(C) Bryophyta - Mosses

    Mosses are the most diverse gametophyte-dominant lineage, with more than 10, 000 species in 800 genera.  Though the earliest true moss fossils date to approximately 300 m.y.a., the moss lineage is thought to have diverged from the horworts perhaps around 420 million years ago.  Mosses can be found in almost all terrestrial environments, and they are often important pioneer organisms, being among the first to colonize newly exposed or barren land.  As such, they have an important role in stabilizing soil and inhibiting erosion.  The so-called "granite mosses," which grow on exposed rock surfaces above 8, 000 ft. in elevation,  play a part in rock decomposition and soil formation.  Xerophytic mosses -- those that grow in arid environments -- are highly resistant to desiccation; some can dry completely and then revive with moisture after long periods of dormancy (as long as 19 years, in some cases).

    The moss gametophyte is, in contrast to the thalloid liverworts, more or less erect or elongate.  The moss gametophyte also demonstrates a suggestion of cell specialization: in the "stem" portion, some elongated cells seem to act as a primitive vascular system.

Examine a piece of moss with both a dissecting scope and under higher power.

1. Note the central "stem" with numerous "leaves" attached.  Are the leaves similar in structure to the liverwort thallus?  How do they differ?
2. Carefully remove a whole gametophyte from the soil, wash, and locate the rhizoids attached to the basal portion of the plant.
3. Examine a gametophyte with an attached sporophyte.  You will examine the sporophyte in more detail later. For now, note that it is attached to the gametophyte and essentially consists fo a stalk and a sporangium, with a "foot" where it attaches to the gametophyte.