When originally described from deeply shaded banks in Malaysia and Borneo the thalloid liverwort Mizutania riccardioides was tentatively assigned to a new family in the Metzgeriales. However, phylogenetic inferences from sequence data indicate that it arose within the Calypogeiaceae. The present ultrastructural study of Mizutania reveals that a unique tripartite wall structure underlies the verrucose surface of the thalli. Outside an inner fibrillar layer, like that in other bryophytes, its middle stratum comprises closely packed lamellae. This recalls similar wall structures in blue iridescent pteridophytes and is most likely responsible for the same phenomenon. Unlike the rounded papillae contiguous with the outer wall layer in other liverworts, the outer part of the wall in Mizutania comprises irregular blocks of dense material, many of which become detached but remain embedded in a thick investiture of mucilage. These dense blocks contain high concentrations of aluminum and silicon; this is the first time that both these elements have been found co-localized in bryophytes. Mizutania has swollen, fungus-containing rhizoids similar to those of the Lepidoziaceae, Cephaloziaceae and Calypogeiaceae but unlike the last family, its gemmae are endogenous rather than exogenous. Whilst the fungal association is in line with the affinities determined from sequencing, its endogenous gemmae, unique wall structure and apical organization are robust morphological criteria for retaining a subfamial rank within the Calypogeiaceae.
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Vol. 114 • No. 1