Ruta-Muraria

In my opinion at least, the smaller a plant is, the more character it has. Wall rue (Asplenium ruta-muraria) is a wonderful demonstration of this. The genus of ferns to which it belongs, "Asplenium," is rather large, containing somewhere along the lines of 700 species worldwide. 

Wall rue can be found growing both in North America and Europe. Its distribution is a reminder of the great land bridges that once connected the continents back when ocean levels were much lower than they are today. The specific epithet "ruta-muraria" roughly translates to "bitter herb of walls." Along with its common name, these seem to hint at where this tiny fern likes to grow. Indeed, at least in Europe, this is a fern of stone walls, growing among the myriad cracks and crevices where microclimates are favorable for its spores to germinate. 

In North America, however, wall rue seems to be a bit more picky. Wall rue is a calciphile meaning it can really only be found in abundance on natural limestone outcroppings. As a result, it is considered a threatened or endangered species throughout most of the continent. The aspect of its habitat I find most interesting is that the limestone it relies upon is the result of an ancient sea that covered parts of North America during the Silurian Period some 443.8–419.2 million years ago. Along with plenty other plants in these habitats, if it were not for the solidified remains of ancient marine organisms, these extant plant lineages would not be here, at least not in the way in which we know them. 

Another interesting aspect of wall rue biology is that this little fern is helping paleontologists in Europe discover potential glacial refugia - ice free areas where plants and animals were able to survive during the height of glaciation. Refugia were likely epicenters of biodiversity, which expanded to recolonize the continents once the ice sheets receded. 

Wall rue, as well as other rock ferns in the genus Asplenium occur in two forms in nature - a diploid form with two sets of chromosomes and a polyploid form containing multiple sets of chromosomes. Polyploids arise from mutated diploids and can be found growing over a wider range than their more restricted diploid parents. By studying the relatedness of different diploid populations, researchers are able to deduce where some glacial refugia may have been located. In this way, these tiny little ferns are offering a rare but clear window into the Earth's long gone past. 

Further Reading:
http://www.sciencedirect.com/science/article/pii/S0024406698902577

http://www.jstor.org/stable/41756114?seq=1#page_scan_tab_contents

http://plants.usda.gov/core/profile?symbol=asru2