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The process of producing offspring in plants by fusion of gametes is called the sexual reproduction of plants. Sexual reproduction is always considered as a superior method as it brings variation and evolution. The fusing of gametes results in offspring that are genetically not the same as the parent plant. The reproduction happens by different processes that occur in plants and give rise to a descendant plant. Fruits and seeds are the results of sexual reproduction. The seeds further contribute to the growth of a new plant.
Process of Sexual Reproduction
Sexual reproduction of plants happens in flowers. Flowers produce both female and male gametes. Meiosis is a crucial phase in the sexual reproduction of plants. It makes the rearrangement of genes and also decreases the number of chromosomes. The process of sexual reproduction mainly consists of phases called pollination and fertilization. After fertilization, fruit, and seeds form the flower. The supportive parts fall off after a specific time.
A typical flower consists of the following layers:
The calyx is a set of sepals. They are small florets that occur in the first three layers of a flower starting from the base. Sepals are usually green, but in some plants, it is colorful. The primary function is to safeguard the flower.
Corolla is a collection of petals. The petals are colorful, hence an attraction to birds and insects that can help in pollination.
The androecium consists of stamens. Stamen is the male reproductive part made of parts called filament and anther. Anther looks like a sac, and it stores and produces pollen. Filament acts in support of an anther.
The gynoecium is a collection of carpels. The pistil is the female reproductive part that consists of stigma, ovary, and style. The topmost part of a flower is a stigma. A long tube that connects ovary to the stigma is called style. Ovary holds a vast number of ovules. Seed formation in a plant takes place at the ovary.
A flower, in general, may comprise of pistil or stamen or both. A flower is classified into unisexual or bisexual depending on the fact whether it contains stamen or pistil. A flower that is bisexual consists of all the parts, as said above. For example, China rose. Plants such as cucumber and papaya produce unisexual flowers.
Pollination and Fertilization
For the formation of a zygote, pollen grains containing male gametes must fuse along with the egg in the ovule. The process of fusing of male gametes with eggs is called pollination. We can also say pollination as the transfer of pollen grains on stigma from the anther. Based on the landing of pollen, we classify pollination into two types:
Self-pollination: If the pollen grain landed on stigma and is from the anther of the same flower, then we call the process self-pollination. There are many advantages as well as disadvantages of self-pollination. Hence some flowers use mechanisms to avoid self-pollination.
Cross-pollination: The process of pollination where pollen grain on the stigma falls from the anther of another flower from a different or same plant is called cross-pollination. This process helps in bringing out a genetic variation in plants allowing them to defend against environmental changes that do not favor them.
The process of pollination involves some agents that are known as pollinators. Pollinators can be insects, birds, wind, and water. The pollination through wind happens by pollen grains traveling through the wind and landing on the stigma. As pollen grains are open to the wind, they are sticky so that they stick to the stigma tightly. Sometimes honey bees that come in search of nectar aid in the process of pollination.
On the transfer of pollen to stigma, the male gametes release from pollen grain and fuse with the egg in the ovule, leading to the formation of zygote. The process of formation of a zygote by the fusion of gametes is known as fertilization.
Sometimes in angiosperms, a male gamete fuses with ovule having two nuclei; this process is called double fertilization. One male gamete forms the zygote, but the other one fuses with diploid polar nuclei to create endosperm. The zygote develops the future plant, and endosperm helps in nutrition for the embryo. The best example for endosperm is tender coconut water, and it is nothing but endosperm which supplies nutrients to the growing embryo.
Once fertilization is over, the ovary turns into a fruit, and ovules help in the formation of the seeds. The remaining parts peel off eventually.
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The result of this process is the generation of male gametes. The pollen mother cell present in the anther undergoes meiosis and forms microspores.
A pollen mother cell or microspore mother cell is 2n-diploid.
Further microspore mother cell divide to form a tetrad.
Upon maturation of the microspores or pollen grains, another leaves or releases them.
The result of megasporogenesis is the formation of female gametes known as megaspores or eggs.
The megaspore mother cell is diploid.
The megaspore mother cell gives rise to four haploid megaspores, out of which only one is useful, and others degenerate. Ovules are called the megasporangium.
Development of a Seed
Mature ovules form the seeds. A typical seed has a seed coat, cotyledons, single embryo, and endosperm.
There is a difference in the storage of food reserves between monocots and dicots. In plants such as wheat and corn, scutellum is the name given to the single cotyledon. Xylem and phloem directly connect scutellum to the embryo.
Aleurone is a cell layer inside a seed coat that secretes enzymes upon germination, and it surrounds endosperm and embryo.
The seed with ovule is covered by a seed coat, which forms due to integuments of ovule sac and helps in protection. In the case of dicots, the seed coat is of two layers, inner tegmen, and outer testa.
Features of Sexual Reproduction
The process of sexual reproduction takes a longer time than asexual reproduction.
The offspring produced in case of sexual reproduction are not similar to their parent plant genetically.
The fusion of gametes from both male and female parts forms the zygote, which further divides to grow into a new plant. The parts that are involved in reproduction get modified into fruit and seed of the plant.
The process of reproduction requires the contribution of both sexes, whether from the same plant or a different plant.
The process of sexual reproduction in plants comes with its benefits. All the fruits that we consume are a result of sexual reproduction, even the seeds used to grow a plant are a result of this process. The type of life cycle of plants is called alternation of generations because a haploid plant known as gametophyte produces diploid gametes that make the offspring diploid. Sporophytes generate haploid gametes again, giving rise to gametophytes. Many natural agents help in the process of pollination knowingly or unknowingly. Reproduction is essential as it helps in the continuation of the species. Naturally, pollen grains of some flowers have a fragrance that attracts the agents that will help in pollination. Pollination happens after the pollen grain is mature to fuse and form the zygote.