A Typical Bryophyte Life Cycle
Although individuals of the three bryophyte groups differ from one another morphologically and in other details, the moss life cycle shown in Figure is typical of the group in general.
As do all plants, bryophytes alternate a gametophytic generation with a sporophytic one (a sporic meiosis, a life cycle in which meiosis gives rise to spores, not gametes). Each of the haploid (1 n) spores is capable of developing into a multicellular, haploid individual, the gametophyte. The first structure formed from spores in most mosses and many liverworts is a filamentous, algal-like, green protonema (plural,protonemata). In some mosses the protonemata are long lived with rhizoids and aerial filaments and they often form dense green mats in suitable sites. Cells in the protonema, probably stimulated by red light and kinetin, give rise to shoots, which enlarge and become the mature gametophytes. In the bryophytes, these are the dominant, independent (photosynthetic) plants.
The gametophytes initiate gametangia on special branches or at the tip of the main shoot. In these structures the gametes— eggs and sperms—are produced during the sexual portion of the cycle. The female gametangium—called an archegonium—and the male antheridium may be produced on the same plant or on different plants. In both kinds of gametangia, a protective layer of non-reproductive tissue—a sterile layer—surrounds the inner reproductive cells. (A sterile layer is absent in algal gametangia and is considered an upward evolutionary step towards the protective seed coats of flowering plants.) Mature sperm, released from the tip of the antheridia when dew or rainwater is present on the surface of the plants, swim to the archegonia and down the necks to reach the eggs. One fuses with the single egg in each archegonium—the process of fertilization—thus combining the sperm and egg nuclear and cytoplasmic material. The resulting cell, a zygote, has a diploid (2 n) chromosome number and is the beginning of the sporophytic generation. This reproduction is termed oogamy—a large, nonmotile egg is fertilized in the archegonium by a small, motile sperm that swims to the egg. In the bryophytes, an external film of water on the surface of the plant is the passageway for the biflagellate sperm; in more advanced plants, sperm move internally within special structures (pollen tubes) to reach the eggs.
After fertilization, the zygote remains in the archegonium and divides by mitosis repeatedly to form a multicellular, diploid embryo, the young sporophyte. Sugars and other materials are translocated from gametophyte to the developing sporophyte through placental tissue, a type of nutrition called matrotrophy. (No plasmodesmata connect the gametophyte and sporophyte; movement of material is along the cell wall, that is, it is apoplastic movement). The sterile jacket cells also divide and in mosses form a tight cap, the calyptra, over the tip of the developing sporophyte. The mature sporophyte in both liverworts and mosses consists of a foot, seta, and capsule. The moss capsule has modifications to assist in spore release: a cap, the operculum, covers the opening, and peristome teeth form a ring around the mouth of the capsule. Sterile cells, elaters, within the capsule are hygroscopic and as they alternately absorb water and dry out, they twist and turn pushing the spores upward and outward.
The hornwort sporophyte that develops from the zygote is an erect, long, green cylinder with an absorbing foot embedded in the gametophyte thallus. The sporophyte is photosynthetic and has stomata so it doesn't depend entirely upon the gametophyte for sustenance. Spores are produced in the cylinder around a central columella of sterile tissue and are released as the mature tip of the sporophyte dries out and twists in the air. At the base of the foot, a zone of meristematic tissue continues to divide and the sporophyte is thus continuously renewed from the base.