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Definitely Dryopteris. (Oreopteris limbosperma, as implied by the epithet, has pinnule marginal sori.) I think it might be Dryopteris borreri (pinnae parallel sided for half their length, truncate pinnules), but I can't be certain that it's not another Dryopteris.
I tend to agree with lavateraguy that it is Dryopteris borreri in the latest definition with its truncated ends. I would prefer to agree with D. affinis agg. which covers more options!
I'm quite new to all of this; I'm a first year student studying Conservation and Environment and one of my assignments is a natural history diary of native flora and fauna, so I need to be certain which species it is before putting it in my diary. To me, both Dryopteris borreri and D. affinis look exactly the same?? If it helps, I found this fern growing out of a stone wall next to an ~6m high waterfall in Wales, about 2m off the ground and naturally a very wet environment. Please help??
I think I'm going to go with Dryopteris borreri- looking at the pictures on www.ferns.rogergolding.co.uk/ferngenus/dryopteris/affinis_agg.html the slightly alternate pinnae seem to match those of my photos more closely than D. affinis.
Identification from comparison with photographs is potentially unreliable - you don't know whether the characters you are depending on are taxonomically significant.
You could identify this plant as Dryopteris affinis agg., or Dryopteris affinis s.l. (which might be what Matt Prince meant), or Dryopteris cf borreri.
I've found that Internet Archive (archive.org) now has copies of Pteridologist and Fern Gazette, including Fraser-Jenkin's 2007 treatment (Fern Gazette 17(1): 1-26 (2007), which includes a dichtomous key, and descriptions with field marks.
Otherwise you can use the keys in Stace or Poland (or Merryweather). But I've struggled with these keys - until one is familiar with the plants one has difficult matching the traits in the field to those in the key.
There is commentary by Merryweather in Pteridologist 4(6): 173-177, but while it presents some of the pitfalls it doesn't provide any practical aid to identification.
Does your course expect you to identify everything to species? That would prevent you adding dandelions, hawkweeds and brambles to your natural history diary.
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