Mosses usually grow close together in clumps or mats and can be found throughout most parts of the world including the arctic. Along with their cousins liverworts and hornworts, mosses are known as bryophytes and classified into the plant division Bryophyta, which is estimated to contain more than 18,000 identified bryophyte species. Dating back over 450 million years, bryophytes are considered by botanists to be some of the oldest plants in existence. Moe advanced vascular plants like ferns didn’t appear on earth until about 50 million years later.
Like many plants, mosses are small, green and photosynthetic. One way they differ is that instead of producing blooms and seeds, mosses reproduce by growing spores. They also lack any real vascular structures like leaves, stems and roots. Instead, they have root-like appendages called rhizoids. Rhizoids are thin filaments whose main function is to anchor the moss to its substrate. Water and nutrient intake occurs on the outside of the plant.
Although moss plants are typically found in wet, shady areas, mosses are actually one of the most drought resistant plants in existence. Some species have therefore adapted to survive in seasonally dry climates such as alpine rocks or stabilized sand dunes. During lengthy dry periods, mosses are able to completely dry-out and suspend all growth. When moisture returns, moss plants will essentially come back from the dead and resume their photosynthetic metabolism.
Scientists estimate that there are approximately 12,000 species of moss classified in the Bryophyta and about 1,000 of those can be found in the United States.