Mosses are primitive plants without specialized tissues to transport water within the plants, so they're dependent on their thin leaves--often only a single cell thick--to absorb water. Though they sometimes have root-like filaments attaching them to the soil, these are not true roots. Some mosses do have thickened leaves that resist drying and, if dried, can revive quickly. Additionally, some mosses colonize sunny spots, but in general they prefer shade.
In spite of their limited ability to withstand drought, mosses are amazingly successful plants, with species adapted to certain rocks, to bogs and even underwater environments. Botanically called "bryophytes"--together with their cousins, the liverworts--mosses are the oldest land plants on Earth and are often found as pioneer plants in disturbed habitats. The sphagnum mosses found in acid bogs have created the deep deposits of peat used for fuel and soil improvement.
Mosses have two different life stages. The familiar leafy moss plant is called a
gametophyte because it produces both eggs and sperm. The fertilized egg grows into a spore-producing sporophyte generation. The sporophyte remains attached to the gametophyte and sends up spore-containing capsules on stems that stretch up above the leaves. The spores germinate into new gametophytes.
Mosses as Groundcover
Moss is fairly tolerant of foot traffic and never needs mowing, so you can use it as a lawn substitute in shady areas, the spots where grass refuses to thrive. It also makes an attractive addition to a rock garden or Japanese-style landscape. Moss is a traditional groundcover in Japan and is tended and weeded as carefully as lawns are in Europe and North America.
Moss isn't fussy as to conditions. It needs shade--the more the better--moisture, acid and rather poor soil; exactly the opposite, in most ways, from grass. You can try transplanting moss from the woods, but the best way to acquire species that will enjoy your site is to make sure the soil is acidic, water it a bit in dry spells and wait to see what turns up. One exception is lawn moss. This is often a more sun-tolerant species and you can usually get quite a bit just by raking it out of a lawn. Try it as a mulch around flowers or shrubs.