Asexual reproduction takes place by the following methods:
1. Binary fission:
In unicellular organisms like amoeba, bacteria and paramecium, the parent cell divides into two daughter cells, each forming an independent organism. This type of asexual division is called binary fission. Fig. 1 (a) and (b).
In amoeba, the nucleus first divides into two equal parts. This is followed by division of the cytoplasm into two parts, each containing a nucleus. The two daughter cells separate and grow to form new amoebae.
Budding is commonly observed in yeast and hydra Fig. 2 (a) and (b). In this method of asexual reproduction, a small bulb-like projection called bud is formed on the parent body, which separates to form new individual.
In yeast, a tiny outgrowth appears on one side of the yeast cell. The nucleus of the parent cell divides into two and one of these enters the bud. The outgrowth soon detaches from the parent cell to form a new individual. Sometimes the bud does not separate from the parent yeast cell and chain of yeast cells is then produced.
In hydra, a bud is formed from one side of the parent body. The bud grows for a day or two and then separates from the parent hydra.
Budding also occurs in corals and sponges. But these buds do not separate from the parent body and begin to reproduce while they are still attached to the parent. A colony of corals and sponges is thus formed.
In some organisms the whole body of the organism breaks up into several parts. Each part or fragment then develops into a complete organism. In spirogyra, a filamentous alga, the filament breaks into two or more fragments and each fragment grows into a new individual.
In some worms, sponges and flatworms, fragmentation occurs as a method of reproduction while in hydra and planaria (a flatworm) if the body is cut up into small pieces, each piece can grow into a complete individual.
4. Spore formation:
In non-flowering plants like bacteria, fungi, mosses and ferns, reproduction occurs by spore formation. Spores are small microscopic reproductive bodies protected by a hard protective coat. Spores are produced by the parent plant as asexual reproductive units and grow by themselves into new individuals.
The thick outer covering of the spore protects it against unfavourable conditions like scarcity of water and food and extremes of temperature. When the conditions are favourable, the spore germinates to form a new individual.
Spores are seen on a stale piece of bread. You must have seen a white cottony growth on moist stale bread. This is the fungus rhizopus, commonly called bread mould. When the fungus is a few days old, you would notice tiny black bodies on the white patches. The black bodies are the spores produced by rhizopus which are blown by wind. Sooner or later, the spores germinate under favourable conditions to form a new bread mould.
The ability of an organism or replace or regenerate lost body parts is called regeneration. Unicellular organisms like amoeba and paramecium can regenerate their body parts. The capacity of regeneration is greater in plants than in animals. A small part from the body of hydra can regenerate the entire animal. A grasshopper can regenerate its broken leg. A starfish can regenerate a lost arm. The power of regeneration is greater in simpler animals. As the animals become more complex the power of regeneration decreases.