Final answer:
Mosses and ferns reproduce sexually through alternation of generations, which includes both a sporophyte and a gametophyte stage, where fertilization requires a moist environment for flagellated sperm to swim to the egg.
Step-by-step explanation:
The only reproductive method available to mosses and ferns is sexual reproduction through a process known as alternation of generations. This involves a visible sporophyte stage and a less conspicuous gametophyte stage. Ferns, for example, have a dominant sporophyte stage characterized by leafy fronds. On the underside of these fronds are clusters of sporangia that produce spores. When these spores germinate, they develop into gametophytes, which are tiny and heart-shaped.
Fern gametophytes contain both antheridia, which produce flagellated sperm, and archegonia, which produce eggs. Sperm swim to the egg for fertilization to occur, and this process typically requires a moist environment as a medium for sperm transport. The resulting zygote develops into an embryo, leading to the formation of a new sporophyte plant, thus completing the cycle.