Fabulous ferns

“Let’s go to Arnside for a girl’s weekend…” said Sue, “…and teach ourselves to identify some ferns”.  Now that sounds more like it.  Ferns are out of my comfort zone but definitely not so far out as a weekend of shopping!  Having been lucky enough to grow up in the Lake District, I’d always regarded the Arnside area as something of a poor relation but it’s one of Sue’s favourite places, and I can now see why.  A miniature gem of an AONB, at only 75 km2, Arnside and Silverdale packs a lot in – limestone pavement and grassland, ancient woodland, salt marsh and wetlands.   Most of the underlying rock is Carboniferous limestone.  Glacial erosion and deposition, water flow and the dissolution of limestone by rain water have shaped the landscape subsequently and produced a range of habitats for many different animals, birds and plants, perhaps the most famous plant being the Lady’s-slipper orchid, Cypripedium calceolus, reintroduced to Gait Barrows NNR from a tiny population of the wild plants found in the Yorkshire dales, after the orchid was believed to have become extinct.

Photo: Arnside and Silverdale AONB

Our fern target species are much less eye-catching, though equally beautiful. “They all look the same, I hear you cry,” but, of course, they don’t once you know what to look for.  We found ferns from four different families without much difficulty and they are all quite distinctive.

Ferns differ from the flowering plants I usually focus on because they reproduce using haploid spores rather than pollen and egg cells and so have an additional stage in their life cycle.  Like pollen and eggs, the spores just have one of the two pairs of chromosomes necessary for the adult plant to grow but, unlike the flowering plants, these spores germinate to produce a small intermediate, haploid, plant known as a gametophyte. It is the gametophyte which produces the eggs and sperm which, after fertilisation, grow into the fern we are familiar with (the sporophyte).  For the most part, the gametophyte is too small to be noticeable.

Fern life cycle

Ferns are generally found in wet habitats because, whilst the eggs stay put in the gametophyte, the sperm need water to swim to them for fertilisation.  The mild, damp coastal habitats of much of the west coast of Britain, especially around Arnside and Silverdale, are perfect for ferns and mosses (which also need water for reproduction) hence our choice of destination!

Our first target on Friday evening was a population of Maidenhair ferns, Adiantum capillus-veneris, on the rocks just west of Arnside which Sue had been shown on a field trip some years ago.  These scarce, limestone-loving ferns are probably the exception to the “they all look the same” rule, but many people in the UK will think their natural habitat is the bathroom!  I’ve only seen them growing outdoors before in the Mughul gardens in Kashmir.  The pinnae (leaflets) on the fronds look rather similar to my favourite Ginkgo leaves.

The other small ferns we found growing on stone walls and in the grikes of limestone pavement on Warton Crag and Gait Barrows were both spleenworts – Maidenhair spleenwort, Asplenium trichomanes, and Wall rue, A. ruta-muraria. We can see the spore producing sporangia clustered into linear sori on the underside of the Wall rue pinnules – these will darken to a rusty brown colour later in the season as the spores inside them mature, ready for release.  These linear sori are both one of the features of this family of ferns and the source of their common name. According to the 15th and 16th Century medical practice of ‘Doctrine of Signatures’, a plant could be used to treat ailments of that part of the body which it resembled, in this case the blood-filtering spleen.

Asplenium ruta-muraria

The spleenworts are typically small ferns like Wall rue with each frond divided into pinnules but, as always in biology, there is an exception.  The Hart’s tongue fern common in damp woodlands is also a spleenwort – Asplenium scolopendrium.  Later in the season its linear sori will be very obvious.

Left and centre: Asplenium scolopendrium growing amongst moss-covered rocks in Gait Barrows woodland. Right: sori from a plant at Raisby Hill in January this year

The other ferns we identified belonged to two different genera of the Dryopteridaceae family – Dryopteris (Male ferns)and Polystichium (Shield ferns).  Both genera have bipinnate fronds; that is, the leaflets or pinnae into which the fronds are divided are then further divided into pinnules – effectively pinnae on pinnae.  The easiest way to distinguish between these two genera is by the shape of the pinnules.  In Shield ferns, these have one prominent thumb-like lobe at the base and the pinnule lobes are sharply-pointed whereas in Male ferns the pinnule lobes are only serrated and they lack a conspicuous ‘thumb’. 

Polystichum aculeatum frond and pinnae – note the distinct ‘thumb’ at the base of each pinna (leaflet) and the spike at the tip of each pinnule.

We found plenty of Hard shield fern growing out of the cracks (grikes) in the limestone pavement at Gait Barrows.  The soft, new growth belies its common name but the remains of last year’s fronds are tough and dry.  The young fronds already have sori on the underside, this time circular groups of sporangia covered with a translucent indusium.

The Male fern, Dryopteris filix-mas, is the robust, upright fern common on woodland floors and shaded ground throughout the UK.  It has the same bipinnate fronds as the Hard ferns but the pinnules are blunt rather than spike-tipped and the longest are without a distinct ‘thumb’. This time the sori are kidney-shaped rather than circular, too.

Male fern, Dryopteris filix-mas

The related Scaly-male or Golden-scaled male fern, Dryopteris affinis, which we also found in the woodland on top of Warton Crag is, as its name suggests, covered in golden scales. It also has a dark brown spot at the point where each pinna joins the central stipe or stalk, though these photos don’t show that. 

Golden-scaled male fern, Dryopteris affinis

This seems like a rather small number of ferns to have learned to identify but I’m working on it and there were, of course, plenty of flowering plants to enjoy too, including several new to me such as Lily-of-the-valley, Convallaria majalis, and Angular Solomon’s-seal, Polygonatum odoratum and some kind of helleborine yet to come into bloom.

Convallaria majalis, Polygonatum odoratum and an unknown helleborine, Gait Barrows nature reserve

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