Asexual Reproduction in Living Organisms!
The type of reproduction that takes place without the process of gamete (sex cell) formation is called asexual reproduction.
This type of reproduction takes place commonly in lower plants and animals, where the body is not very complex.
Binary fission occurs under favourable environmental conditions. Binary fission is the division of one cell into two similar cells. This is the simplest method of asexual reproduction. It occurs in unicellular organisms like bacteria, yeast, Euglena, Amoeba and Paramecium. In some organisms (e.g., Leishmania, which causes kala-azar) binary fission takes place in definite orientation due to their specific body structure.
Take a permanent slide of Amoeba showing binary fission. Observe it under a microscope. You will see that the nucleus first divides amitotically into two, followed by the division of the cytoplasm. The Amoeba finally splits into two daughter cells.
Under unfavourable circumstances some unicellular organisms develop a hard protective covering over the cell, called cyst. The nucleus of the cell divides repeatedly, producing many nuclei.
Each nucleus is surrounded by a small amount of cytoplasm and many daughter cells are produced within the cyst. When favourable conditions return, the offspring are released. Multiple fission is seen in many algae and the malarial parasite (Plasmodium).
Sometimes new individuals develop from the body wall of the parent as bulblike projections called buds. The buds may be unicellular or multicellular depending upon the type of parent organism. The buds finally separate to form new individuals. Budding occurs in yeast. Hydra and sponges.
Put some yeast in 10% sugar solution kept in a glass. Cover the glass and keep it in a warm place for a day. The yeast cells grow and reproduce in the sugar solution. These cells are known as a culture of yeast cells. Take a drop of the yeast culture solution on a slide and cover it with a coverslip. Examine it under the microscope. You will see buds on the yeast cells.
Obtain some pond water. You may see green filamentous structures floating in it. Take some of these structures on a slide. Put a drop of glycerine on them and cover them with a coverslip. Observe under a microscope.
The green filamentous structures you see are an alga named Spirogyra, which grows in ponds, ditches and springs. Each filament has a single row of cylindrical cells. Each cell has spiral bands of chloroplasts.
When a Spirogyra filament breaks into pieces, each piece grows into a new filament by cell division. This process is fragmentation. During this process the body of an individual breaks up into two or more parts and each part develops into a complete organism.
Some animals like sponges. Hydra and flatworms (Planaria) also reproduce by a similar method known as regeneration. If they are cut into pieces, each piece can regenerate into an entire individual.
In complex organisms all cells are not similar. The cells are organized into tissues and tissues into organs. The different organs are placed at definite positions. If such an organism breaks off at any point, the broken part cannot grow into a complete organism with all organs.
Spores are asexual reproductive bodies enclosed in a thick-walled structure called sporangium, which can tide over unfavourable conditions such as extreme heat, dryness, acidity, and so on. Spore formation is a common method of asexual reproduction in many lower forms of life such as algae, bacteria and fungi.
Under favourable conditions, the spores are released by the breaking of the thick wall of the sporangium. The spores then germinate into new individuals. In fungi, sporangia burst and release spores. By this method of asexual reproduction, organisms can overcome unfavourable conditions. Some fungi, e.g., Rhizopus and Mucor reproduce by producing spores.
Take a piece of moistened bread and keep it inside a polythene bag for 2-3 days in a warm, humid place. You will observe yellow and black patches on the bread slice. After 4-5 days you will see a powdery substance with spores. The spores are of bread mould (Rhizopus). The moulds also have threadlike structures called hyphae through which they draw nutrients from the bread.
Vegetative Propagation in Plants:
The vegetative parts of a plant such as the root, stem, leaf, etc., can produce new plants. You must have seen gardeners taking cuttings from the stem of a rose plant and planting them in the soil. Under suitable conditions, the cuttings grow into new rose plants.
Vegetative propagation is common in plants like orchids, ornamental plants and grasses. Plants such as banana, rose, jasmine, etc., which do not produce seeds, can be grown by vegetative methods. The new plants are genetically similar to and bear the characteristics of the parent plant.
In some plants like Dahlia, sweet potato, etc., the adventitious roots become swollen due to storage of food. Adventitious buds are also present on them. When roots bearing such buds are planted in the soil, new plants are produced as a result of vegetative propagation.
Observe a potato closely. You will see ‘eyes’ on its surface. These eyes are actually buds. You know that the stem has buds from which leaves and smaller branches arise. Cut a potato into small pieces. Plant some of the pieces with eyes and some without eyes in moist sand.
Observe the changes taking place in these pieces over the next few days. You will find stems, leaves and roots growing from the potato pieces that had eyes.
Some plants produce sub aerial stems which develop as lateral branches from the mother plant and give rise to a new plant after getting detached from the mother plant. For example, in runners such as grasses the stem grows along the surface of the soil and produces roots where it touches the ground to give rise to a new plant.
In some plants the underground stem gets modified for storage of food, and under favourable conditions it produces shoots and gives rise to a new plant. Such stems include rhizomes, tubers, bulbs and corms.
The fleshy leaves of Bryophyllum bear adventitious buds in the notches along the leaf margin. These buds develop into small plants (plantlets) under favourable conditions. These plantlets can be easily separated to grow as independent plants.
Artificial modes of vegetative propagation:
Farmers, gardeners and horticulturists have developed various artificial methods of vegetative propagation, like grafting, layering, cutting and tissue culture for growing plants in gardens and nurseries.
Cutting is a very simple method of propagation in which a piece of the parent plant’s stem with nodes and internodes is placed in moist soil. This grows into a new plant. In grafting the cutting of a plant is attached to the stem of a rooted plant.
The attached cutting becomes a part of the rooted plant, draws nutrition from it and grows roots at the joint. Now if it is separated, it grows into a new plant. In layering, one or more branches of the parent plant are bent close to the ground and covered with moist soil. The covered portions grow roots and develop into new plants.
Cut two pieces from a money plant—one with leaves (i.e., a portion with nodes) and the other without leaves (i.e., a portion of an internodes). Place these with one end immersed in water kept in a transparent bottle. Leave them like this for a week.
You will see that roots and new leaves grow on the piece with leaves, while the other piece gradually withers. This is because a plant can grow new leaves and branches only if it has nodes. (New leaves and branches arise at the nodes.) The piece of money plant which does not have nodes cannot grow because it cannot produce new leaves.
In this technique some tissue from a desired plant is placed in a suitable nutrient medium under proper conditions. The tissue grows into an unorganized mass, known as callus. A small part of this is put in another medium, which contains growth hormones that induce the formation of plantlets from the callus.
When plantlets grow, they can be transplanted in the soil or in pots for developing to maturity. Tissue culture allows us to grow a whole plant from cells taken from any part of the plant body. Many plants can be grown from one parent plant in the laboratory under controlled, disease-free conditions.