When two parents (of opposite sex) are involved in reproduction, it is called sexual reproduction. Gamete formation and fusion of male and female gametes are necessary for sexual reproduction.
In all organisms; which reproduce sexually; the basic mechanism of reproduction is same. Main steps of sexual reproduction are as follows:
The process of production of gametes is called gametogenesis. Gametes are formed as a result of meiosis and hence gametes have haploid number of chromosomes. Depending on the relative size and appearance of gametes, an organism can be termed as homogamous (isogamous) or heterogamous (anisogamous).
When the appearance and size of gametes is similar, the condition is said to be isogamy, e.g. cladopora. When the appearance and size of gametes is dissimilar, the condition is said to be anisogamy. Anisogamy is the norm in most of the organisms. In most of the cases, the female gamete is much larger than the male gamete.
In some organisms, the male and female gametes are produced by the same individual. Such organisms are called unisexual or hermaphrodite, e.g. earthworm. It is important to remember that the term hermaphrodite is used for animals only. Unisexuality is quite common in plants. When gametes are produced by different individuals, the organism is called bisexual. Bisexuality is the norm in higher animals.
In flowering plants, the unisexual male flower is called staminate flower and the unisexual female flower is called the pistillate flower. In several fungi and simple plants, the bisexual condition is called homothallic or monoecious, while the unisexual condition is called heterothallic or dioecious.
The fusion of male and female gametes is called fertilization. Fertilisation is also called syngamy. The product of fertilization is called zygote.
The female gamete is usually non-motile and male gamete is usually motile. The male gamete needs to travel up to the female gamete so that fertilization can take place. But most of the male gametes perish on the way. To ensure fertilization, the male gametes are produced in a very large quantity. The male gametes need some medium through which they can travel to the female gamete. In aquatic animals and plants, water serves as the medium for the travel of the male gametes. In terrestrial plants, air is the usual medium. In higher animals, body fluid is the medium through which male gametes can travel.
When fertilization happens out of the body of an organism, it is called external fertilization. External fertilization is the norm in aquatic animals, e.g. fish and frogs.
When fertilization happens inside the body of an organism, it is called internal fertilization. Internal fertilization is the norm in reptiles, aves and mammals.
Pollination is the phenomenon associated with flowering plants. Transfer of pollen grains from anthers to stigma is called pollination. Pollination is of two types, viz. self pollination and cross pollination. When pollen grains are transferred to the stigma on the same flower, this is called self-pollination. On the other hand, when pollen grains are transferred to a different flower, this is called cross-pollination.
A flowering plant needs the help of some agent to carry out cross-pollination. Air, water, insects and animals are the various agents of pollination. Pollination by air can be seen in maize and other plants from the grass family. Pollen grains are hairy so that they can easily float on air and pollination can be facilitated. Pollination by water is seen in aquatic plants and in some plants which grow near water, e.g. coconut and water lily. Insects are the major agents of pollination. Flowers are colourful and many have nectar to attract insects so that pollination can take place.
The zygote undergoes several rounds of mitotic cell division and forms the embryo. This process is called embryogenesis. The embryo subsequently develops into a new individual.
Animals which show internal fertilization exhibit two modes of producing their offspring, viz. ovipary and vivipary.
Animals which produce eggs are called oviparous. The embryo develops inside the eggs and comes out at a suitable time.
Animals which give birth to young ones are called viviparous. The embryo develops inside the uterus and is born when it resembles the parent organism.
Vivipary has certain advantages over ovipary. While eggs are at the mercy of the vagaries of nature and are prone to be destroyed by potential predators, young babies are much safer. In most of the viviparous animals, the young ones are taken care of for a considerable time till the new individual is well developed to lead an independent life.
In flowering plants, zygote is formed inside the ovule. After fertilization; sepals, petals and stamens wither and fall off. The pistil remains attached to the plant. The zygote develops into the embryo and ovules develop into the seed. The ovary develops into a fruit with a thick wall called pericarp. Pericarp is protective in nature. Seeds get dispersed once they become mature. Subsequently, seeds germinate and give rise to new plants.
When a plant bears fruit without fertilization, this is called parthenocarpy. Banana fruits are good examples of parthenocarpic fruits. New individuals are also borne without fertilization in many animals, e.g. the worker bees in a beehive are borne from unfertilized eggs.