In the previous Chapters, we have studied those life processes which help an organism (plant or animal) to keep 'alive'. Now, all the living organisms grow old with time and ultimately die. In fact, every living organism remains alive on this earth for a limited period of time and then dies.
So, new organisms have to be produced in place of those who die. The production of new organisms from the existing organisms of the same species is known as reproduction. In most simple words we can say that reproduction is the creation of new living things (from the existing living things).
Actually, one of the most important characteristics of living organisms is their ability to reproduce more members of their species. Reproduction is essential for the survival of a species on this earth. So, living organisms produce more organisms of their kind to maintain the life of their species on this earth.
The process of reproduction ensures continuity of life on earth. For example, human beings reproduce by giving birth to babies (sons and daughters). These babies grow and ultimately become adults. So, when the old parents die, their sons and daughters keep living on this earth. These sons and daughters also reproduced by giving birth to more babies, and this process goes and on. Thus reproduction by human beings ensures that the human species will continue to exist on this earth for all the time to come.
Similarly, cats reproduce by giving birth to kittens so that their species may live forever. And dogs reproduce by giving birth to puppies so that their species may continue to live on this earth. Most of the plants reproduce by producing seeds to grow more plants so that their species may continue to live on this earth.
It is clear from the above discussion that for a species of a plant or an animal to continue living on this earth, it must reproduce itself. Reproduction gives rise to more organisms with the same basic characteristics as their parents. For example, human beings always produce human babies; cats always produce kittens; and sunflower seeds always produce sunflower plants. If, however, some species of the living organisms cannot reproduce due to certain reasons, then the organisms of this species will gradually die out and disappear from this earth one day. In this chapter we will discuss the various methods of reproduction in plants and animals.
Please note that the existing organism or organisms are called parents and the new organisms produce by them are called offspring's. For example, our father and mother are parents and we (their children) the offspring's. The parent or parents of organisms can be animals as well as plants. Sometimes, the word daughter (or daughter cells) is also used for the offspring's of an organism.
There are many different ways in which new organisms are produced from their parents. Some organisms like Amoeba just split into two parts to produce new Amoebae; some organisms like Hydra and yeast grow out of the parent's body in the form of a bud; some organisms like birds and snakes hatch out of the eggs laid by their parents; whereas some organisms like human babies, cubs, kittens and puppies are born from their mother. This means that each species of organisms reproduces in a different way. All the different ways of reproduction can be divided into two main groups: asexual reproduction and sexual reproduction.
Thus, there are two main methods of reproduction in living organisms:
(i) asexual reproduction, and
(ii) sexual reproduction.
We can now say that the living organisms reproduce mainly by two methods: 'asexual reproduction' and 'sexual reproduction'. This means that new living organisms (new plants and animals) can be made either by the method of 'asexual reproduction' or by the method of 'sexual reproduction'.
We will now discuss the meaning of asexual reproduction and sexual reproduction. In order to understand this, please! keep in mind that certain organisms contain 'reproductive cells' (called 'sex cells', 'gametes' or 'germ cells') in their bodies whereas some other organisms do not contain 'reproductive cells' ('sex cells', 'gametes' or 'germ cells') in their bodies.
In asexual reproduction, the offspring arises from a single parent. The production of a new organism from a single parent without the involvement of sex cells (or gametes) is called asexual reproduction. It is called asexual reproduction because it does not use special cells called 'sex cells' (or gametes) for producing a new organism. In asexual reproduction, a part of the parent organism separates off and grows into a new organism.
Thus, in asexual reproduction, only one parent is needed to produce a new organism. But no sex cells are involved in asexual reproduction. Some of the examples of asexual reproduction are : binary fission in Amoeba; budding in Hydra; spore formation in Rhizopus fungus (or bread mould); regeneration in Planaria (flatworm); fragmentation in Spirogyra and vegetative propagation in flowering plants (like rose plants).
Please note that asexual reproduction is the simplest method of reproduction. It takes place mainly in those organisms whose bodies have a simple structure. So, the simple animals, simple plants and micro-organisms (like bacteria) reproduce by asexual reproduction methods. Actually, asexual reproduction takes place in unicellular animals and plants, micro-organisms (like bacteria) and simple multicellular animals (like Hydra and Planaria) and some multicellular plants (like Eryophyllum and rose plants, etc.).
In sexual reproduction, the offspring arises from two parents of different sexes : a male sex and a female sex. The male parent contains male sex cells (or male gametes) and the female parent contains female sex cells (or female gametes).
The production of a new organism from two parents by making use of their sex cells (or gametes) is called sexual reproduction. In sexual reproduction, the sex cell of one parent fuses with the sex cell of the other parent to form a new cell called 'zygote'. This zygote then grows and develops to form a new organism.
Thus, in sexual reproduction, two parents are needed to produce a new organism. The two parents which are involved in sexual reproduction are called male and female. Our father is a male and our mother is a female. The male and female parents have special organs in them which produce male sex cells and female sex cells respectively (which are required in sexual reproduction).
The humans, fish, frogs, hens, cats, dogs, cows, horses, deer, rabbits, lions and tigers all reproduce by the method of sexual reproduction. Most of the flowering plants also reproduce by sexual reproduction. As we will study after a while, some organisms use both the methods (asexual and sexual) for reproduction whereas other organisms use only one of these methods for reproduction.
The basic difference between asexual and sexual reproduction is that only one parent is needed in asexual reproduction whereas two parents are needed in sexual reproduction. Another difference is that no sex cells (or gametes) are involved in asexual reproduction but sex cells (or gametes) take part in sexual reproduction. We will now study the asexual reproduction and sexual reproduction in detail, one by one. Let us start with asexual reproduction.