Images below courtesy UK Microscopy Moss shoots	Moss sporangia
Diagram courtesy The New York Botanical Garden

Class Musci, the mosses, is the largest and most diverse class of bryophytes, with more than 9,000 species. Mosses (the gametophyte plants) are anchored in the soil with thin rhizoids, which facilitate nutrient uptake. A short stem grows up from the rhizoids, and is covered by tiny leaves arranged in a spiral pattern around the stem, reaching no more than six inches in height.

Though mosses contain no vascular tissues, many species have a midrib vein, running from the base of the plant up the spiral, that distributes water and nutrients. There are two types of gamete-producing organs that grow in the gametophytes: the antheridia, which produce sperm cells, and the archegonia, which produce egg cells. About half of the mosses contain both of these on the same plant.

The sporophytes are stalks attached to the gametophytes, with a sporangium on top of each stalk. The mouth of the sporangium, in most species, is covered by an operculum, which functions like a lid. Controlling operculum activity, and thus the release of spores from the sporangium, is the peristome, composed of many small, toothlike structures around the operculum.

Mosses often grow together in vast mats that hold up all of the organisms, compensating for the lack of a rigid structure. Mats of mosses are spongy and can hold in water. In one group of mosses, the peat mosses (Sphagnum), decaying mats accumulate to form peat, which can be used as a fuel, in addition to having several other domestic uses.

Mosses are found worldwide, especially in moist, tropical areas, but also in drier climates. Some even live on ponds and lakes. Mosses have a lot of biological importance in their habitats. They serve as food for small animals, and their ability to hold vast quantities of water helps prevent soil erosion and flooding.