Flora alpina: Common ferns

Polypods all the way down

Welcome back to the Flora alpina, an evolutionary tour of the plants in the Alps.

Previously in the series, we’ve gotten an overview of Alpine plant diversity, stood in awe of the Tree of Life, expanded on the meaning of plant families and Floras, and learned about the evolutionary history of plant sex (trust me, it’s relevant), and started our proper exploration with the lycophytes. The last installment featured the weird ferns.

Today: the common ferns.

Ferns are, for many of us, like wallpaper for the forest floor. No flowers or fruits to draw the eye, just pattern and shadowy green. At first glance, all ambience.

Time to take a closer look.

Young frond of Pteridium aquilinum (bracken) (Photo by Rolland Douzet)

Last time, we met the “weird” ferns, which means today’s post features the “normal” ferns. This is subjective, but not arbitrary: we’re climbing from the long, old, spindly branches of the Fern Tree of Life into a cluster of denser, younger branches. The shapes and habits of these ferns—order Polypodiales—are more familiar to us because they take up the most fern-space on earth today (~80% of the ~10,000 fern species), as well as in the Alps.

Despite their evolutionary youthfulness (originating around 100 million years ago), Polypodiales ferns are diverse. They can be considered a resurgence of the fern dynasty, overtaking the old Carboniferous monarchs that mostly died out along with the giant lycophytes. They’ve even held their own alongside another recent explosion of plant diversity, their contemporaries, the flowering plants.¹   

Fern features

In the Alps, most (but not all) Polypodiales ferns are variations on the familiar shape: tapering fronds divided into feathery pinnae, which unfurl from young fiddleheads every year. Their most distinctive features—the sori, clusters of sporangia where they store their spores—are tucked away on the undersides of fronds. Whether arranged in stripes or polka dots like clumps of caviar, or coating the edges of pinnae like salt on the rim of a cocktail glass, sori are often used to tell species apart. So are insidia, flaps of tissue that may or may not grow over the sori to protect them. (For a refresher on how fern reproduction and spores work, see this post.)

Frond and pinnae; fiddleheads; sori; sori with insidia (Photos 1, 3 & 4 by Rolland Douzet; 2 by Anne Thomas)

In the pages of Flora alpina, locating the common ferns is easy because they’re all lumped into a single family, Polypodiaceae. However, fern taxonomists have been busy since the Flora was compiled 20 years ago. Now, although they’re still arguing about it,² it’s safe to say the 60-odd Alpine fern species should now be divvied up among at least ten families (out of the 26 families in Podypodiales.)

So let’s sample a few.

Thanks to Rolland Douzet, director of the alpine botanical garden Jardin du Lautaret, for generously providing many of the beautiful photos in this post.

Pterideaceae (Brake family)

• Worldwide: ~45 genera, ~1150 species

• Alps: 6 genera, 7 species

The ferns in the Pterideaceae family are a bit on the weird side, possibly because they’re one of the basal families of Polypodiales—an older and more isolated branch, just like the weird ferns were basal to the larger fern class.

For example, we have the maidenhair fern, Adiantum capillus-veneris, with minute fan-shaped leaves cascading softly down the rocks.

There’s parsley fern, Cryptogamma crispa, whose sterile fronds indeed resemble parsley, while the fertile fronds look entirely different, elongated pinnae curling around their thick clusters of sori.

And Pteris cretica, ribbon fern, and its congeneric, Pteris vittata, with long, thin pinnae reminiscent of cycads or palm leaves.

Adiantum capillus-veneris (1: Rolland Douzet, 2: Luke Armstrong), Pteris Vittata (Iris Yu), Cryptogamma crispa (4: whole plant, 5: fertile and sterile fronds; Rolland Douzet)

Dennstaedtiaceae (Bracken family)

• Worldwide: 10 genera, 240 species

• Alps: 1 genus, 1 species (Pterdium aquilinum)

A single representative of another basal family, Dennstaedtiaceae, takes up a disproportionate amount of space. That is the bracken fern, Pterdium aquilinum.³ One of the most cosmopolitan and pioneering ferns out there, bracken can colonize whole hillsides via underground rootstocks. (This clonal, vegetative growth is a handy alternative to the gametophytes that grow from spores and rely on moist conditions to reproduce.) It can also suppress the growth of surrounding plants by exuding allelopathic chemicals into the soil, not to mention toxins that can cause stomach cancer, and hormones that cause insects to molt uncontrollably until they die. It’s a very common sight blanketing moorland in places like Wales, but not hard to find in the Alps either.

Bracken is deciduous, so in autumn the fronds get crispy.

Pteridium aquilinum, fresh (by Rolland Douzet) and crispy (by Anne Thomas)

Polypodiaceae (Polypody family)

• Worldwide: 65 genera, ~1650 species

• Alps: 1 genus, 3 species

I have to mention the polypodies (Polypodium) if only because they’ve given their name to the whole class of ferns. Polypody means “many feet,” but I have yet to fully conceptualize that image (something about the stems along the rhizome…). They occupy the pared down version of the family Polypodiaceae—so, polypods all the way down.

And also, they’re beautiful. Their pinnae are distinctively smooth-edged and stylish. Their sori, too, are stunning specimens, perfectly round copper-colored clumps of sporangia in tidy rows along their pinnae.

On closer examination the clumps may appear to consist of tiny curled up millipedes. Actually, they’re catapults. These are leptosporangia, spore-filled pods ridged with a structure called an annulus (the part that looks like a millipede). Through the workings of cell architecture and changing internal pressure as the cells dry out, the annulus will pull back and then spring to launch the contents of the sporangium. The physics are exactly the same as a medieval catapult. (The video below shows this in action.)

(All Polypodiales ferns, as leptosporangiate ferns⁴, use this mechanism.)

Polypodium cambricum (1-2) and Polypodium vulgare (3-4) (Rolland Douzet)

Learn more about catapulting sporangia in this video and this commentary.

Aspleniaceae (Spleenwort family)

• Worldwide: 2 genera, 740 species (OR 24 genera + many more species, depending on the taxonomist)

• Alps: 1 genus, 20 species

Asplenium is the most species-rich and diverse genus of ferns in the Alps. These ferns tend to be laced with small, delicate pinnae, or else totally lack pinnae and instead feature broad, undivided fronds. Their defining feature is their sori, which form in stripes or commas rather than round blobs. Apparently, this is how the name “spleenwort” came about, back when people believed that plants with shapes resembling organs were somehow medically useful to those organs.

You may have seen the maidenhair spleenwort, Asplenium trichomanes, with its black wire of a stem and regular, petal-like pinnae spilling out of into a crack in a rock.⁵ Flora alpina lists several subspecies, as a well as a hybrid with Asplenium septentrionale, forked spleenwort. Their offspring, Asplenium x alternifolium, is indeed something between the rounded filigree of the maidenhair and the elongated tendrils of the forked spleenwort. (Aspleniaceae is not an evolutionarily tidy family.)

Another iconic Asplenium is the hart’s-tongue fern, Asplenium scolopendrium, which, with its shiny, rippling fronds, looks like a houseplant growing on the forest floor. Its sori are as stripy as they get.

Asplenium also features our first semi-endemic species, or one that’s pretty much only found in the Alps: Asplenium seelosii.

Asplenium trichomanes (1: Anne Thomas, 2-5: Rolland Douzet)

Asplenium x alternifolia (Rolland Douzet)

Asplenium scolopendrium (Anne Thomas)

Male-ferns and lady ferns

Dryopteridaceae (male-ferns)

• Worldwide: ~25 genera, 1100 species

• Alps: 3 genera, 15 species

Athyriaceae (lady ferns)

• Worldwide: 3 genera, ~600 species

• Alps: 1 genus, 2 species

There are two common species of ferns, classically shaped and rather similar despite belonging to two different families, which are a bit unfortunately named. Someone (probably the same person who thought Asplenium sori looked like spleens) decided that the larger, less delicate fern, Dryopteris filix-mas, embodied masculinity, while the daintier one with more demurely hidden sori, Athyrium filix-femina, was deemed ladylike.  

With no botanical context, someone could be forgiven for confusing male-ferns and lady ferns with actual reproductive complements. However, the names are, of course, an absurd bit of whimsy. Not only are these separate species, but also, fern sporophytes (the plants we see) have no “gender,” since their job is to produce asexual spores. Those spores grow into tiny gametophytes, which do reproduce sexually, but are typically hermaphrodites with both male and female organs.

The genera (and families) of these two species are also often given these same common names. In the Alps, there is also an alpine lady-fern, Athyrium distentifolium.

Dryopteris filix-mas, with fresh sori in #1 and dessicated sori in #4 (1-2: Rolland Douzet, 3-4: Anne Thomas)

Athyrium filix-femina (1&2); Athyrium distentifolium (3&4) (Rolland Douzet)

The highest mountain fern

Cystopteridaceae (fragile fern family)

• Worldwide: 3 genera, 37 species

• Alps:  2 genera, 7 species

When I asked the director of the Jardin du Lautaret, Rolland Douzet, what species of fern is found at the highest altitudes, he hazarded Cystopteris alpina and the closely related Cystopteris fragilis, which were recorded above 3000m in Italy. These are bladder-ferns, named for inflated insidia around their sori.

Not far away were also found a weird fern, Botrychium lunaria (moonwort), and a lycophyte, Huperzia selago (alpine clubmoss).

Cystopteris fragilis (Rolland Douzet)

Other fern families in the Alps

Thelypteridaceae

• Worldwide: ~30 genera, ~900 species (very difficult to classify)

• Alps: 3 genera, 3 species

Woodsiaceae (cliff ferns)

• Worldwide: 1 genus, ~35 species

• Alps: 1 genus, 3 species

Onocleaceae

• Worldwide: 4 genera, 5 species

• Alps: 1 genus, 1 species (Matteuccia struthiopteris, Ostrich fern)

Blechnaceae (deer ferns)

• Worldwide: 24 genera, ~250 species

• Alps: 1 genus, 1 species

Phylogeny

For an excellent overview of where the lycophytes and ferns fit in the Plant Tree of Life, see this poster. For a more detailed look at how the fern families from today’s post are related to each other and the other (weird) ferns, see the red section (Polypodiales) of the phylogeny below. The photos are specific to the purposes of the original article, but some of them were featured here.

Fern phylogeny from Petlewski and Li 2019, BioOne.

Up Next in Flora alpina

Onward to the gymnosperms with Pinaceae, the pine family—including not only pines, but most of the coniferous trees you can think of.

(Still working on a home page/guide for Flora alpina and the Plant Tree of Life!)

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Glossary

Genus (plural genera), family, order, class: In taxonomy, progressively higher organizational levels above species

-aceae (ay-see-ee): Suffix of a plant family

-ales (ay-lees): Suffix of a plant order

Phylogeny: a visual representation of branching evolutionary relationships in the tree of life, similar to a family tree

Basal: describes a branch in a phylogeny that splits from the rest of the tree near its “base,” farther back in time than most of the other branches.

Vascular plants: Plants with veins, including lycophytes, ferns, gymnosperms (e.g. conifers), and angiosperms (flowering plants)

Spore: A reproductive cell produced by asexual splitting (lots more on that here)

Sporangium: Sac where spores form

Leptosporangia: A sporangium that forms from a single cell and characterizes a subgroup of the fern class, including Polypodiales

Annulus: a worm-shaped structure that launches the spores inside a leptosporangium when it dries out

Sorus (plural sori): Clusters of sporangia in ferns

Insidium (plural insidia): A flap of tissue that covers a sorus to protect it

Frond: A fern leaf

Pinna (plural pinnae): One of the leaflets on a frond

Fiddlehead: A young fern frond curled in a fiddlehead shape before unfurling (also known as a crozier); this shape results from a growth pattern called circinate vernation

Gametophyte: One of the two generational phases of plants, the organism that produces gametes for sexual reproduction, tiny (made up of only a few cells) in ferns (lots more on alternation of generations here)

Sporophyte: The generation that produces spores, larger and more vegetative than the gametophyte in ferns (what we think of as a fern)

Cosmopolitan: A species that is found across most of the terrestrial earth

Allelopathic: Describes a chemical released by a plant to inhibit the germination or growth of nearby plants

Hermaphrodite: An organism with both male and female reproductive parts

1. Du et al., 2021. Simultaneous diversification of Polypodiales and angiosperms in the Mesozoic. Cladistics.

2. Ferns are the poster child for taxonomic revision. Two years after the Flora alpina was published in 2004, a foundational paper (Smith et al 2006) proposed a much more detailed division of ferns into families. This evolved further in papers published in 2011 and 2014 (this paper also gives a nice overview of ferns if you’re interested in reading more). Then, in 2016, a scientific paper with 94 authors was published by a consortium known as the Pteridophyte Phylogeny Group (like the similar Angiosperm Phylogeny Group); after weighing the latest phylogenetic and botanical data, this latest reorganization was their first official flag planted in the taxonomic terrain: PPGI. Despite including so many scientists, not everyone likes what they came up with or agrees that it represents the consensus; papers in support of one system or the other continue to trade arguments.

Why does this matter? Obviously, taxonomists will never agree on one way of sorting, and ferns go on being their ferny selves. But which system you choose to organize your understanding of ferns can change how you look at them. It tells you a different evolutionary story about the diversity you see.

3. Some taxonomies split Pteridium aquilinum into multiple species, but Flora alpina treats it as one.

4. The “leptosporangiate ferns” are an evolutionary grouping that includes several other fern orders, including some of the weird ferns.

5. Maidenhair spleenwort is coincidentally a similar shape to the maidenhair fern Adiantum; even more coincidentally, the ginkgo tree is also similar and is sometimes called the maidenhair tree.