Wildlife on Walls: 5. Wall Rue

There aren't many plants that are so intimately associated with man-made walls that this tough little fern, wall rue Asplenium ruta-muraria.

It's likely that human constructions in brick and stone have extended its range because it's a species that prefers an alkaline substrate - and the mortar between bricks and rocks provides and excellent substitute for limestone in areas with a more acidic natural geology. Stretching the point a little, you could argue that the Romans helped widen the distribution of this fern because they introduced mortar as a means of constructing walls. Without this inadvertent human assistance, this would be a much rarer fern.

It produces spores under those leathery, drought-resistant fronds which have a distinct coincidental similarity to rue Ruta graveolens.