Southern Marsh Orchids

Southern Marsh Orchids

Wednesday, 8 April 2015

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.
Polypodium cambricum frond shape
The very broad fronds of Polypodium cambricum...

Polypodium cambricum pinna toothing
...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!

Polypodium cambricum paraphyses branched hairs
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.
Polypodium cambricum hedgebank habitat
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!

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