Theory:

The broken filaments of the organism that develops into a new organism are known as fragmentation.
Each of these fragments contains a protoplasm, so they grow into a new filament of the algae by cell division. Fragmentation occurs when the parent is matured and broken into pieces. It is seen in Spirogyra.
 
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Fragmentation in Spirogyra
 
The organism with simple body organisation can reproduce by cell-by-cell division. But, yet it is not possible with the organisms with the complex structural organisation.
 
When the organisation of life is complex, then the reproduction process will be complex. In these circumstances, multicellular organisms reproduce as fragments or pieces, such as spirogyra.
 
In the favourable conditions, fragmentation is the preferred mode of reproduction.
  • After fragmentation, the filament grows into a new filament. To generate a new filament, each fragment undergoes several divisions and elongation.
  • Fragmentation can occur as a result of mechanical injury or the disintegration of the central lamella as the water's salinity and temperature vary.
  • The filament can be broken when one cell's centre lamella protrudes into an adjacent cell.