As discussed earlier, sexual reproduction involves two parents and gamete formation. Gametes are special cells which are formed after meiosis. There are two types of gametes, viz. male and female gametes. The number of chromosomes is haploid in the gametes. When gametes fuse during fertilization, the number of chromosomes becomes diploid. This is important for maintaining the unique identity of a particular species which reproduces by sexual method.
In sexual reproduction, the offspring gets sets of genes from two different individuals. This leads to subtle variation through subsequent generations. These variations accumulate over thousand of generations and finally may give rise to a new species. That is how all complex organisms have evolved from a common ancestor.
DNA replication is the process by which DNA makes a copy of itself. DNA replication happens during the S – phase (synthesis phase) of the cell cycle. This is important because the daughter cells would need additional copies of the DNA. The process of DNA replication is a foolproof process, yet some alterations do take place. These alterations may lead to some variations in the characters of the daughter cells.
Additional copies of DNAs are made after DNA replication. As a result, number of DNA becomes double after DNA replication so that equal number (halves) get distributed among daughter cells. This is the reason, number of DNA is maintained in daughter cells after cell division. But you have read that sexual reproduction involves fusion of gametes that results in formation of zygote. So, a zygote will have double the number of chromosomes compared to the number in a gamete. Some mechanism needs to be in place to maintain the number of chromosomes in zygote to be equal to the number in somatic cells. This is ensured by meiosis during gamete formation so that number of chromosomes in gametes becomes half of that in somatic cells.
Flower is a modified leaf which bears special organs and plays the role of the reproductive system in plant.
A typical flower is composed of four distinct whorls, viz. calyx, corolla, androecium and gynoecium.
Calyx: The outermost whorl of the flower is called calyx. It is composed of green leaf-like structures, called sepals.
Corolla: The second whorl of the flower is called corolla. It is composed of colourful leaf-like structures, called petals. Petals are colourful so that insects and birds can be attracted; to assist the flower in pollination.
Androecium: This is the third whorl in the flower. It is composed of stamens. Stamen is made of a slender stalk and anthers on top. Anthers produce the pollen-grains. Pollen grains are the male gametes.
Gynoecium: This whorl is at the centre of the flower. It is composed of a swollen base, called ovary. A slender style stands upright on the ovary. It has a flat top, called stigma. Ovules are inside the ovary. Ovules are the female gametes.
Pollination: The pollen grains need to be transferred to the stigma so that fertilization can take place. The transfer of pollen grains from anther to the stigma is called pollination. If the pollen grains from the same flower or the same plant are transferred to the stigma, it is called self pollination. If pollen grains from a different plant are transferred to the stigma, it is called cross-pollination. Cross pollination is better, from the perspective of variations. Many agents help plants in cross pollination, e.g. insects, animals, air, water, etc. Insects are the main pollinators for the plant kingdom.
The fusion of male and female gametes is called fertilization. The product of fertilization is called zygote. Zygote undergoes several rounds of mitosis and develops into an embryo. Subsequently, the embryo develops into a new individual.
After landing at the stigma, pollen grains absorb moisture and germinate. A pollen grain develops a pollen tube, which penetrates through the tissue of the style and reaches the ovule. Pollen nuclei are transferred through the pollen tube. After fertilization, zygote is formed, which finally develops into the embryo.
Post-Fertilisation Changes in Flower: The calyx and corolla wither and fall off and so do the stamens. The ovary turns into the fruit. The embryo turns into seed. Once the seed becomes mature, fruit dries up so that dispersal of seeds can take place.
Structure of Seed: A seed contains an embryo, some reserve food and is enclosed by a protective covering, called seed coat. The reserve food is stored in the cotyledons. The embryo has two pointed parts. The upper part is called plumule which gives rise to the shoot system. The lower part is called radicle which gives rise to the root system. Cotyledons supply food when the embryo needs it during germination. Seed germination is the process by which the embryo in the seed kick-starts a new life.
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