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Monday, 3 December 2018

Asexual Reproduction





In asexual reproduction, an individual produces an offspring by itself, I.e. only one parent is present. There is no fusion of nuclei, and the cell(s) which give rise to the offspring usually divide by means of mitosis. As a result , asexual reproduction often produces clones, offspring which are identical to the parent. In extremely rare cases, the offspring may not be identical. This is due to mutations, changes in the genes or chromosomes.

Fission

The simplest type of asexual reproduction is the fission of unicellular organisms such ad bacteria and protists. Here, the parent organism simply divided into two or more parts, each of which can exist by itself. In bacteria, where the cell just divides into two identical parts, the process is called binary fission.

      Under favourable conditions, fission occurs rapidly, giving rise to numerous identical offspring. This type of reproduction allows organisms to colonize new habitats very fast.

Budding

In budding, the offspring develops as an outgrowth of the parent. The bud may form on an external or an internal surface of the parents. Internal buds are formed in some sponges, and are released when the parent dies. External buds break off from the parent without causing any injury and lead an independent life.

     The simplest type of budding is seen in yeasts. The bud may separate immediately or remain attached to the parent cell, and in turn form a bud. In multicellular animals like the hydra, the cells in the bud undergo differentiation to form the complex body of the organism.

Spore formation

Spores are small unicellular bodies which are produced in large numbers. They are small, light and easily dispersed by air. Under favourable conditions, each spore can develop into an independent organism. Spores are commonly produced by bacteria, fungi, protists, algae, mosses and ferns.

Fragmentation

In this process , a part of an organism breaks up or fragments and gives to a new individual. This is a form of regeneration. It occurs in simple organisms like algae, coelenterates and sponges. In Spirogyra, when a filament reaches a certain length, parts of it break away and grow into new filaments.

Vegetative propagation

This process occurs mainly in higher plants. Here, a new plant grows from any portion of an old one other than the seeds. The parts involved must have a store of food and must be able to produce all the organs of a complete plant, namely roots, stems, leaves and flowers. The store of food will be used by new plant during its early stage of growth until it can photosynthesize.

    In many flowering plants, certain organs, such as stems and roots, are specially modified for vegetative propagation. In some plants, these  these organs also act as perennating organs a d enable the plant to survive from one growing season to the next. Perennating organs lie dormant in the soil during the unfavourable season. They are usually swollen with excess food produced during the previous growing season. At the onset of the favourable season, the perennating organ starts to grow and send up an aerial shoot. Stolons, rhizomes, tubbers, bulb and corms are perennating organs

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