The answer is The flagellated sperm would not be able to swim to the eggs for fertilization.
Their life cycle of mosses has alternation of generation -
a haploid gametophyte and a diploid sporophyte. Gametophyte produces haploid
gametes which fuse to form a diploid zygote. The zygote grows into diploid
sporophyte which produces haploid spores. The spores grow into gametophyte and
the cycle begins again.
Mosses require water for reproduction. Before fertilization, the sperm swims to an egg through the water. Therefore, mosses require water for successful fertilization. In drought periods, without water, the sperm cell could not swim to the egg cell.
