1. REPRODUCTION IN ORGANISMS
- Reproduction is a process in which an organism produces young ones (offspring) similar to itself.
- The period from birth to the natural death of an organism is known as its lifespan.
- No individual is immortal, except unicellular organisms. There is no natural death in unicellular organisms
Life spans of some organisms
- Based on the number of participants, reproduction is 2 types: Asexual reproduction & Sexual reproduction.
- It is the production of offspring by a single parent.
- It is seen in unicellular organisms, simple plants & animals.
- The offspring are identical to one another and to their parent. Such morphologically and genetically similar individuals are known as clone.
Types of asexual reproduction
a. Fission: In this, the parent cell divides (cell division) into two or more individuals. E.g. Protists and Monerans. Fission is 2 types:
- Binary fission: It is the division of parent cell into two individuals. E.g., Amoeba, Paramecium.
- Multiple fission: It is the division of parent cell into many individuals. E.g. Plasmodium, Amoeba.
Under unfavourable condition, Amoeba withdraws its pseudopodia and secretes a 3-layered hard covering (cyst) around itself. It is called encystation. Under favourable conditions, encysted Amoeba undergoes multiple fission to give many minute amoeba or pseudopodiospores. The cyst wall bursts out and spores are liberated to grow up into many amoebae. This is called sporulation.
b. Budding: In this, a bud appears and grows in the parent body. After maturation, it is detached from parent body to form new individual. E.g. Hydra, Sponge, Yeast etc.
c. Fragmentation: In this, the body breaks into distinct pieces (fragments) and each fragment grows into an adult capable of producing offspring. E.g. Hydra.
d. Vegetative propagation: It is the production of offspring from vegetative propagules in plants.
Vegetative propagules are units of vegetative propagation.
Examples for vegetative propagules:
- Buds (‘eyes’) of the potato tuber.
- Rhizomes of banana & ginger.
Buds & Rhizomes arise from the nodes of modified stems. The nodes come in contact with damp soil or water and produce roots and new plants.
- Adventitious buds of Bryophyllum. They arise from the notches at margins of leaves.
- Bulbil of Agave.
- Offset of water hyacinth.
- Runner, sucker, tuber, bulb etc.
Other asexual reproductive structures: E.g. zoospores (microscopic motile structures in some algae and protists), conidia (Penicillium) and gemmules (sponge).
Asexual reproduction is the common method in simple organisms like algae and fungi. During adverse conditions, they can shift to sexual method.
Higher plants reproduce asexually (vegetative) & sexually. But most of the animals show only sexual reproduction.