Like other members of the nonvascular plant phylum Bryophyta, mosses require water (even in the form of heavy dew) to assist in reproduction. They are simple, rootless green plants that flourish in moist and shaded terrestrial habitats. Mosses were the first plants to grow on the Earth's mineral substrate, and can be found worldwide clinging to soil, rocks, and trees, in locations as biologically and climatically diverse as the rainforests of the Amazon and coastal Antarctica.

View a high magnification image of the moss capsule.

As with many other types of primitive plant, the bryophytes reproduce through alternating sexual and asexual generations. Water facilitates transport of sperm from the male antheridium to the female archegonium, where the sperm fertilizes an egg to produce a zygote. From zygotes, elongated structures (setae) begin to grow out of the clumps of moss. Upon reaching maximum length, the tips of the setae begin to enlarge to form capsules, or sporangia. The sporangium, a spore-bearing structure, contains minute, developing spores and is attached to the seta by a structure called a foot. Inside the capsule, spores divide by meiosis, mature, and are shed by wind currents and breezes. Close examination of the capsule reveals a protective cover called the operculum, which falls away from the parasitic sporophyte upon maturity. In order to ensure their release into a moisture-laden environment, mature spores must pass by humidity-sensitive peristomal structures known as the teeth. The teeth increase and decrease the size of the opening at the end of the capsule by swelling and contracting in accordance with the moisture content in the air.

Mosses provide benefit to their ecosystems by breaking down exposed soil, thereby freeing nutrients for uptake by the roots of complex plants. Additionally, these low-lying evergreen plants reduce the damaging effects of water and wind erosion on soil structure by providing vegetative cover and absorption of water in moist terrestrial environments. Many animals depend on mosses as a source of nutrition, or in the case of some small invertebrates, as cover habitat providing a moist microclimate.

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