Fern frond

Fascinating Ferns

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They may not produce flowers, but ferns are nonetheless a fascinating and beautiful group of plants. First emerging around 360 million years ago in the late Devonian period, they are also some of our oldest, predating the dinosaurs. Since those first ferns appeared, they have evolved into a diverse range of species. They now grow on every continent except Antarctica, although fossils show that they once grew there as well. And while we often associate them with damp, woodland settings, some are remarkably at home in much drier habitats, including manmade structures like walls. Read on to find out more about this incredible group of plants.

Prehistoric Pioneers

Although liverwort-like bryophytes appeared around 450 million years ago, at approximately 360 million years old, ferns are one of our oldest groups of vascular plants. Vascular refers to the transport systems that move water and essential nutrients around plants internally and which mosses and liverworts lack. Having these systems mean that ferns could grow larger than their bryophyte predecessors. By the Carboniferous period some had evolved into large swamp growing tree ferns with tall, trunk-like stems and leafy fronds. Along with the giant horsetails and club mosses also growing at this time, these Carboniferous tree ferns were compressed and heated over the subsequent hundreds of millions of years. This process produced large coal beds that man has since exploited for fuel, hence the ‘carbon’ part of the geological period’s name.

Dicksonia antarctica tree fern
Dicksonia antarctica is a present-day species of tree fern

The fossil record shows that ferns really started to diversify during the Cretaceous period, around 145 million years ago, although many older families became extinct as the climate cooled and flowering plants began to dominate. This is when many modern fern species’ lineages began. There are now more than 10,000 species globally. Fern fossils don’t just tell us about plant evolution, though. They also help to reveal climate and geographical conditions from the past. For example, fossil seed ferns in rocks collected on Robert F Scott’s ill-fated Antarctic expedition of 1911 to 12 showed that ferns once grew on the continent. Dating from around 250 million years ago, they proved that Antarctica was once warm enough to support plants. In addition, they showed that today’s continents were previously joined, on multiple occasions, in larger super-continents, allowing the spread of organisms between regions that are now geographically and biologically isolated.

Male fern
Some modern ferns, like this male fern, grow in shuttlecock-like clumps

Boy Meets Girl

Fern reproduction is a two-stage process unlike that of the flowering plants that evolved later. In common with their bryophyte predecessors, ferns reproduce via single-celled structures called spores. These tiny containers of genetic material often, but not always, appear in distinct clusters called sori on the underside of a fern’s fronds. The shape and arrangement of the sori varies from species to species and can help with identification. Once released, the spores develop into small plants called gametophytes. These look very different from the ‘adult’ plants we call ferns. Water is crucial for the next stage of the cycle. The gametophytes release sperm which need water to swim to the egg-producing area of the same or, ideally, another gametophyte. Once the sperm has fertilised an egg, the more familiar fern structure, called a sporophyte, develops. The gametophyte eventually withers away.

Male fern sori
This male fern’s brown sori are clearly visible here

Flowering plants, or angiosperms to give them their technical name, developed a different approach to reproduction. Instead of spores, angiosperms produce multi-cellular seeds which contain a developing embryo. These embryos develop directly into sporophytes without the intermediate gametophyte stage. Seeds also have much more protective casings than spores. These cases make them more resilient to dry or unfavourable conditions, meaning they can lie dormant until ready to germinate. This may be one reason that flowering plants were quickly able to spread and achieve dominance over ferns and other early plants once they appeared at the start of the Cretaceous. Other theories suggest the way flowers attract pollinators makes reproduction easier or that smaller cells allow angiosperms to photosynthesise better.

Common polypody sori
These sori are on a common polypody

Fiddle-de-dee

As fern sporophytes grow, many larger species’ individual fronds first appear as tightly wound spirals. These are called fiddleheads or croziers because they look very much like the scroll at the top of a violin or the end of a bishop’s staff. Some species, including the ostrich and lady fern, can be cooked and eaten at this stage. They are especially popular in parts of Asia and North America. If foraging yourself, though, always make sure you know what you are collecting and pick only what you need. Some species, such as bracken, is toxic if eaten in any quantity.

Bracken fiddlehead
Unfurling fern fronds resemble a violin’s scroll, hence the name ‘fiddlehead’

Bracken is the UK’s largest, and perhaps most familiar fern, and it likewise produces these coiled structures in spring. This extremely common species can form dense patches by sprouting from vast networks of rhizomes. These horizontal stems lie either on or just under the ground and can remain dormant until favourable growing conditions arrive. This tactic has helped bracken to spread to every continent except Antarctica, and it is thought to be the world’s commonest terrestrial plant. The rhizomes also mean bracken is less reliant than some species on spore production.

Foxgloves and bracken
Bracken can form dense stands, crowding out nearly all other species

Unfurling Fronds

Once unfurled, fern fronds often divide into leaflets, or pinnae, either side of the frond’s vertical axis. Some, like common polypody, have one leaflet on each side of the frond. These are called pinnate ferns. Other species, including male fern, are bipinnate and have pinnae that then divide again into smaller pinnules. A further group, including broad buckler-fern and bracken, have pinnules that are then also divided into sections called pinnulets, making them tripinnate. Just as with the sori size and arrangement mentioned earlier, the fronds’ divisions and subdivisions can help differentiate very similar looking species.

Broad buckler-fern
The fronds on this broad buckler-fern divide three times making it tripinnate

A few species, however, have completely undivided fronds. These include the hart’s-tongue fern, a common species along damp banks and walls. More unusual are members of the adder’s-tongue family which don’t look like ferns at all. The adder’s-tongue fern itself produces a single oval leaf along with a spore-bearing spike, making it look a bit like a smaller version of a flowering cuckoo pint plant. Moonwort, meanwhile, has a fleshy, pinnate leaf frond. This sterile frond divides into half-moon shape crescents, hence the name. In addition, it has a fertile frond with clusters of globular spore-producing structures on.

Hart's-tongue fern
Hart’s-tongue ferns have undivided fronds. Note the brown sori lines on the frond undersides
Moonwort
Moonworts have a fertile frond bearing grape like structures and a sterile frond of leafy pinnae

Finding Ferns

We might think of ferns as most often associated with damp, dark habitats such as wet woodlands, gorges and sheltered riverbanks. However, they have adapted to a wide variety of environments, although they do need water to reproduce. Some of the UK’s rarest species are the filmy ferns, such as the protected Killarney fern. They are also some of the most typical in terms of habitat requirements. These strange, delicate, almost translucent ferns like growing on damp rock faces, beside waterfalls or on trees in temperate rainforests. Many of our common species, meanwhile, are less fussy. Hard fern, for example, often grows in damp woods and gorges but can also be found on open ground such as moors and heathland at relatively high elevations. Bracken is also, unsurprisingly, fairly adaptable. In fact, pretty much the only places that it won’t grow are bogs and marshes.

Hard fern
Hard fern, also called deer fern, can grow in a variety of habitats including damp woods and open moorland

Some ferns, like the common polypody, will grow as epiphytes on trees if conditions are moist enough. As such, they are good temperate rainforest indicators. Our spleenworts, meanwhile, are most at home on limestone rocks, although the largest, hart’s-tongue fern also grows on shaded banks and in woodland. All have adopted walls as perfect substitutes for natural rock faces and can often be seen growing together in our towns and cities.

Glasdrum woods polypody
Common polypody grows on trees in the right conditions

The smallest, wall-rue, has club-shaped, dark green leaflets. Maidenhair spleenwort has a distinctive narrow, black leaf stalk, while the aptly named rustyback fern’s rough, brown sori cover the back of the fronds, giving a rusty appearance. Black spleenwort is perhaps the most fernlike, with its feathery two to three pinnate fronds. In contrast to these wall-dwellers, and unusually for ferns, adder’s-tongue and moonwort prefer open, unshaded habitats such as meadows, grasslands and dune slacks.

Wall-rue and maidenhair spleenwort
The darker green wall-rue is here growing right alongside the lighter maidenhair spleenwort
Rustyback fern
Rustyback fern truly lives up to its name

Born Survivors

Once flowering plants arrived in the Cretaceous with their new-fangled insect attractors and fancy seeds, ferns may have lost out in the dominance stakes. But they managed to survive and diversify to fit their own niches in the increasingly competitive world they found themselves in. So much so, in fact, that one species has probably become the commonest terrestrial plant on Earth.

Black spleenwort
Black spleenwort is one of many wall-growing fern species

Their delicate beauty has also entranced humans. In the Victorian era, there was even a mania for collecting them that almost certainly led to local extinctions of some species such as the Killarney fern. Now, like much of our wildlife, they face multiple new threats, including climate change, habitat loss and pollution. Let us hope that these resilient early plants continue to survive in an ever-changing and challenging world.

Further Reading

Ferns can be difficult to identify so this comprehensive field guide to Britain’s 57 species, along with their relatives the horsetails, clubmosses and quillworts, is invaluable. The book is user-friendly and eschews jargon to help the novice. There are also handy comparisons between similar species.

Although by no means just about ferns, this fascinating narrative account of Earth’s history provides a useful backdrop to ferns’ origin story. And who could resist that beautiful fern image in the cover?

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