When haploid (n) or gametophytic generation and diploid (2n) or sporophytic generation repeat alternately in the life cycle of any organism, it is called alternation of generation.
The life cycle of Spirogyra Consists of the following phases
i) Gametophytic generation or haploid (n) phase, and
ii) Sporophytic generation or diploid (2n) phase
i) Gametorthytic generation (n): Spirogyra is a gametophytic plant. Through conjugation, lateral or scalanform, diploid (2n) zygospore is formed after the union (n+n) of gametes (male and female gametes) Gametophytic generation ends just after the formation of zygote (2n) and sporophytic generation start.
ii) Sporophytic generation (2n): Zygote (2n) is the only step of the sporophytic generation. The diploid generation ends just after the meiosis division.
Formation of new plant from haploid spore (n): Out of four cells formed from meioses division, only one remains active and three others are degenerated.