Moss, any of a class of small, simple, green plants. They grow in cool, moist places, often forming dense, velvety masses of vegetation. Individual plants as well as the masses are called moss. Mosses are related to liverworts and hornworts.

Several other plants, unrelated to the mosses, commonly are called “moss” because they grow in mosslike clusters.

About 14,000 species of mosses occur throughout the world, from the polar regions to the tropics. Most species live on shady ground, on rock ledges, or on tree trunks. (Moss clumps are most common on the side of the tree that gets the most shade and moisture, often the north side.) A number of species live in rivers and ponds. Moss plants do not grow in saltwater.

The moss plant is hardy. During a dry spell it turns brown or black, and looks dead; but it becomes green again as soon as rain falls. The mosses are among the first plants to establish themselves on rocky ground. They slowly break down the rock, preparing the way for more highly developed plants. Moss plants absorb many times their weight in moisture; they soak up rainfall on hillsides, helping to prevent erosion. The soil-building and moisture-conserving work of the mosses is indirectly of great importance to humans. The only type of moss of direct use to humans is the bog moss, or sphagnum. It forms peat, a fuel, and peat moss, a garden mulch and soil conditioner. Dried sphagnum is used as packing material in shipping plants.

How A Moss Plant Grows

The mosses are bryophytes—primitive, nonflowering plants lacking true roots. They are anchored to the material on which they live by slender filaments called rhizoids. A moss plant contains chlorophyll and manufactures its own food by photosynthesis. It reproduces by alternation of generations. A single-celled reproductive body called a spore develops into a threadlike, branching structure, called a protonema. The protonema produces a gametophyte —the stemmed moss plant of the sexual generation. The gametophyte bears cuplike structures containing either male or female reproductive cells, or both. Each fertilized female cell gives rise to the asexual generation—a sporophyte, or stalked capsule containing spores. The spores are released into the air to repeat the cycle.

Mosses of the United States

Among the 1,000 species of mosses in the United States, the following are common.

Bog Moss, or Sphagnum,

has an erect stem bearing three or four branchlets at regular intervals. Color and leaf shape vary. This moss forms thick mats on the surface of ponds.

Broom Moss,

common in woodlands, has all its leaves on one side of the stem. It forms lustrous dark green tufts or cushions three to four inches (7.5 to 10 cm) high.

Cord Moss,

a weed, is especially common in burnt fields. It has reddish brown stalks bearing dark brown, pear-shaped capsules. The stalks rise from budlike clusters of leaves.

Fern Moss

grows on wet ground or tree trunks or in flowing water. It has sprawling branches that resemble fern sprays.

Haircap Moss, or Pigeon-wheat,

is dark green the year round. Its erect stems often grow to six inches (15 cm) in height. The leaves are silvery at the base. It grows in the woods and at the edge of bogs.

Pincushion Moss

a very tiny plant, forms a cushion at the base of trees in damp woods.

Plume Moss,

or Feather Moss, covers logs and stumps. Its flattened branches look like tiny evergreen sprays.

Purple Ceratodon,

common in dry places, has three-inch (7.5-cm) branching stems whose lower parts are covered with fine leaves curved slightly backward at the tips. The moss is bright green in winter and turns purplish in summer and fall.

Silvery Bryum

is common in pavement cracks and on dry compact ground. It has silvery shoots and dark green leaves.

Tree Moss

is an erect plant forming dark-green clumps in woods and swamps. It is six inches (15 cm) tall and looks like a miniature tree.

Water Moss

lives beneath the surface of streams and ponds. Its long, slender branches are covered with scaly, brownish-green leaves.

Woodsy Mnium

is common on moist, shaded lawns. Some of its leafy green shoots arch over sideways; other are erect and may produce capsules.

Mosses make up the class Musci of the division Bryophyta. Bog moss belongs to the genus Sphagnu; broom, Dicranum; cord, Funaria; fern, Thuidium; haircap, Polytrichum; pincushion, Leucobryum; plume, Hypnum; purple ceratodon, Ceratodon; silvery bryum, Bryum; tree, Climacium; water, Fontinalis; woodsy mnium, Mnium.