What is a Moss?

Dried specimen of Brachythecium rivulare from the Herbarium's collection of bryophytes.

Mosses are small primitive plants, that together with liverworts and hornworts, comprise the group of plants called bryophytes. Mosses thrive in wet habitats, but can survive in a wide variety of ecological niches. Like the more familiar flowering plants, mosses have green leaves and produce their own food through photosynthesis. However, in contrast with flowering plants, mosses do not have roots, or well-developed systems for conducting water and food. Dead cells in their tiny stems serve to conduct water. During dry periods, the plants do not die, but merely dry up and become dormant until they are wetted again.

Mosses reproduce not by seeds but by spores produced in capsules elevated on long stalks, the "sporophyte" (see bottom photo). To produce the spores, the male sperm must swim through water to reach and fertilize the female egg cells. Through meiosis, haploid spores (with only one set of chromosomes) are formed, which are dispersed to grow into new plants. Thus the major, leafy part of the moss plant is a haploid "gametophyte", while only the small sporophyte is diploid (with the full two sets of chromosomes). In contrast, the major portion of a flowering plant is the diploid sporophyte, and only the pollen and ovules represent the gametophyte.
Despite their small size, mosses represent a significant amount of the plant biomass of the world, particularly in moist and wet ecosystems, and they play an important role in the global carbon cycle.
The University of Colorado Museum of Natural History's Herbarium contains an excellent collection of over 116,000 specimens of bryophytes, mainly mosses, from all over the world.
To find out more about Colorado mosses and learn how to identify them, look for the just-published book Bryophytes of Colorado: mosses, liverworts and hornworts, by W.A. Weber and R. Wittmann, 2007, Pilgrim's Process Inc.
Mosses, Lichens and Ferns of Northwestern North America, by D.H. Vitt, J.E. Marsh, and R.B. Bovey, 1988, Lone Pine Publishing.