English: Polypody (a fern) - the underside This species of fern was growing on a tree, beside one of the paths in the woodland walks at Ardardan.

There are three British species of Polypody: Southern (Polypodium cambricum), Common (P. vulgare), and Western (P. interjectum); there are also three hybrids (one for each of the three possible pairings of these species).

Visible here on the underside of the frond are the golden-coloured "sori"; a sorus is a cluster of tiny spore capsules ("sporangia"). The species shown here is not one of the hybrids; these cannot produce viable spores, and have only small purple sori. In addition, Southern Polypody does not occur in this region, so the fern found here is either Western or Common Polypody.

Distinguishing between these two species is not easy, since they are very similar. The features that can aid identification include the shape of the sori (either round or oval), and the shape of the finger-shaped leaflets, known as "pinnae", which may end in a sharp or a blunt tip, and which may have serrated edges or not. However, only a microscopic examination will give an identification with certainty. [See "The Fern Guide" by James Merryweather (3rd ed., 2007, Field Studies Council)]