Hornworts, liverworts, and mosses - commonly referred to as bryophytes - are considered to be a pivotal group in our understanding of the origin of land plants because they are believed to be among the earliest diverging lineages of land plants. Mosses, liverworts and hornworts are found throughout the world in a variety of habitats, from the harsh environs of Antarctica to the lush conditions of the tropical rainforests. Bryophytes are unique among land plants in that they possess an alternation of generations, which involves a dominant, free-living, haploid gametophyte alternating with a reduced, generally dependent, diploid sporophyte. Bryophytes are small, herbaceous plants that grow closely packed together in mats or cushions on rocks, soil, or as epiphytes on the trunks and leaves of forest trees. Bryophytes are remarkably diverse for their small size and are well-adapted to moist habitats and flourish particularly well in moist, humid forests like the fog forests of the Pacific northwest or the montane rain forests of the southern hemisphere.