Ans.
Gametophyte shows haploid, gamete-forming stage of plant's life cycle, while sporophyte shows diploid, spore-forming stage of plant's life cycle. In green algae, gametophytes and sporophytes are isomorphic (indistinguishable). In bryophytes, such as mosses, gametophyte shows dominant stage as it is most visible stage in their life-cycle. In vascular plants, such as ferns and seed plants, sporophytes show dominant stage.
Thus, 'seed plants show briefest gametophyte stage' and 'mosses show longest gametophyte stage.'
