Growing on brickwork of railway embankment.
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Chris, do the stalks darken later in the season?
... the petioles (not rachises) are reddish-brown to blackish. But looking at my photographs of this, they have green petioles, as do many (but not all) photographs online.
Technically the stem below the frond is called a stipe in ferns - not that it matters. Most descriptions refer to the stipe in this species as being red-brown or nearly black. But as has been noted, many seem to have this character missing. Often the stipe is only dark at the very base and it's necessary to dig about in the centre of the fronds to see what's happening. I don't know, I'm afraid whether the stipes darken with age. Quite possible. This species is extremely variable however and perhaps the lack of darkening is typical of some populations. Sounds like some research might be undertaken....
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I'll look for this next time I see this fern.
Lat/Lng: 52.4954, -0.9069
OS grid ref: SP743891