A TAXONOMIC REVISION OF THE PROPAGULIFEROUS SPECIES OF POHLIA (MUSCI) IN NORTH AMERICA

Abstract

  The genus Pohlia is represented by about 29 species in North America. Nine species in the section Pohliella are characterized by asexual, axillary propagula produced in the leaf axils of sterile, or rarely fertile gametophytes. Species of this propaguliferous complex are notably uniform in most sporophytic features, such as exostome and endostome morphology, exothecial cell pattern and annulus morphology, and this group probably forms a monophyletic assemblage. Each of the nine North American propaguliferous Pohlia species can best be defined on the basis of the developmental pathway(s) which characterize its propagula. Because of their similarity in position, appearance, and manner of development, these propagula are considered homogous with axillary branches. A continual gradient of structural complexity exists between propagula of different species, and is interpreted as a reduction series. Evolutionary reduction in propagulum complexity probably occurred through progressive modification of ontogenetic pathways.