The bryophytes are a diverse group of "lower" land plants, with some 23,000 described species worldwide, making it the largest group of land plants excepting flowering plants. The group includes three quite distinct lineages (i.e, mosses, hornworts, and liverworts) of familiar species frequently encountered in mesic forests and along streams, as well as a number of less familiar species of tropical rainforests, arctic tundra, and desert boulders. The bryophytes are generally considered a "key" group in our understanding of how the modern land plants (the embryophytes) are related to each other phylogenetically and how they came to conquer the hostile land environment from their primitive home in fresh water (habitats still occupied by relatives of the land plants, the green algae). This is because of the apparent basal phylogenetic position of the bryophytes among the extant embryophytes, the remnant lineages present today from the spectacular radiation of the land plants in the Devonian Period, some 400 million years ago.

Their phylogeny has attracted interest over a long period of time (e.g., Campbell, 1905; Bower, 1935; Khanna, 1965; Crandall-Stotler, 1980; Miller, 1982; Crandall-Stotler, 1984; Schofield, 1985; Duckett and Renzaglia, 1988). Based on recent morphological cladistic analyses, the classical group "bryophytes" appears not to be monophyletic. Instead, the mosses seem to be more closely related to the "vascular plants" (= tracheophytes) than to the hornworts or liverworts (Mishler and Churchill, 1984; Mishler and Churchill, 1985; Mishler, 1986a; Bremer, et al., 1987). Even more recent molecular cladistic analyses confirm the non-monophyly of the bryophytes, and the relatively basal position of the liverworts, although the precise interrelationships of the mosses, hornworts and tracheophytes remain problematic (Mishler, et al., 1992; Waters, et al., 1992).

The bryophytes represent some of the most species-rich lineages of land plants, presenting challenges (as well as opportunities) for understanding processes of diversification. They have several biological features making them particularly suited to serve as study organisms in macroevolutionary, population genetic, and ecological research (see Shaw, 1991). The plants are complicated enough in ontogeny and mature structure to serve as model systems for studying land plant evolution, and yet simple enough morphologically (constructed as they are from easily traceable cell lineages derived from single apical cells) and genetically (given the haploid vegetative plant body) to be readily studied with current techniques. They are, in the great majority of cases (except for some ephemeral species), observable throughout the year; they are also small in size, easily regenerated from fragments, and thus also easy to study in culture. For a few examples, the suitablity of bryophytes has been demonstrated for studies of: population genetics (Shaw, 1987; Wyatt, et al., 1988); species cohesion (Mishler, 1990); niche differentiation (Watson, 1980; Slack, 1990); reproductive biology and phenology (Stark, 1983; Stark and Stephenson, 1983; Kimmerer, 1991a; Kimmerer, 1991a,b; Mishler and Oliver, 1991); physiological ecology (Longton, 1981); relationships between ontogeny and phylogeny (Mishler, 1986b; Murray, 1988); and cladistic biogeography (Churchill, 1981; Churchill, 1985; Vitt 1995). Despite this intrinsic interest and potential for scientific study, a major limitation in the use of bryophytes as study systems for evolutionary and ecological processes has been the lack of basic floristic and alpha taxonomic knowledge of the plants in many regions, especially the tropics.

Bryophytes tend to have distributional ranges that correspond to historical biogeographic patterns of tracheophytes (Crum, 1966; Schofield, 1969; Crum, 1972; Schofield, 1983), but intriguingly, species of bryophytes (at least with currently prevailing species concepts in the group) usually are relatively more widely distributed. Bryophyte species tend to be highly specific for particular microenvironments (responding to such features as temperature, light and water availability, substrate chemistry, etc.), making them good ecological indicator species. Thus, bryophytes are attracting much attention recently from applied ecologists and conservation biologists. For example, bryophytes appear to be an ecological keystone group in rainforests; the temperate rainforest which extends along the coast from southern Alaska to northwestern California has been studied in some detail. Nadkarni (1984) suggested that bryophytes in such areas provide a major buffer allowing measured loss of minerals from the tree to the soil -- a loss slow enough to allow almost complete reabsorption by the tree. Tropical rainforests have been studied much less well, but there is a suggestion that epiphytic bryophytes serve many important ecosystem functions there as well (Pócs, 1982; Richards, 1984; Gradstein and Pócs, 1989).

Accurate classifications and correct identifications are a necessary preliminary to any ecological comparisons among regions, yet the bryoflora is probably the most poorly documented element of land vegetation in most environments, especially in the tropics. Knowledge of the systematics and evolution of bryophytes has generally lagged behind that of the larger land plants, primarily because of their small size and the few scientists specializing on the group. In the United States there are about 76 systematists that work on mosses (actual count from the directory of the American Bryological and Lichenological Society, while about 1390 systematists work on tracheophytes (actual count from the directory of the American Society of Plant Taxonomists).

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