Bryophytes are nonvascular seedless plants that include mosses, liverworts, and hornworts. There are more than 20,000 species of moss, with some species microscopic in size and others more than 1 meter in height. Liverworts are leafy, flowerless, rootless plants. Hornworts are aquatic plants that have forked leaflike structures and form branched clusters. These three plants seem to have little in common except for their life cycles. Many mosses are small and live in dense, compact mats that make them appear to be one plant rather than thousands of individual plants. A blanket of soft, spongy moss is the gametophyte (sexually reproducing) stage of the moss life cycle.
In the life cycle of bryophytes, the gametophyte, or the multicellular haploid form taken by a plant during alternation of generations is larger and lasts longer than the sporophyte, an organism that produces spores in a multicellular diploid form. Bryophytes create offspring by sexual reproduction but are also capable of asexual reproduction. Many gametophytes are capable of producing offspring without sporophytes. When this occurs, specific multicellular structures break away from the original plant. With liverworts and mosses, tissue clusters detach from the initial plant and are carried away by water to form new plants. The water may be raindrops or splashes of water from nearby streams, rivers, or waterfalls. It only takes small droplets to carry off the clusters of cells that will be the basis for new growth.An organism is heterosporous if it has two or more different types of spores. Bryophytes are heterosporous because they have two different types of spores. One type of spore develops into male gametophytes, and the other type develops into female gametophytes. The standard bryophyte life cycle begins with a germinating bryophyte spore. The spores begin their development, forming a mat-like structure called a protonema. The protonema, in turn, develops into the gametophyte stage, in which the plant develops leaflike structures. The gametophytes develop branches, and separate branches may be either male or female. Formed from male spores, the male reproductive organ in mosses and ferns is the antheridium (plural, antheridia), which releases sperm. Formed from female spores, the female reproductive organ in mosses and ferns is the archegonium (plural, archegonia), which holds the egg. The sperm fertilizes the egg and forms the basic diploid cell that is the embryonic sporophyte. The sporophyte develops three structures: a foot, a spore capsule, and a stalk. The foot serves as a hold-fast, anchoring the young sporophyte to the gametophyte. At the opposite end of the sporophyte, a spore capsule forms, and the stalk provides support for the sporangium (plural, sporangia), which is a sac for producing spores. The foot absorbs nutrients from the gametophyte, and the stalk conducts nutrients to the capsule, where they are used to produce spores. The capsule releases spores, and the life cycle continues.