Bryophytes
Mosses (Bryophyta)
The mosses are a large and diverse group of plants with 10,000-12,000 species. Mosses are generally considered to be “non-vascular” (lacking the specialized conducting tissues xylem and phloem), although some mosses contain functionally similar conducting cells (hydroids for conducting water and leptoids for conducting photosynthates). Because of their general lack of “true” tissues and (consequently) organs, the body of a moss is traditionally referred to as a thallus. That said, the thallus consists of stem-like and leaf-like portions, and these terms are usually applied for convenience. Mosses are anchored to their substrate by strands of cells called rhizoids.
Mosses, liverworts, and hornworts have a similar variation of the sporic life cycle in which the haploid gametophyte stage is perennial (long-lived), photosynthetic, and free-living. For this reason the gametophyte is described as the dominant generation. The sporophyte generation is much reduced (small), short-lived (in mosses and liverworts living just one growing season), and nutritionally dependent on the gametophyte. The sporophyte of mosses and liverworts lives just long enough to form and release the spores, then dies. The spores are released directly to the environment (hence these plants are free-sporing).
Although mosses are common in most habitats they are small and easily overlooked. They are familiar in shady, moist habitats, but are also very common in dry, often exposed habitats. Some are common components of cryptobiotic crusts in our deserts of southern Utah. Other moss species are pioneer species that grow on bare rock, sand, or clay (or fence posts, roofs, or brick walls). They are often commensals on tree trunks of branches. Mosses tend to be substrate specific and may be used in ecological studies as indicators of microclimates.
Life Cycle: A generalized moss life cycle is summarized below. Liverworts and hornworts have similar life cycles.
Gametophytes may be either monoecious (one individual forming both sperm cells and egg cells) or dioecious (male individuals and female individuals). Sporophytes are homosporous (form just one type of spore).
Liverworts (Hepatophyta)
There are about 9,000 species of liverworts. Liverworts are often mistaken for mosses due to their similar small size and morphology. They also are abundant in the same moist, shady habitats favored by many mosses. Liverworts share additional features with mosses including: lack of true tissues and organs, life cycle with a dominant gametophyte generation and a reduced sporophyte that is nutritionally dependent on the gametophyte.
Liverwort gametophytes may be “leafy,” with stem-like and leaf-like portions or “thallose” with a flattened thallus. Leafy liverworts may be difficult to distinguish from mosses, but are usually dorsoventrally flattened with their leaves in three rows, the bottom row of leaves smaller than the other two. Gametophytes are attached to the substrate by unicellular rhizoids. Liverworts form a thin cuticle, but are the only members of the plant kingdom that lack stomata. Some thallose liverworts form air chambers on their upper surfaces that are connected to the outside environment by tiny pores. Air chambers contain short filaments of photosynthetic “cactus cells.”
Gametangia are usually borne on the surface of the thallus or leafy stem, but in some are raised above the thallus on antheridiophores or archegoniophores. Sporophytes develop within the archegonium and consist of a foot, seta, and a simple sporangium. At maturity the seta elongates, the spores are released, and the sporophyte rapidly withers.
Hornworts (Anthocerotophyta)
Hornworts represent a relatively small clade of plants (120-180 species) that are morphologically similar to thallose liverworts. Their life cycle is similar to that of mosses and liverworts with a long-lived dominant gametophyte and a smaller sporophyte that is nutritionally dependent on the gametophyte. Archegonia and antheridia are sunken in the thallus. Unlike mosses and liverworts, the sporophyte of a hornwort is perennial and will live and continue to grow as long as the thallus stays alive. The sporophyte is cylindrical and anchored in the gametophyte tissue by its foot. New sporophyte tissue is formed from a basal meristem located just above the sporophyte foot. Even while the older, distal portions of the sporophyte are releasing spores, the newly formed proximal portions of the sporophyte are still developing.
