Plants: Types of Moss

Among Earth's oldest plants, mosses grow from desert climates, such as California's Mojave Desert, to mountains with frigid winter temperatures, including the Alaskan arctic. The upper green portion of moss is the capsule, and moss varieties range in size from those with capsules smaller than pinheads to those with fernlike capsules about 3 feet long. Mosses can be identified by their growing habits and by the habitats where they grow.

Biologists call mosses that grow more upright than horizontal "acrocarpi," from the Greek words "acros," meaning highest, and "karpos," which means fruit. Small capsules appear at the end of the stemlike sporophytes, giving these mosses the appearance of tiny trees. Common tree mosses include haircap mosses (Polytrichum spp., Pogonatum spp., Atrichum spp. and Timmia spp.), cave moss (Schistostega spp.) and sword moss (Bryoxiphium spp.)

Named for the Greek word "pleura," meaning rib or side, "pleurocarpi" mosses' capsules grow from the sides of branches, with each entire moss appearing to be horizontal or prostrate. Pleurocarpi mosses grow flat along tree roots and rotting logs. Varieties include sheet moss (Hypnum spp.), so named from the Greek word meaning sleep; fern moss (Thuidium delicatulum), with fernlike branches; and callicladium (Callicladium haldanianum), a common woodland moss.

Far fewer kinds of mosses grow in desert environments than elsewhere. Those mosses, including Syntrichia caninervis and Pseudocrossidium

crinitum, survive by becoming dormant during the dry months and can even survive for years without water. In deserts, most of these mosses are brown, tan and blackish brown, turning green when rains begin. Low elevations in the desert contain more clumping, acrocarpi mosses, and high elevations contain more creeping, pleurocarpi species.

The structures of water-tolerant, or poikilohydry, mosses can change to accommodate different levels of water in their environment, a characteristic from their prehistoric past when they began moving from the sea to land. At the same time, the mosses still have the desiccation ability of desert mosses to withstand dry periods. One such moss, Syntrichia ruralis, can appear dead and dry at its base but have a living section at the top. Other types can go through multiple wet and dry cycles each day.