A magnificent fern grows on an old Ash-tree on the banks of the Dordogne. It is a Polypody, but which one? There are three species in Perigord and sometimes a hybrid between two of them is found. The species are very much alike. To be sure of your identification you have to look at the sores (the lids of the sporangia) under a microscope et count the cells. Well, in the field you normally don't carry a microscope.
Hybrids are often polyploids - the number of chromosomes has doubled or tripled - and thus the organs (leaves, flowers) can be much larger than those of the parents. This one is really big, it has fronds up to 70 cm. So probably it is the Hybrid Polypody (Polypodium x mantoniae).
It has other characteristics of its presumed parents. The big pinnate leaves are long triangles with toothed margins as in the Intermediate Polypody (Polypodium interjectum). The leaves are planar, the pinnae - the segments of the leaves - are a bit hanging in the picture but they are basically all in two dimensions as in the Common Polypody (Polypodium vulgare).
In this fallen tree lives one of the two parents, Common Polypody, as in an apartment building with several floors.
They are rather small and the fronds are long with parallel margins.
Often Polypodies grow on trees, but you can find them also on the forest floor. Or maybe this one lives on a dead trunk hidden under fallen leaves? Possibly this is an Intermediate Polypody (Polypodium interjectum) considering the shape of the leaves, slightly triangular with the basal pinnae uplifted. But this is just a guess!