Reproduction is a function through which a living organism is able to produce more of its own kind. It is one of the fundamental attributes of living organisms. In this process new individuals of a species are produced and perpetuated. Thus, the population of a species is increased by reproduction. A species is defined as a group of organisms (plants or animals) that can interbreed to produce offspring.
Continuity of life, from the time of its origin (millions of years ago) to the present day, has been possible through reproduction. The offspring, thus, produce take in food, grow, and reproduce again. Hence, reproduction is an essential life process which helps an organism to perpetuate its kind on this earth. Reproduction is living organisms takes place by two general methods: asexual and sexual.
Asexual Reproduction in plants and Animals
The basis characters of this reproduction are: (i) only one individual of an organism is involved, (i) opposite sexes are not involved, (iii) cell divisions are either amitotic or mitotic, (iv) the new individuals produced, are genetically identical to the parent, and (v) it represents a rapid mode of multiplication. This the common mode of reproduction only in lower plants and animals, where the body is not very complex. As there is no fusion of sex cells (gametes), the offspring is produced without the process of fertilization. The different forms asexual reproductions are mentioned below.
1. Binary Fusion
The type of division, in which two individuals are formed from a single individual, is called binary fusion. This is the simplest method of asexual reproduction which is found in unicellular organisms. The cells divide into two daughter cells. The daughter cells are morphologically and genetically identical. In this method, the nucleus first divides amitotically into two, which is followed by the division of the cytoplasm. The cell finally splits into two daughter cells. Binary fission occurs under favorable environmental conditions. In plants binary fission can be seen in bacteria, yeast and euglena. Among animals, Amoeba and paramecium can be taken to study binary fission.
2. Multiple Fission
The type of division, in which many individuals are formed a single individual, is called multiple fission. Under unfavorable circumstances, some unicellular organisms developed a protective covering over the cell. The nucleus of the cell divides repeatedly producing many nuclei. Latter on, each nucleus is surrounded by small amount of cytoplasm and many daughter cells are produced within the hard covering called cyst. When the favorable condition return, the cyst breaks up and small offspring are liberated. Multiple fission is seen in many algae (lower plants). A common example of multiple fission in animals is malarial parasite (plasmodium). In both types of division, the whole body of an organism represents the reproductive unit.
In this process, a bulb-like projection is formed on one side of the body. The bud may be unicellular or multi-cellular depending upon the type of the parent organisms. One or more such buds may be produced from a single parent body. The nucleus divides and one of the daughter nuclei passes into the bud. The bud is finally separated from the parent body and becomes a new individual after growing to full size. Formation bud is called budding. This process is found in yeast among plants. In animals, budding can be commonly seen in hydra and sponges. In the former the buds are external, in this latter buds may be internal.
4. Fragmentation or Regeneration
Fragmentation is the process in which the body of an individual breaks up into two or more parts and each part develops into a complete organisms. In case of plants, filamentous algae like spirogyra regenerates in this way. In animals, worms like flatworms (planaria) and tape worms also reproduce by this method.
Spore formation is a common method of asexual reproduction in lower forms of life such as bacteria, lower plants, etc. in many algae, a single parent cell divides 2 to 3 times to produce 4to 8 or more motile (capable of moving as a whole) zoospores. The cell gets enveloped by a tough covering and the entire structure is known as sporangia or zoosporangia. Under favorable conditions, the spores are released by breaking the thick wall of the sporangia. Spores then germinate into new individuals. In fungi, sporangia burst and release spores. Spores are light in weight and very small in size. By this method of asexual reproduction, plants overcome unfavorable conditions of heat, dryness, high temperature, high acidity, alkalinity, etc.