3. Sexual Reproduction

Sexual Reproduction: 

• Sexual reproduction occur in plants & in human beings. The mode of reproduction that takes place with the involvement of two individuals of two different sexes i.e. male and female.
• During sexual reproduction, male organism having male sex organs produces male gametes i.e. sperms which are small and motile and the female organism having female sex organs produces ova which are generally large and store food.
• Male and female gametes fuse to form a zygote that grows into a new organism.

Significance of Sexual Reproduction:

• Sexual reproduction involves DNA as well as cellular apparatus of two different organisms which promotes diversity of characters in the offspring.
• Since gametes are derived from two different organisms, it results in a new combination of genes which increases the chances of genetic variations.
• Sexual reproduction results in the origin of new species.
• Sexual reproduction involves division in the sex organs that reduces the DNA matter to half so that the zygote formed after fusion has the same amount of DNA as the parents it maintains DNA in a species.

Limitation of Sexual Reproduction: Sexual reproduction involves the process of combining DNA from two different organisms which may bring some undesirable features also.
Sexual reproduction in flowering plants

• The reproductive parts are present in the flower.
• The parts of the flower are sepals, petals, stamens and carpels.
• Sepals are green structures that protect the inner parts when the flower is in bud stage.
• Petals are colourful and attract the insects for pollination.
• Stamens are male reproductive parts and produce pollen grains that contain male gametes. Each stamen has two parts—
• Filament i.e. stalk and Anther i.e. swollen top part which has large number of pollen grains.

The carpel is the female reproductive part and produces ovules that contain female gametes. It has three parts—Stigma which is top sticky part and receives pollen grains during pollination. Style which is the middle long part and ovary which is the swollen part and contains ovules. Each ovule has an egg cell i.e. female gamete.

The flowers may be bisexual i.e. having both stamens and carpels for example; Mustard China Rose (Hibiscus).The flower may be unisexual i.e. paving either stamens or carpels for example; Papaya, Watermelon.
Pollination: The process of transfer of pollen grains from an anther to the stigma of the flower is pollination. Two types of pollination are:
(i) Self-pollination: The transfer of pollen grains from the anther to the stigma of the same flower or another flower of the same plant.
(ii) Cross-pollination: The transfer of pollen grains from the anther to the stigma of another flower or another flower of a different plant of the same species. It generally takes place with the help of some agents like insects, birds, wind and water.
Fertilization: Fertilization is the process of fusion of male and female gamete to form a zygote during sexual reproduction. Pollination is followed by fertilisation in plants.
The events are as follows:
Pollen grains land on the stigma of the ovary.
Pollen tubes grow out of the pollen grains, travel through the style and reach the ovary, through micropyle.
Pollen tube has two male germ cells. Each ovule has two polar nuclei and a female germ cell (egg).
Pollen tube releases two male germ cells inside the ovule, one of them fuses with female germ cell and forms a zygote which grows into the baby plant i.e. embryo, the fusion is known as syngamy.
The other male germ cell fuses with two polar nuclei, the process is known as triple fusion. So in flowering plants two fusions take place during fertilisation. It is called double fertilisation.
Post-fertilisation changes: After fertilisation the following changes takes place in the flower.
Zygote divides several times and forms an embryo inside the ovule.

• The ovule develops a tough coat and changes into the seed.
• The ovary grows rapidly and ripens to form a fruit.
• Petals, sepals, stamens, style and stigma shrivel and fall off.

Seed and its parts: The advantage of seed is that it protects the future plant i.e. embryo.
Seed has two parts: Cotyledons and Embryo Cotyledons store food for the future plant.
Embryo has two parts: Plumule and radicle. Plumule develops into shoot and radicle develops into root.
The process of development of a seedling from the embryo under appropriate conditions is known as germination.
Reproduction in Human Being: 

• Human beings show sexual reproduction.
•  Male parent produces male gametes called sperms.
• Female parent produces female gametes called ova.
• Sperms have tail and are therefore, motile. They are produced in large numbers in the testes.
•  Ovum is bigger, non-motile and only one ovary produces one ovum in one month.
• There is no food stored in the sperms whereas ova contain stored food.
• Both the gametes are microscopic unicellular and have half the number of chromosomes as compared to the body cells.
• Human beings become reproductively active from the onset of puberty.
• Puberty is the period during adolescence when the rate of general body growth begins to slow down and reproductive tissues begin to mature.
• Onset of puberty in human males is between 11 to 13 yrs of age, while in human females is between 10 to 12 yrs. of age.
•  Puberty is associated with many physical, mental, emotional and psychological changes in boys and girls which occur slowly over a period of time. These are called secondary sexual characters.
• For instance thick dark hair start growing in new parts of the body such as arm pits and genital area between the thighs. Thinner hair appear on legs, arms and face. Skin becomes oily and pimples may appear on the face. Individuals become more conscious of their bodies become more independent, more aggressive etc.
• In case of boys, beard and moustache start appearing, voice begins to crack, reproductive organs develop and start producing releasing sperms.
• In case of girls, breast size begins to increase, skin of the nipples darkens, menstruation starts.
• The act of mating between the male and female partner is termed as copulation.