When out and about in Dorset's deciduous woodland from time to time you will find ferns growing up in the trees, quite often in mature oaks. I had seen this elsewhere but it was not until I went through details of the Dorset Wildlife Trust reserves when we first moved here to Dorset some years ago that I spotted something called 'polypody' listed on some woodland reserve species lists. My curiosity aroused I read up about it and went looking for it.
There are three types of polypody fern; common, southern and intermediate (also called western) and they are very difficult to tell apart especially when they are high up in a tree! To make things worse, where two different species of polypody grow close together they can also hybridise and you get something in between! Yelling them apart is a job fore the experts and so I just refer to them as polypody ferns (Polypodium agg.).
Polypody ferns can also be found on walls and rocks and even on roofs of old farm buildings. it also occurs on the ground on acid soils and it likes shady banks. They are not rare but they are certainly unusual so keep an eye out for them, especially if you are enjoying a woodland walk.