ABSTRACT
The division Bryophyta includes three classes of small terrestrial plants: Musci (mosses), Hepaticae (liverworts), and Anthocerotae (hornworts). Bryophyte associations with nitrogen-fixing cyanobacteria are of specific interest because the cyanobacteria function in supplying fixed nitrogen to the plant tissue and are, in many ways, analogous to the symbiotic relationship between agronomically important legumes and rhizobia. Epiphytically associated cyanobacteria tend to be morphologically similar to growth forms in free soil, while those in specific endophytic association have distorted vegetative cell shapes and higher frequencies of heterocysts. In the liverworts and hornworts, as in other cyanobacterial associations involving a photosynthetic eukaryotic partner, the symbiotic Nostoc have a filament and cellular morphology markedly different from free-living cultures. The bryophytes respond to the endophytic presence of Nostoc by an increase in the cavity surface area, in part by proliferation of multicellular filaments of eukaryotic origin.
