Asplenium ruta-muraria
Introduction '''''Asplenium ruta-muraria''''' is a species of fern commonly known as '''wall-rue''' (but which, as a fern, is not by any means closely related to common rue). It is a very small epipetric species, growing exclusively on limestone and other calcareous rocks. Its fronds are bluish-green and are heavily sub-divided, becoming up to 12 cm in length. Sori of ''A. ruta-muraria''
Taxonomy Linnaeus first gave wall-rue the binomial ''Asplenium ruta-muraria'' in his ''Species Plantarum'' of 1753.A global phylogeny of ''Asplenium'' published in 2020 divided the genus into eleven clades, which were given informal names pending further taxonomic study. ''A. ruta-muraria'' belongs to the "''Pleurosorus'' subclade" of the "''Pleurosorus'' clade". The ''Pleurosorus'' clade has a worldwide distribution; members are generally small and occur on hillsides, often sheltering among rocks in exposed habitats. ''A. ruta-muraria'' is exceptional within the subclade for its lack of hairs on the leaf. The phylogeny suggests that multiple cryptic species may be present among plants now called ''A. ruta-muraria''. Within the subclade, the ''A. ruta-muraria'' aggregate (including ''A. dolomiticum'') is sister to the species formerly segregated in the genus ''Pleurosorus''.
Distribution and habitat ''Asplenium ruta-muraria'' is native to Europe, East Asia, and eastern North America. In North America, its range extends from western New England southwest along the Appalachian Mountains to Alabama and Tennessee. It is also found in the Ozarks and in southern Indiana and Ohio, with outlying occurrences in Michigan, Ontario, Quebec, and Clinton County, Iowa. In Europe, it commonly invades masonry and is a common species, while in the United States, it is more difficult to find, favouring pockets in very well-weathered and rather dry limestone outcrops.
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