Mosses, hornworts and liverworts
Mosses, hornworts and liverworts are often collectively referred to as bryophytes. Bryophytes require water to reproduce sexually, produce spores rather than seeds and lack the vascular tissue found in ferns and “higher” plants. They differ considerably in other respects, however, and recent research indicates that the bryophytes do not form a natural taxonomic group in the sense that they do not share a single common ancestor, i.e., they are not monophyletic.
Hornworts (Anthocerotophyta) have irregular lobed or branching bodies, known as thalli, the tissue of which is not organised into organs. Guard cells form on the underside of the thallus. Cavities form under these that are typically filled with cyanobacteria. Only one species of hornwort has been found in Iceland, Carolina phaeoceros (Phaeoceros carolinianus). Its distribution is confined to geothermal areas.
Liverworts (Marchantiophyta) are either made up of a thallus or leafy stems; unlike many mosses, liverwort leaves do not have ribs (costas). Liverworts are much more common than hornworts in Iceland: 139 liverwort species have been found here to date.
Mosses (Bryophyta) have thin stems around which simple leaves branch out. Moss leaves frequently have a midrib. A total of 460 moss species have been found in Iceland.