Very many ferns have evolved to live on rock surfaces and are well adapted to doing so. Indeed, more than half of the British fern species are regularly found in this habitat. Some, such as the polypodies, are equally at home as epiphytes (growing on other plants), their creeping rhizomes providing both the main means of adherence but also acting as storage organs for nutrients and water.
Rocky features are increasingly a scarce natural commodity, particularly in much of lowland Britain. The arrival of humanity has led to their exposure, exploitation and the creation of an ever-increasing built environment. Fred Rumsey describes the Wall Ferns that have succeeded in conquering the urban environment.