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. They lack a distinctive kind of fluid-conducting cell—the tracheid—and are referred to collectively as nonvascular land plants.
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.
A moss, like all other plants, undergoes a complex life cycle in which the generations alternate between haploid and diploid forms. The most familiar form of the moss is the "leafy" haploid form, called the gametophyte.
Gametes form within specialized sex organs, called gametangia. The male gametangium is an antheridium. It produces sperm. The haploid gametophyte produces these haploid sperm by mitosis.
The female gametangium is an archegonium. Each archegonium produces an egg. Like the sperm, the eggs are haploid and are produced by mitosis.
Archegonia and antheridia are produced on the same individual in many species, so each individual has both male and female reproductive structures. However, adjacent individuals often fertilize one another's gametes, which helps maintain genetic diversity in the population.
For a moss, sexual reproduction requires water, which is one reason mosses are typically found in moist environments. Sperm cells require water for transport to egg cells. Once in the vicinity of an egg, a sperm cell swims to the egg and fertilizes the egg by fusing with it. The fusion produces a diploid zygote—the first cell of the next generation, called the sporophyte generation.
The zygote divides by mitosis and grows into a multicellular sporophyte. During the life of the sporophyte, it remains attached to the gametophyte and depends on the gametophyte for water and nutrients.
As the sporophyte continues to grow and elongate, the surrounding gametophyte tissue keeps pace, but eventually cracks at its middle. The remaining hoodlike tissue at the tip protects the growing sporophyte.
Underneath grows a sporangium filled with thousands of spores. The spores are haploid and are produced when diploid cells of the sporophyte undergo meiosis.
When the top of the sporangium is shed, it reveals a ring of teethlike structures holding in the spores. These "teeth" are highly responsive to humidity. They arch in and out again as they absorb water and then dry out. Each time the teeth open out, spores are released to the wind.
A spore that lands on moist ground will germinate, producing a filamentous, elongating structure called a protonema. This protonema gives rise to buds, which, in turn, develop into the next haploid gametophyte generation. The gametophytes in this generation will mate and continue the alternation of generations.
Alternation of generations is a feature of all plants. The nonvascular plants 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 haploid gametes from gametophyte plants fuse during fertilization. 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.