Porella platyphylla is a species of liverwort belonging to the family Porellaceae. It has a Holarctic distribution, occurring across Eurasia and North America, where it typically grows on tree bark and rocks in areas with adequate rainfall. The species is most common in regions receiving at least 600 millimetres of annual precipitation. The species forms part of a complex taxonomic group that includes several closely related species and hybrids, with populations showing distinct genetic differences between continents despite their morphological similarity. The species is characterised by its obtuse-rounded leaf lobes with flat margins, and irregular teeth at the perianth mouth when mature. It reproduces both sexually, with separate male and female plants, and asexually through leaf fragments. P. platyphylla is known for its ability to survive long periods of desiccation, being able to recover normal photosynthetic function within hours of rehydration. The species produces unique chemical compounds, including pinguisane sesquiterpenoids and sacculatane diterpenoids, which help distinguish it from related species. Recent molecular studies have revealed significant genetic variation within the species, particularly in European populations, though this variation is not always reflected in physical characteristics.
Taxonomy
The species was first described by Carl Linnaeus, with early descriptions and illustrations provided by Pier Antonio Micheli (1729) and Johann Jacob Dillenius (1741). P. platyphylloidea was later described by Lewis David von Schweinitz in 1821 from a North Carolina collection, who distinguished it primarily by its occurrence on rocks rather than bark, larger plants with often tripinnate branching, quadrate leaves with inflexed margins, and differences in the relative sizes of the plant's structural features. Historical taxonomic treatments varied in their interpretation of these plants. While some researchers like Marshall Avery Howe (1897) and Theodore Christian Frye and Lois Clark (1946) treated them as a single variable species, others such as Alexander William Evans (1916), Karl Müller of Freiburg (1915), and Rudolf Mathias Schuster (1980) maintained them as separate species. The perianth mouth structure and elater spiral patterns were particularly emphasised as diagnostic features by later authors. A comprehensive 1998 study examining both morphological and genetic variation in Porella platyphylla and P. platyphylloidea led to a significant taxonomic revision. The research, which analysed 35 populations across North America and Europe, revealed three distinct genetic groups within what had previously been considered two separate species. To help stabilize the taxonomy of this complex group, researchers later designated a DNA voucher specimen from Germany (Heinrichs and Feldberg 4600) as an epitype for P. platyphylla.
Related species Porella platyphylla forms part of a complex of closely related taxa, with its closest relative being P. cordaeana. DNA analyses have revealed two main clades within P. platyphylla: one predominantly North American and another widespread in Europe but also present in North America and Asia. Both species maintain distinct European and North American lineages that form sister groups in phylogenetic analyses.
Porella cordaeana can be distinguished from P. platyphylla by its crenulate-sinuous perianth mouth and acute, ventrally reflexed and twisted lobules. While both species have leaf lobules that fold under, in P. cordaeana these are twisted and taper to a point, whereas in P. platyphylla they are merely folded under without twisting and have rounded or obtuse tips. These species can sometimes be found growing together in the same habitat, and multiple species may even grow intermixed within a single colony. This close physical association, combined with their ability to reproduce sexually, has led to hybridisation between species. A hybrid between these species, P. × baueri has been documented in Europe. This hybrid is an allopolyploid, meaning it originated through hybridisation between two species and underwent chromosome doubling. While P. platyphylla and P. cordaeana are typically haploid with eight chromosomes (occasionally nine in P. platyphylla), their hybrid derivative P. × baueri is polyploid. Genetic evidence suggests this hybridisation event was relatively recent, and the hybrid may have originated multiple times. Analyses of chloroplast and nuclear DNA have produced conflicting evolutionary trees, suggesting that the North American populations formerly known as P. platyphylloidea may be the result of ancient hybridisation between P. cordaeana and P. platyphylla, though through a different and much older hybridisation event than that which produced P. × baueri.
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