Ascomycetous fungi, identified by the presence of simple septa and Woronin bodies, form a range of highly specialized associations with the unicellular rhizoids of 46 (16.2%) of the 284 species of British liverworts. The majority of these are in the two jungermannialian suborders Lepidoziineae and Cephaloziineae. Thirty three species (11.6%) also possess flagelliform axes. These extend to depths of 20-30 cm in peaty substrata and in most species bear abundant fungus-infected rhizoids. When grown in sand or water culture, isolated from their natural substrata (sandstone rocks, peat, Sphagnum, rotten wood), the hepatics are free from rhizoid infections. On returning the plants to their native substrata, the rhizoid-fungus associations are re-established. Each rhizoid or fascicle of rhizoids is infected independently and a continuous strand of fungus-containing cells is absent. Internally the flagelliform axes contain elongate parenchyma cells with abundant plasmodesmata in their transverse end walls. The apices of the axes are invested by mucilage and subapical amyloplasts appear to act as graviperceptive particles. The morphology of the flagelliform axes and the ultrastructure of the rhizoid-fungus association varies greatly between taxa. The Lepidoziaceae and Calypogeiaceae produce fascicles of rhizoids from the bases of rudimentary leaves whereas the flagelliform axes in the Cephaloziaceae are leafless. In Cladopodiella the fungus forms a pseudoparenchymatous sheath around the rips of swollen rhizoids. Development of swollen rhizoids precedes fungal infection in Lepidozia, Kurzia and Telaranea but not in other genera. Hyphae penetrating the basal walls of the rhizoids in Cephalosia, Nowellia, Calypogeia and Odantoschisma are ensheathed by host Wall material. These diverse patterns of fungal infection suggest independent, and probably relatively recent, evolution in the various genera. The flagelliform axes in hepatics appear to be major organs of assimilation and perennation structurally and functionally analogous in the former capacity to vascular plant mycorrhizas and particularly the hairs roots in Ericaceae.
Keywords: Bryophytes; liverworts; mycorrhizas; rhizoids; ultrastructure.