2. Modes of Reproduction-Single Organisms

Types of Asexual Reproduction is Unicellular organism

(i) Binary Fission: 

• Seen in bacteria, protozoa like Amoeba, Paramecium.
• In these first pseudopodia withdrawn (karyokinesis) the nucleus of the parent cell divides and then the cytoplasm divides (cytokinesis) resulting in the formation of two daughter cells.
•  It occurs during highly favourable conditions.
•  The cell division can occur in any plane as in case of Amoeba.
• However, organisms like Leishmania. (Cause Kala-azar), which have a whip like flagella at one end, binary fission occurs in a definite orientation in relation to the flagellum.

Cytokinesis: Division of cytoplasm.
Karyokinesis: Division of Nucleus.
(ii) Multiple Fission: 

• Seen in Plasmodium, (a malarial parasite).  
• During unfavorable conditions, the parent cell develops a thick resistant wall around itself forming a cyst.
• Within the wall, the cytoplasm divides many times to form many plasmodia.
• When conditions become favorable, the cyst wall breaks and the Plasmodium are released.

(iii) Budding: 

• Seen in Yeast (a fungus). 
•  The parent yeast cell develops a protrusion or an outgrowth at its upper end.
•  The nucleus of the parent cell divides and one of them moves into the outgrowth which grows bigger and finally separates from the parent cell to lead an independent existence.
• Very often if the conditions are highly favorable, a chain of buds is formed.

Types of Asexual Reproduction in Multicellular Organisms:

(i) Fragmentation: 

• Seen in multicellular organisms which have a relatively simple body organisation like Spirogyra.
• Spirogyra has a filamentous body. (If it breaks into smaller pieces or fragments). Each fragment has the capacity to form a new individual.
• However, all multicellular organisms cannot show cell-by-cell division as cells from tissues which form organs.
•  These organs are placed at definite positions in the body. Hence, they need to use more complex methods of reproduction.

(ii) Regeneration: 

• It is the ability of organisms to develop their lost parts. 
• Some organisms show have high regenerative capacity it is also a means of reproduction for example; Planaria.
• Regeneration is carried out by specialized cells which re divide to form a mass of cells from which different cells undergo changes to become different cell types and tissues.
• These changes occur in an organized sequence known as development.

(iii) Budding: 

• Seen in Hydra.  
• Parent Hydra develops a bud at its lower end.
• This grows in size and finally breaks off to live independently.

(iv) Spore Formation: 

• Seen in Rhizopus (a fungus).   
• Rhizopus body is made up of thread-like structures called hyphae.
• The erect hyphae bear sporangia inside which reproductive structures called spores are formed.
•  Spores are asexually reproducing bodies having a thick protective wall.
• They are produced during unfavourable times and help to tide over the unfavourable environmental conditions.
• When the spores fall on a suitable medium, each one forms a new individual.

(v) Vegetative Propagation: 
Method by which plants reproduce by their vegetative parts such as roots, stems, and leaves.
Types of Vegetative Propagation: It is two types

• Natural vegetative propagation.
• Artificial vegetative propagation (Tissue culture).

Mint reproduces naturally by roots. Sugarcane, jasmine by stems and Bryophyllum by leaves. In bryophyllum, buds are produced in the notches along the leaf margins and when they fall on the soil, they develop into new plants.
Importance of Vegetative Propagation

• Plants can bear flowers and fruits earlier.
• Plants which have lost the ability to produce viable seeds can also reproduce by vegetative propagation.
• All plants are genetically almost similar to the parent plant.
• Seedless varieties can be obtained.
• The property of vegetative propagation is used by horticulturists in developing methods like layering, grafting to grow many plants like sugarcane, roses, or grapes.