This is one of the 12 large plant divisions. It includes the mosses, liverworts and horned liverworts. Most are small land plants. They have no true leaves or stems. They are held in the soil by rhizoids. They do not have conducting tubes. However, some cells are similar to sieve tubes in higher plants.
Class Musci, the mosses, has about 15,000 species. At their most familiar stage, they are green and leafy. Mosses reproduce by forming multicellular spores. These spores develop into the leafy green gametophyte plants. Gametophytes produce both male and female cells. The fertilized egg develops into a stalk and capsule during the sporophyte generation. This stage of development is partly parasitic on the gametophyte. Mosses are found in moist places from the tropics to the polar areas.
The class Hepaticae, or liverworts, is the most primitive. There are about 8,000 species. They grow in moist shade, mainly in the tropics. Some grow as epiphytes, or nonparasitically, on trees. The plant body is a thallus. The common liverwort found in the temperate regions is called Marchantia. The sexual organs form a head of rays on top of a tiny stalk. The sporophyte capsule forms under the female rays.
The class Anthocerothe or horned liverworts, includes only fifty species. The small, flat thallys is only 1 inch log. Horned liverworts are almost the only higher plants with pyrenoids. They are homothallic.