Pottering around with Polypodies

Polypodies (Polypodium spp.) are a group of ferns with something of a reputation for being a bit difficult to tell apart. Nevertheless, with a bit of experience it's possible to distinguish quite quickly between the two common species - P. interjectum (Intermediate Polypody) and P. vulgare (Common Polypody). In the field, this can be done on the basis of frond outline and the shape of the blobs of spore-cases (sori).

When in doubt, though, microscopic examination is needed, and so the other day I was working through half-a-dozen specimens that I had pressed during the last year.

One of these, which I had collected in March 2014 from Aberthin, I had initially thought might be a form of P. interjectum. It was pure ignorance on my part that I didn't suspect the much rarer P. cambricum (Southern Polypody) at the time, because it was a textbook example, with much wider fronds, and strongly toothed pinnae.

The very broad fronds of Polypodium cambricum...

...and the strongly toothed pinnae.

I'd not seen P. cambricum before, so I was quite pleased when under the microscope, it proved to have, as it ought to, only about 4-6 thick-walled cells on each spore-case (sporangium).

However, another well-documented feature peculiar to P. cambricum - the branched hairs or 'paraphyses' amongst the spore-cases - took a bit more effort to find. Although 'Stace' (edition 3) says these are >0.5mm long (and 'Flora Europea' says 0.6-1.2mm), you would have a job to see these with even a x20 hand-lens, as they're even thinner than the stalks of the spore-cases, and well-hidden amongst them. Colin Page's helpful 'Guide to Cumbrian Ferns and Fern-allies' says they are "very easily overlooked, especially when infrequent".

My persistence paid off, though, and I eventually found a couple of these hairs. I even managed to take a photo of one through my ancient brass microscope, by messing around with the settings on my Canon PowerShot. The surprising results of combining 19th-century and 21st-century technology!

One of the branched hairs that are diagnostic for Polypodium cambricum. Just above is the flap of a opened spore-case. The field of view is about 0.4mm.

P. cambricum is quite a local plant in Britain, with a southern/western, mostly coastal distribution. In Glamorgan there are a number of recent sites in the Gower, but apparently very few localised records from East Glamorgan. At Aberthin there are two metre-long colonies within a short distance of each other, growing on an old laneside bank made of limestone blocks - in firm agreement with its reported habitat of calcareous rocks.

One of the Aberthin colonies of Polypodium cambricum (left-most, brighter green clump), growing next to Polypodium interjectum (right-most, dull-green clump). Surely it's just a matter of time before the hybrid appears!