How do Organisms Reproduce? | Class 10 CBSE | Web Notes | Part 3 - Sexual Reproduction in Flowering Plants
SEXUAL REPRODUCTION
Why Sexual Mode of Reproduction?
It is the reproduction in which both sexes (male and female) are needed to produce new generations.
The errors during DNA copying create variations.
Variations do not protect all individuals in a population but help for survival of the species. If the DNA copying mechanisms are less accurate, the resultant DNA cannot work with cellular apparatus and will die.
Sexual reproduction can generate more variations and speed up the making of new variants by combining DNA copies from two individuals.
All these variations are accumulated and inherited from generation to generation. It produces individuals with different patterns of variations in a population.
New generations will not have twice the amount of DNA due to a process of specialised cell division called meiosis. It occurs in sex organs.
During meiosis, gametes (germ cells) are formed in which the number of chromosomes and amount of DNA are half compared to the non-reproductive cells.
Gametes from two individuals combine to form a zygote that becomes a new individual. It re-establishes the chromosome number and DNA content in the new generation.
In very simple organisms, the two germ cells are almost similar. In complex organisms, one germ cell is large and contains food stores. It is called the female gamete. The other is small and motile, called the male gamete.
Sexual Reproduction in Flowering Plants
Reproductive parts of angiosperms are located in the flower.
Parts of a flower: sepals, petals, stamens, and pistil.
Longitudinal section of a flower
Sepals: Outermost part which protects the bud.
Petals: Brightly colored to attract pollinators.
Stamen: Male reproductive part, formed of anther and filament. It produces yellowish and sticky pollen grains. Pollen grains produce male germ cells.
Pistil: Female reproductive part. It has three parts:
Ovary: The swollen bottom part. It contains ovules. Each ovule has an egg cell (female gamete).
Style: Middle elongated part.
Stigma: Terminal part which may be sticky.
Unisexual flower: Contains either stamens or pistil. E.g., papaya, watermelon.
Bisexual flower: Contains both stamens and pistil. E.g., Hibiscus, mustard.
Pollination: It is the transfer of pollen from stamen to the stigma. It is two types:
Self-pollination: Transfer of pollen in the same flower.
Cross-pollination: Transfer of pollen from one flower to another by agents like wind, water, or animals.
After pollination, a pollen tube grows out of the pollen grain, travels through the style, and reaches the ovary. The male germ cell from the pollen grain reaches the female germ cells through this tube.
Germination of pollen on stigma
Male germ cell fuses (fertilization) with the female gamete in the ovule to form a zygote.
The zygote divides several times to form an embryo within the ovule.
The ovule develops a tough coat and becomes a seed.
The ovary grows rapidly and ripens to form a fruit.
Petals, sepals, stamens, style, and stigma shrivel and fall off.
Seed contains the embryo (future plant) which develops into a seedling under suitable conditions (germination).
A cut-open germinated seed (e.g., Bengal gram, chana) shows the seedling parts.
