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 Polypodium cambricum frond shape](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEjbzXVpVWxWvcA5yx4CKDJJ3dO5TcS7CG71sKc4xBxIOalKpNp_gufYBHvfFU2A06tKokle7n_vP2xiiN0tzOpNRCYdj1m1IMW022k8oQbjJ7cv4NRAqq7yKHk1-OJba4ZTLj8dBpp7_wU/s1600/IMG_2281.JPG) |
The very broad fronds of Polypodium cambricum... |
![Polypodium cambricum pinna toothing Polypodium cambricum pinna toothing](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhT1n2HKxZz5DG0ucG_a3sdTb8sTmDxou17FNWJpoY60lhiA7uVwVftu0ozs9AOmWgqSVcsgQR9na0wpqzEPdJ3ZLvGMarrAE5YWDUXfkopdZFktbnDluZxTM14LIG4P4BZG4gHt6HAkgU/s1600/IMG_2283.JPG) |
...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!
![Paraphyses in Polypodium cambricum Polypodium cambricum paraphyses branched hairs](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEhDZREGb9u951TJ4Oo44x15mnNZjaH679p1b4lT_yF3H5QTJhnsbVzSC7mdXmey-flHp9f5h0k6yejd_dMHNjNDlP_HfittQgjNnu62iXMLYN-MT2Hmqk9nxkDA_Co2InjUbubilIj0V38/s1600/IMG_2276.JPG) |
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 Polypodium cambricum hedgebank habitat](https://blogger.googleusercontent.com/img/b/R29vZ2xl/AVvXsEg6GUG05ztTeM1vKN96WESGcZ0rxcnY01aFHuOES2KxTh3O1agK1MSQHYe5CnrOe5TX6ir_Wa1kv3dcnjzbLocuGlp3qfBcQFda5RS3aHHB0TbhrKn2HeoQfbIpkIj4voOA1yhln0V5_3c/s1600/IMG_2279.JPG) |
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! |