A moss is a member of the plant phylum Bryophyta. These plants, along with the liverworts (Hepatophyta) and hornworts (Anthocerophyta), lack well-developed vascular systems. For their lack of a distinctive kind of conducting cell—the tracheid—they are called nontracheophytes.
The life cycle of a moss, like all plants, is characterized by an alternation of generations. A diploid generation, called the sporophyte, follows a haploid generation, called the gametophyte, which is in turn followed by the next sporophyte generation.
Alternation of generations is a feature of all plants. The nontracheophytes are unique, however, in that the gametophyte generation, rather than the sporophyte generation, is the most conspicuous.
The green, "leafy" mosses on the banks of streams are all haploid gametophytes. The diploid generation of the plant arises after a male and female gametophyte mate and fuse their haploid gametes. The resulting diploid zygote grows into the sporophyte—the long stalked structure bearing a sporangium. This sporophyte is a new generation in the life cycle, yet the new organism can never leave the gametophyte, because it depends on the gametophyte for its nutrients.
When the sporangium breaks open and releases its haploid spores, a new generation of gametophytes can germinate.
Textbook Reference: Concept 21.2 Key Adaptations Permitted Plants to Colonize Land