MODES OF REPRODUCTION USED BY SINGLE ORGANISMS

The modes of reproduction by various organisms depend on their body design.

Creation of new generations from a single individual is called asexual reproduction.

Types of asexual reproduction: Fission, fragmentation, regeneration, budding, vegetative propagation & spore formation.

1. Fission

It is the cell division of unicellular organisms to form new individuals. E.g. Many bacteria and protozoa.

It is 2 types: Binary fission & Multiple fission.

1. Binary fission:

In this, parent cell divides into 2 daughter cells. E.g. Amoeba, Leishmania.

o In Amoeba, binary fission takes place in any plane.

o Some unicellular organisms have complex body organisation. E.g. Leishmania (cause kala-azar), have a whip-like structure. Here, binary fission occurs in relation to these structures.

2. Multiple fission:

In this, some single-celled organisms divide into many daughter cells. E.g. Plasmodium (malarial parasite).

2. Fragmentation

In this, an organism simply breaks up into smaller pieces (fragments) and they grow into new individuals. E.g. Spirogyra (greenish, filamentous multicellular structures seen in pond or lake).

All multicellular organisms cannot simply divide cell-by-cell because the cells are organised as tissues and organs. Here, different cell types perform different functions. So, reproduction is also performed by a specific cell type. It can grow, proliferate and make other cell types.

3. Regeneration

In this, a differentiated organism is cut into many pieces and they grow into separate individuals.

E.g. Hydra & Planaria.

During regeneration, some specialised cells proliferate to form a cell mass. From this, different cells develop to form various cell types and tissues.

However, regeneration is not the same as reproduction, since most organisms do not normally depend on regeneration for reproduction. Also, many organisms use this only to regenerate lost parts.

4. Budding

In this, a new organism is developed from a small part of the parent’s body. E.g. Hydra, Yeast etc.

Budding in Yeast:

· Dissolve about 10 gm sugar in 100 mL of water.

· Take 20 mL of this solution in a test tube and add some yeast granules. Put a cotton plug on the mouth of the test tube and keep it for 1 - 2 hours in a warm place.

· Put a drop of yeast culture from the test tube on a slide and cover with a coverslip. Observe under microscope. We see yeast reproducing by forming buds.

Organisms such as Hydra use regenerative cells for budding. In Hydra, a bud develops as an outgrowth due to repeated cell division at one specific site. Buds develop into tiny individuals and when fully mature, detach from parent body and become new individuals.

5. Vegetative Propagation

In this, plant parts like root, stem and leaves develop into new plants under appropriate conditions.

Advantages:

o It is used in layering or grafting to grow plants like sugarcane, roses or grapes for agricultural purposes.

o Plants raised by this method can bear flowers and fruits earlier than those produced from seeds.

o Seedless progenies can be produced. E.g. banana, orange, rose, jasmine etc.

o All plants are genetically similar to the parent plant.

Potato tuber has buds for vegetative propagation.

· Cut a potato into small pieces such that some pieces contain a notch or bud and some do not.

· Place them on some wet cotton on a tray.

· After few days, potato pieces with buds give rise to fresh green shoots and roots.

In Bryophyllum, buds produced in the notches along the leaf margin fall on the soil and develop into new plants.

In money plants, vegetative propagation occurs through stem cutting with at least one leaf (for photosynthesis). It can be demonstrated by the following activity:

· Cut some pieces of a money-plant such that they contain at least one leaf.

· Cut out some other portions between two leaves.

· Dip one end of all the pieces in water.

· After few days, it is observed that portion of the Money plant with at least one leaf grows and gives rise to fresh leaves. But money plant without leaf dies.

Tissue culture

It is a method of growing tissues or cells from a plant in artificial medium. Cells are divided rapidly to form callus (a small group of cells). It is transferred to another medium containing hormones for growth and differentiation. Then the plantlets are placed in the soil. They grow into mature plants.

Thus, many plants can be grown in disease-free conditions. This technique is commonly used for ornamental plants.

6. Spore Formation

It is present in organisms like bread mould (Rhizopus).

Keep a slice of wet bread in a cool, moist and dark place.

Observe it for a week.

A layer of white mass of thread-like structures is seen on the slice. It is the hyphae of Rhizopus.

Hyphae include some blob-on-a-stick structures. They are involved in reproduction. The blobs (sporangia) contain cells (spores) that can develop into new Rhizopus.

Spores are covered by thick walls that protect them until they come into contact with a moist surface and can begin to grow.