Fern Friday will be a very occasional series as I try to learn to identify the various fern species I encounter during my meanders. Common polypody is one I see quite often but it turns out not to be as straight forward as I was hoping. It seems that the various Polypodiaceae look very similar and require expert examination to be sure of their species. So, I’m lumping mine together as Polypodium vulgare sensu lato (i.e. broadly speaking, these are Common polypody).

As you can see, this specimen was growing on an old stone wall, acidic rocks and walls being their preferred habitat, though they can also grow as epiphytes on trees. The fronds of a fern are divided into pinnae (like leaflets): if divided just once, like the Common polypody, they are described as pinnate; if twice, they are bipinnate; if three times, tripinnate (further Fern Friday posts will show examples of these multi-pinnate species).

On the underside of a fern leaf the sorus (plural sori) can be found. The sorus is a small cluster of sporangia, where the spores the fern uses for reproduction develop. In the Common polypody, the sori, which are usually round, start off a pale yellow and change to an orangey brown when mature. You may recall that these sori are sometimes home to the larvae of a tiny moth, Psychoides filicivora (see my 19 December post, Leafmines: Psychoides filicivora,2).