Question

What happens when a mature spirogyra filament attains considerable length?

Answer 1

when a matured spirogyra reaches considerable length then,
they simply break down up into smaller pieces. These pieces or fragments grow into new individuals.
see attachments for better understanding

Answer 2

It will start to reproduce by dividing itself into number of fragments. Later these fragments will grow into an individual spirogyra. This method is known as fragmentation.