Rare fern found at Westonbirt

After finishing an early job, I dashed over to meet John W. at midday. We had arranged to visit Westonbirt Arboretum, the wonderful national asset of all things wooded, which is only a ten mile drive from Stroud. Woodpeckers was very annoyed that I was going there without her, but I wanted to visit with John to explore an idea which might be to both his and their advantage.

I have blipped John before, and mentioned that we have known each other since being at the same school a long way back. On the way i even drove the long way round via the village of Uley, where John stayed with Andrew Wood, when he first came to the Cotswolds.

We set off in the sunshine, and it stayed bright until mid-afternoon, by which time we had eaten a sandwich at the cafe, before heading off into the old woodlands. The arboretum is huge and we only walked through a small part of it. But there is so much to see; specially planted rare trees from all over the world, native common species and their variants, as well as ancient coppiced woodland still managed in the traditional way. We also had a quick look at the large charcoal burners that are placed beside a ride close to some recently coppiced hazel, and are used to convert the cut wood into charcoal for sale.

I was particularly interested to walk there with John, as he has a great deal of botanical knowledge being is particularly interested in lichens and liverworts! It is rubbing off on me now as well. We had hardly walked fifty yards in fifteen minutes after entering the old woodland as there were many cut and fallen trees which we could get access to and see the rich variety of lichen species often side by side. I wanted to see the recently coppiced small-leaved lime trees that are purported to be over 2,000 years old, so we headed on to the far side of the estate.

Early on our walk we noticed a collection of beautiful small ferns that were growing high up on the moss covered limbs of several oak trees in various places. John hadn't seen ones quite like this before and he explained much about their life story and of how they spread their spores. But he also told me about fungi we saw as well as spending some time collecting tissue samples of various of these lichens, mosses and the ferns. His reason is that he intends to make slides from the samples, which he can use under his various microscopes to photograph them at high magnifications, producing amazing stereoscopic 3D pictures, which he then can show us using his stereo glasses on his computer screen.

I took lots of pictures of John as he was taking his samples, as well as hand-held close-ups of mosses and liverworts, roots and branches as well as rare contorted 'weeping' trees, and finally the ancient lime tree. We reached that at the end of our walk, and we talked to one of the assistants to the artist, or architect, who was building the specially commissioned artwork. This was being made from the coppiced lime branches, next to the tree stools from which they came, which are very extensive. I will hopefully photograph that when it is finished, and being 'used' by families and lots of children, as almost certainly it will be very popular.

I am choosing to blip this picture of one of the ferns we saw, which John thinks must be particularly successful at Westonbirt, as it appeared on a whole range of tree species, both close to the ground, like this one, as well as high up in the branches of tall trees. John told me how much he likes the ingenuity of their spore dispersal system and proceeded to explain how they work, but I got lost and can't now remember the details. No news there, sadly. But what I think he said is that each of these brown protuberances, called Sporangia, which grow beneath each frond, are composed of at least 100 spores that are uniquley dispersed when the conditions are right. I will let you explore this link from wiki, which does explain more about them. But I will also ask John to view this blip, and hopefully get him to send me an addendum in his own words, to tell the amazing story he told to me this afternoon in explanation, and wonderment.

John has now sent me an explanation:
'Brief' descriptions of things are often difficult, without the aid of diagrams, but I will do my best. The fern turns out to be Polypodium vulgare, Common Polypody. In early texts it is justly referred to as the Oak Fern, but in Victorian times that name was unaccountably transferred to the much rarer P. dryopteris, which more often grows on the ground. Polypodium vulgare is widely distributed through temperate regions on both sides of the equator. It figures prominently in herbals especially for pulmonary complaints.

The golden/orange spots on the underside of the frond are sori. Each sorus comprises a bunch of about thirty to fifty sporangia, little sacs with walls one cell thick and enclosing 32 or 64 spores. This represents 5 or 6 divisions of the original spore. Other ferns go to 128 or 256 spores or more. Each sporangium is attached to the sorus by a short stalk. Up one side, from the point of attachment to the apex or 'free end' of the sac, there is a row of cells with thickened walls, called the annulus. The immature sorus is covered by a thin membrane, the indusium, enclosing the sporangia, but as it matures the indusium splits and curls back, exposing the sporangia to the air. On a dry day the cells forming the thickened annulus dry out. Tension develops and at a certain point the sac splits and the annulus snaps backwards catapulting the spores into the air.

If you have access to a low power microscope, magnification X40 is fine, you can place a bit of more or less any 'just mature' fern frond on the stage, sori upwards, and focus the light from a reasonably powerful lamp on them. The sporangia will discharge one after another, flinging spores everywhere, a particularly engaging sight.

Interesting to reflect that while each sorus will release about 50 X 50, or two and a half thousand spores, each one of a Bryum Capillare capsule releases about fifteen million.