Sexual reproduction is the most common method of reproduction in plants and animals. Two parents, one male and other female, are involved in the process of sexual reproduction. It involves the fusion of two types of reproductive cells called gametes to form a single cell called zygote. The zygote multiplies repeatedly and undergoes specific changes to form the new individual.
In most higher animals, male and female gametes are produced by different individuals. Such individuals are said to be unisexual.
However, in some animals like flatworm, earthworm, leech and hydra, both male and female gametes are produced by the same individual. Such organisms are said to be hermaphrodites or bisexual. Both male and female reproductive organs are present in the same individual in bisexual organisms. In most plants, both male and female reproductive organs are present in the same flower.
Sexual Reproduction in Plants:
Flower is the reproductive organ of a flowering plant. A flower has four whorls namely- petals, sepals, stamens and carpel in order from outside to inside. Of these four parts, carpel and stamen order are the ones that are involved in the process of sexual reproduction. Stamen is the male reproductive organ of the flower while carpel is the female reproductive organ.
A unisexual flower, as in corn, mulberry and papaya, contains only one type of reproductive organ. A flower may be male or female in accordance with the type of reproductive organ present in it. Such flowers are also called incomplete flowers.
Most flowers, however, are bisexual since they contain both stamens and carpel. Such flowers are called complete flowers. Thus, a flower is said to be complete when all the four whorls are present in it. The carpel produces the female reproductive cell called egg or ovum. The swollen base of the carpel is called ovary. Ovary contains one or many egg-like bodies called ovules. The egg cell or ovum is present inside the ovule. The male reproductive cells are the pollen grains present inside the ovule. The male reproductive cells are the pollen grains present in the anther of the stamen.
Pollen grains from the same flower or another flower fall on the stigma of the carpel. Transfer of pollen grains from the anther to stigma of a flower is called pollination. Pollination may be carried out by various agents like wind, water, insects or birds.
After pollination, the pollen grains grow a thin tube called the pollen tube which moves down towards the ovary. The pollen tube carries the male gamete. Only one pollen tube finally enters the ovule where the male gamete fuses with the female gamete or the egg cell. This process is called fertilization. After fertilization, sepals, petals, stigma and style dry up and fall. The ovary grows and develops into the fruit while the ovules form seeds. The seeds germinate when the conditions are favorable and grow into new plants.
Reproduction in Human Beings
Male Reproductive System:
The male reproductive system consists of a pair of testes (singular testis) epididymis, and vas deferens (plural: vasa deferentia). Testes are located in a pouch called scrotum hanging outside the abdominal cavity. The testes produce numerous sperms at a time. Each testis is attached to epididymis, which temporarily stores the sperms. The sperms attain maturity and become fully motile in epididymis from where they are passed on to vas deferens. The two vas deferentia coming from each testis open into the urethra. In the urethra, secretions of many glands are mixed with sperms. This sperm carrying fluid is called semen. The urethra lies inside an organ called penis which is used to inject the sperms into the body of the female.
Female Reproductive System:
The female reproductive system consists of a pair of ovaries, a pair of fallopian tubes or oviducts and a uterus. The ovaries are present in the lower abdomen. Usually one ovum is produced by one of the ovaries every month and is shed into the abdomen. Ovum is picked up by funnel-shaped openings of the oviduct. Fertilization of the ovum takes place in the oviduct. The oviducts of both the sides unite to form a muscular uterus. The fertilized ovum attaches itself to the muscular wall of the uterus and divides repeatedly to form a mass of cells called embryo. The embryo derives nutrition from the mother’s body and develops into a baby.