Whereas Maidenhair Spleenwort Asplenium trichomanes and Wall Rue Asplenium ruta-muraria are commonly found around here on the sides of mortared walls, their tops are often crowned with the foliage of another fern, Polypody.
Unlike many other ferns, polypodies lack an indusium (membranous cover) over the sori; so when looked at through a hand lens the individual globular like sporangia are immediately visible. Although it was originally thought that there was just one British species of polypody with various sub species, now it is recognised there are two main species; P vulgare and P cambricum which produce a fertile hybrid P interjectum and between these three there are a complex variation of forms.
Although there are certain morphological characteristics that may be useful to help identify what the Polypody may be, they are not definitive. Microscopic examination of the cells on the sporangia called the annulus is required. In the specimen I found there were no fine hairs(parphyses) growing between the sporangia so this suggests it is not at least P cambricum. It may be a hybrid between P vulgare and P interjectum but as I don’t know then it should be described as P vulgare agg. (not P vulgare).
An interesting feature of this specimen is the forking of the apex of the leaf blade and the lowest leaf segments. This ability of ferns to produce unusual variations in leaf shape was highly prized by Victorian collectors, who decimated many wild populations.