It is the reproduction in which both sexes (male & female) are needed to produce new generations.
The errors during DNA copying creates 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.
But 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.
But new generation will not have twice the amount of DNA. It is due to a process of specialised cell division called meiosis. It occurs in sex organs.
During meiosis, gametes (germ cells) are formed in which number of chromosomes and amount of DNA are half as compared to the non-reproductive cells.
Gametes from two individuals combine to form zygote that becomes a new individual. It re-establishes the chromosome number & DNA content in 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 female gamete. The other is small and motile. It is called male gamete.
Reproductive parts of angiosperms are located in flower.
Parts of a flower are sepals, petals, stamens & pistil.
Sepals: Outermost part which protect the bud.
Petals: Brightly coloured to attract pollinators.
Stamen: Male reproductive part. Formed of anther & filament. It produces yellowish and sticky pollen grains. Pollen grains produce male germ cells.
Pistil: Female reproductive part. It has 3 parts:
o Ovary: The swollen bottom part. It contains ovules. Each ovule has an egg cell (female gamete).
o Style: Middle elongated part.
o 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 pollens from stamen to the stigma. It is 2 types:
o Self-pollination: Transfer of pollen in same flower.
o Crosspollination: 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 and travels through the style and reach the ovary. Male germ cell from by pollen grain reaches the female germ cells through this tube.
Male germ cell fuses (fertilization) with the female gamete in the ovule to form zygote.
Then 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 & stigma shrivel and fall off.
Seed contains embryo (future plant) which develops into a seedling under suitable conditions (germination).
A cut-open germinated seed (e.g. Bengal gram, chana) shows the following parts: