The main plant body of Spirogyra is a gametophyte (n). It produces haploid gametes (n). Gametes of two opposite strains fuse form the zygospore (2n) – the first cell of sporophyte.
The diploid nucleus of the zygospore undergoes meitotic division forming haploid nuclei that germinate to give rise to the gametophytes (n).
Thus, the gametophyte generation dominates over the sporophytic generation because the sporophytic generation is strictly confined to zygospore. Hence the life cycle is haplobiontic.