A few months back I explored an interesting-looking firebreak between rows of forestry trees and was surprised to find that it ended with an open vista across boggy heathland. I suspect that the numerous shallow pools and meandering burns that occur here will be full of dragonflies come summertime.
Even more surprising was finding a substantial patch of Greater Tussock-sedge along a wet ditch, not at all a common plant here on Skye. I diligently recorded the number of tussocks and took note of the grid reference, only to find that somebody else had beaten me to it and this patch was already recorded on the BSBI distribution database.
Just ten recent records for Skye, with only three of these since 2020
Today I returned to the spot in question and found the tussocks looking very well and healthy amongst the seemingly dead vegetation surrounding them
I shall wait until they are flowering before doing a proper write-up on the species
I can't help but think that a clump of Greater Tussock-sedges looks exactly like a bunch of army snipers in ghillie suits. I can imagine these silently standing up and suddenly finding myself staring into the barrels of several rifles. Can you see it too, or is it just me?
My quest to find as many ferns, rushes, sedges and their allies as I can has finally begun. At this time of year my options are clearly limited to whatever is green during the winter months. One such group are the polypodies, of which there are three taxa on Skye. Today I headed into nearby Dunvegan and soon found the first few species of ferns of my quest. Here I will deal with just one of them, Polypodium vulgare.
There are three species of Polypodium in Britain, two of which occur here on Skye. Several years ago, the Skye Botany Group undertook a season's worth of recording polypodies using microscopy to categorically identify each sample we logged. The two species present are Polypodium interjectum and Polypodium vulgare. Our aim was to record the habitats and distributions of each and to establish whether or not their sterile hybrid occurs. It does, though at a very low frequency.
Polypodies are notorious for not behaving properly. They exhibit plasticity in their overall morphology, particularly Polypodium interjectum (presumably because it is itself of hybrid origin). So although a 'classic' interjectum is quite readily identifiable, and a 'classic' vulgare is reasonably safe to state as being such, there are a lot of plants which could be one or the other. The surest way to be certain is to check the sporangia and their spores beneath a microscope. And we did a lot of that!
I should mention that we did our microscopy mostly in the summer and autumn months. This is because we wanted to check material that was fresh and in good condition. Checking old and weathered material is not ideal. Guess what, it's all pretty old and weathered at this time of year! However, I did find some fairly decent material to examine. It's definitely easier in the summer, and I may well visit this species again to demonstrate this.
Underside of a frond showing the rows of sori
I'm not going to go into detail regards ferns having spores rather than seeds, and the developmental processes involved regards the production and dispersal of these spores. There are books and websites that explain it all a lot better than I can. But I do obviously need to provide some details. I'll keep it very simple for now. However...
...at the bottom of this page I'll add a YouTube video produced by local fern expert James Merryweather. It covers our native polypodies in detail. He has a style all of his own, perhaps not to everyone's taste, but he does know his stuff and has some 'interesting' graphics to help explain several of the features I mention. Give it a go, and be sure to leave him a thumbs up afterwards.
The circular brown spots shown on the frond above are the sori. Each of these comprises a tight bundle of several hundred sporangia, each of which is attached by a stalk to the leaf surface. The fern's spores are housed within these sporangia.
So basically, each brown spot is where the fern holds its spores in lots of tiny individual packages. The packages and their cargo of spores start to develop in the spring and by late summer they are mature, whereupon the packages split open and release the spores into the wider world. Please do watch James' video, the sequence depicting this process is priceless!
Through a 10x handlens you will be able to see the individual sporangia, hundreds and hundreds of them packed shoulder to shoulder in a single sorus (singular of sori). With a 20x handlens, and a bit of a squint, you can make out some of the features required to identify your polypody to species level. But ideally, a low-powered microscope is required, something like 40x magnification is good. What you are looking for, the feature that varies between the three polypody species, is the row of indurated (thickened) cells that form the annulus (spine) of each sporangium. Each species of polypody has a differing number of indurated cells in the annulus, though individual variation leads to a degree of overlap. This individual variation requires several sporangia to be checked and the number of indurated cells averaged out, ten is the recommended number to check. The second feature to check on the sporangia is the number of basal cells that run between the stalk and the base of the indurated cells. Combine the number of basal cells with the number of indurated cells and you'll have your species identity sorted. Phew!
This looks complicated, but it's not. And it shows everything you need to see
This image is from my copy of Stace 4 (2019). I've annotated it and highlighted the basal cells in yellow pen. I used this extensively when I was helping out on the polypody project, numbers 2 and 3 are the species that occur on Skye. One of them matches my photographs below.
Imagine each sporangium as a tiny ball. Inside the ball are the spores which need to be released from within. The ball needs to somehow rupture in order to release the spores. The process of rupture is pretty damn ingenious actually, but essentially the thickened cells of the annulus act like a spring and, when conditions are just right, these cells suddenly contract their walls, causing the opposite surface to stretch and burst open, jettisoning the spores out and into the atmosphere. Remember Pacman? Now imagine grabbing that football in your hands and pulling your hands apart until one side of the football splits open and becomes a Pacman. That's (kind of!) what happens when the annulus contracts, it splits the sporangia apart. The image above shows the back of Pacman's head, with his open mouth facing away from you. The two rounded lobes were once joined, but now they've split open revealing a gaping mouth, from which the spores escaped.
Here are a couple of images to help you visualise the sporangia and their Pacman profiles.
A Pacman-like split sporangium below, rear view of an adjacent sporangia above
Compare with the text book image above. One basal cell and 11 indurated cells
To be fair, most of the time I think you could get away with just counting the basal cells when it comes to the polypodies that occur on Skye. But let's play it safe. I did indeed check a number of sporangia, averaged out the number of indurated cells (it came to 12) and the number of basal cells remained constant at one. This categorically rules out Polypodium interjectum (2 basal cells and typically 7 indurated cells in Skye populations) and matches Polypodium vulgare perfectly.
Blimey, give yourself a pat on the back if you managed to follow all that! And if you didn't, but you wished you had, maybe watch the video below and then have another read. There are 268 species on my Checklist. Common Polypody was number one. Stay tuned for lots more soon!
There's a bit more polypody info on thisBSBI plant crib, it's well worth a squint. Now sit back and enjoy James' video.
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