The modes for reproduction in unicellular organisms are:
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(i) Binary fission:
It is found in unicellular animals like amoeba, paramecium, euglena, etc. After attaining an optimum size the adult individual undergoes a simple division into two daughters of equal size.
It consists of division of the nucleus followed by cleavage of cytoplasm into equal parts, each having a daughter nucleus.
Binary fission can occur in any place in amoeba. It is longitudinal in euglena and transverse in Paramecium.
The organisms undergoing binary fission are almost immortal since they never grow old but divide to form two young offspring after attaining the adult size.
(ii) Multiple fission:
The mode of asexual reproduction is found in unicellular animals or protozoans, e.g., amoeba, Plasmodium, monocystis. The nucleus of the parent cell undergoes repeated divisions to produce a number of daughter nuclei
The cytoplasm cleaves around each nucleus simultaneously to form a number of daughter cells or individuals inside a parent cell. Ultimately, under favorable conditions parent cell membrane ruptures and daughter individuals are released.
Sometime, the daughter cells derived from multiple fission develop resistant coverings to produce spores. The process is called speculation. In others the parent cell develops a cyst wall around before, undergoing multiple fission
Both the process is seen in amoeba. As in binary fission, the parent individual disappears during the formation of daughters in multiple fission
The modes of reproduction in multicellular organisms are: (i) Regeneration: The ability of an organism to replace its lost body parts is called regeneration. Hydra, planarian and sponges exhibit regeneration. In hydra, 1/6 mm body piece is capable of regenerating into an entire individual.
The mature spirogyra (algae) breaks into two or more pieces (fragments). Each fragment grows into new individual. The process is called fragmentation.
(iv) Sexual reproduction:
This is the mode in which male and female gametes fuse to from the zygote.