollen grains (or microspores).

· The pollen grains are haploid structures and formed by meiotie division.

· Pollen grains now get released from the anther and reach the stigma of flower. This process transfer of pollen from anther to stigma is called pollination. Rest of the development of the pollen grain takes place after its pollination.

· After pollination, pollen grain develops on stigma to form male germ cells/gametes (i.e., sperms).

Meaning Of Pollination

Transfer of pollen grains from anther to the stigma (of a carpel) is called pollination. It is of two main types : (1) Self-pollination and (ii) Cross pollination.

Transfer of pollen from anther of a flower to the stigma of the same flower (or flower of same plant) is called self-pollination.

Transfer of pollen from the anther of a flower to the stigma of another flower, belonging to a different plant, is known as cross pollination. It is mainly found in unisexual flowers, but can also occur in bisexual plants.

There are many factors or agencies, which make this transfer easier. Such factors or agencies are called pollinating agencies. Some of the pollinating agencies are : Air (anemophily), water (hydrophily) insects (entomophily), animals (zoophily), bats (chiropterophily) etc.

2. Megaspore and Development of Female Gamete

· The ovary contains one to many ovules.

· Inside these ovules, certain specialised structures are developed. These are called megaspores. These are haploid structures and are formed by meiosis.

· Usually only one megaspore remains alive in the ovule and it develops to form an embryo sac, which gives rise to female germ cell or egg cell.

· Besides egg cell, other cells in the embryo sac are antipodal cells, synergid cells, polar nuclei.

· The egg cell is haploid and represents the female gamete.

3. Fertilisation and Seed Germination

· The pollen grain, which develops on the stigma, gives rise to a pollen tube. This tube contains male gametes (sperms). The tube now grows towards the ovule (passing through style).

· After reaching the ovule, the tube penetrates the ovule and reaches the embryosac.

· Inside the embryosac, the tip of pollen tube bursts and releases sperms (male germ cells).

· One sperm goes to fertilise the egg cell and results in the formation of zygote, which gives rise to proper embryo.

· Another sperm fuses with the polar nuclei to form a triploid endosperm nucleus, which lateron provides nutrition to the developing zygote / embryo.

4. Post Fertilisation Changes and Seed Formation

· After fertilisation, ovary grows rapidly and becomes ripened. Such a ripened ovary is called fruit.

· Meanwhile, the petals, sepals, stamens, style and stigma may shrivel and fall off.

· Internally zygote starts germination. The ovule becomes the seed. In this way, the seed contains the germinating embryo (i.e., the future plant).

· Seeds when germinate under proper growing conditions, give rise to seedlings. Seedlings contain two important structures plumule and radicle.

The plumule part on further development gives rise to the sh0ot part (stem, leaves etc), of the new plant. On the other hand, the radicle, part gives rise to the root part of the new plant.