Pluristratose leaf lamina in pleurocarpous aquatic mosses is a mysterious morphological character state because of its recurrence among unrelated lineages. It has been found sporadically around the world in phylogenetically distant taxa, and is thought to be a mutation and/or adaptation to aquatic habitats. During an extensive survey of bryophytes in spring habitats in the Italian Alps (Province of Trento), we found different numbers of leaf lamina cell layers among specimens of Palustriella falcata. We carried out a thorough study, measuring a set of morphological characters that identify variability among specimens within the same spring and among multiple springs. The main goals were to assess the amount of morphological variability, to quantify the concordance among morphological traits, and to test to what extent environmental variables account for morphological variability. Our results showed that, in many cases, morphological characters differed even among shoots within a spring. We found positive and significant partial correlation between pluristratose lamina and width of costa, but negative correlation between pluristratose lamina and length of cells. Constrained multivariate analysis showed that 40.3% of this morphological variation was explained by a set of environmental variables, but most importantly, we observed extensive pluristratose laminae in constantly submerged habitats. We interpreted the different numbers of cell layers in the leaf lamina as a phenotypic continuum from P. falcata, with a single layer of cells, to Palustriella pluristratosa Stech & Frahm, with a multilayered lamina. In addition we offer a point of view concerning the evolutionary significance of this trait, its possible origin, and its evolution in aquatic mosses.
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