Mosses have been around for a really, really long time. In fact, these plants -- which scientists believe evolved from green algae -- probably originated 299 to 251 million years ago during the Permian period, though some evidence suggests that they may have appeared as early as 350 million years ago during the Devonian period [sources: Britannica, Richardson]. Today, mosses thrive in moist temperate forests, but can live just about anywhere, from the equator to the Arctic Circle. They're known to survive in all kinds of harsh environments: on rocks and trees, along seashores, on sand dunes, around hot springs and even at the bottom of lakes!
Mosses, along with their close relatives hornworts and liverworts, are members of the phylum Bryophyta. They're further classified into three groups: granite mosses, peat mosses and true mosses. There are only about 100 species of granite mosses, which are generally blackish in color, small in stature, and grow in mountainous or arctic regions. Bright green or reddish peat mosses typically grow in soft, muddy bogs and include around 350 species. True mosses are the most common type of moss by far, boasting approximately 14,000 species. This group includes the green carpetlike plant that most people picture when they think of moss.
What could all these different species possibly have in common? For one thing, they share the same basic structure. All mosses anchor themselves with small tufts of rootlike structures known as rhizoids. A simple stem supports the leaves, which are spirally arranged like the stripes on a candy cane. They're usually lance-shaped with a pointed tip, and are seldom more than one cell thick. The stem and leaves, collectively known as the gametophyte, house the reproductive parts of both male and female plants. Sometimes emerging from the top of female gametophytes is a sporophyte. This structure consists of a thin stalk called a seta that supports a little brown ball known as the capsule. Topping the whole thing off is the operculum, which looks like a hat sitting atop the capsule.
Right now all of these parts may seem pretty strange to you, but on the next page you'll find out how they help the moss feed itself and reproduce.