Leaf architecture and associated laminal cell development as key innovations of pleurocarpous mosses

Pleurocarpous mosses constitute the largest radiation within the Bryophyta (mosses), encompassing approximately 40–45% of extant moss diversity. Pleurocarpy, defined by lateral sporophyte production that is often associated with indeterminate, creeping shoots, has long served as a principal classificatory criterion, and has been interpreted as a major architectural innovation related to ecological expansion. Recent phylogenomic analyses provide strong support for the monophyly of the pleurocarp clade and clarify relationships among its principal lineages. Here I evaluate the hypothesis that uniformly prosenchymatous leaf architecture constitutes a key innovation of the pleurocarp radiation and is associated with unique laminal cell development. Comparative morphological evidence demonstrates that uniform laminal prosenchymatous cells are pervasive across the major pleurocarp lineages but are rare or absent in earlier-diverging moss clades. Mapping this character onto recent phylogenies support a single evolutionary origin at the base of the pleurocarp clade, with limited secondary modification. Prosenchymatous leaf architecture is associated with Hypnobryoid laminal cell development that contains modifications in timing of apical cell divisions and geometry. These modifications provide sustained anisotropic leaf expansion and reduced transverse cell division frequency. Although the ecological consequences of laminal prosenchyma remain untested, its mechanical and hydrological implications are consistent with an indeterminate, mat-forming growth habit and ecological dominance of pleurocarpous mosses. Compelling evidence supports changes in laminal cell development that produces uniform prosenchymatous laminal cells and thus appear to represent an integrated morphological innovation that may have contributed to the evolutionary success of the pleurocarp radiation.