Fernilicious: The many gifts of ferns

Spring is here and while many of my friends are oozing about the cherry blossoms, I’m obsessing about what plants are coming up. It is always a marvel when the ground erupts and leaves and stems pop out. Here in Maryland, the beloved Virginia bluebell is in its full sky blue glory, but as it poked its way out of the ground a few weeks ago its leaves were a unique black-purple hue. Hostas are coming out of the ground now looking like a squad of spearheads and peonies are appearing as deep maroon stalks. But perhaps my favorite of the spring emergents are the ferns. The leaves slowly unfurl from a tight spiral creating one of the most graceful of nature’s displays. Ferns evoke strong feelings in many of us, for me it is a combination of tranquility and bounty, perhaps - quiet exuberance. The reproductive cycle of ferns has always fascinated me, but after having done research on them, I’m even more enthralled. Ferns have shaped life on the planet for far longer than humans have even been around.

In order to understand the greatest gift of ferns, we have to understand how they reproduce. Like all other land plants, ferns have a sexual component to their reproductive cycle. During this phase, an egg is fertilized by a sperm. They create a zygote that will grow into the plant we recognize as a fern. You may recognize this pattern from how human babies are formed, but in our case, the zygote grows into a baby which when grown will have the same form, roughly, as its parents. In contrast, the fern that grows out of the sexual union, is unlike its parents. The parent which produced the egg and sperm that grew into our recognizable fern, are called gametophytes, because egg and sperm are called gametes. Phyte simply means plant. Unlike the fern stage with which we are familiar, fern gametophytes are often shaped like a violet leaf - sort of heart shaped, but thicker and more wrinkly. They tend to lay flat on the ground and only the most avid of naturalists will have ever noticed them. Reproductively successful gametophytes usually grow in damp areas for the sperm need to swim through ambient water looking for a fern egg to fertilize. If you want to picture a fern gametophyte, think of a soggy piece of kale about the size of your fingernail.

But how do we get from the fern fronds we all know and love to the wet-kale-like-gametephytes that make the baby ferns? Magic. Fern fronds develop little capsules called sporangia. The sporangia often appear as little brown spots on the underside of fern’s fronds, and within the sporangia capsule the fern produces spores. Because of this spore production, we call this part of the fern cycle a sporophyte. At some point, the fern opens the sporangia capsule and the spores scatter, often carried by wind to distance places. Usually, male and female spore are the same size, but in some ferns, the female spore is much larger. We’ll return to this disparity in a minute. If the released spores land in a hospitable spot, they grow into a gametophyte. The gametophyte matures and releases an egg and/or a sperm, which may find each other, and unite to make a baby fern. This switching between fern fronds and the wet kale form is known as the alternation of generations. As a matter of fact, all land plants and many algae reproduce using this alternation of generations.

Now that we’ve laid out the reproductive cycle of the ferns, we can understand one of the biggest gifts that ferns have given us. As noted above, while most ferns produce male and female spores of about the same, microscopic size, one family of ferns produce both the familiar microscopic spore, and a much larger female spore. These megaspores are retained inside of the sporangia - the spore production capsule. Within this cozy capsule, the megaspore is fertilized, and, lives out its entire gametophyte stage. The vegetative phase of these gametophytes is nearly eliminated. Although ferns don’t actually produce a seed, this process is edging fairly close to seed production and is the leading candidate for how seeds developed. And if ferns were the pathway to seed evolution, it means ferns have given us flowers, which in turn means ferns have given us fruit and nuts, wheat and rice, tomatoes and cotton.

Now that we’ve labored our way through fern reproduction, know that most ferns that you come across are in fact the result of none of those processes. An isolated fern is indeed probably the result of the above described alternation of generations, but groupings of ferns usually arise from vegetative reproduction. The rhizomes of a pioneer fern spread outward and new fern fronds pop up. Each fern in a given colony is a clone of the first fern, all with the exact same genes as the original plant. Why then the complicated alternation of generations if this simple vegetative reproduction is possible? Think for a moment about the range and diversity of habitats that can be exploited by a plant that has wind borne spore, that can spread far and wide, and has different life stages, that can withstand different soil and climatic conditions. They are clearly a vastly successful group of plants having survived for over 400 million years. Recall that humans have only been around for about 300,000 years which is only one one-thousandth of the time that ferns have been around.

Sexual reproduction is a feature common to almost all multi-cellular organisms, including animals. I’ve often wondered if this arose separately in animals and plants, or if we share a common ancestor. It turns out that sexual reproduction is at least 2 billion years old. That’s almost half the lifetime of planet earth. Sexual reproduction arose in the eukaryotes, perhaps from the reproductive processes of bacteria, which predate the eukaryotes. In case you are wondering, a eukaryote is one of 3 domains of life, the other two being bacteria and archaea. Bacteria and archaea are usually microscopic and both reproduce primarily asexually. They thus pass on their full set of genes to the next generation. Eukaryotes are defined as having a cell nucleus enclosed by a nuclear envelop. From such humble microscope distinctions, eukaryotes have since evolved into plants, animals and fungi each inheriting their sexual proclivities from the earliest eukaryotes, so probably from a common ancestor. It is interesting to ponder that it is the mixing of genes afforded by sexual reproduction in eukaryotes which has facilitated the great diversity of life on Earth.

But lest you think the only gift ferns have given us are seed plants, be amazed that ferns also play a massive role in the history of our planet and in the structure of our forests. To start with, ferns are the main source of coal and natural gas - perhaps a mixed blessing. As ferns were buried during the carboniferous period they sunk into vast swamps where there was little oxygen and hence little decomposition. Continued growth and burial over a few hundred million years led to coal deposits.

But that’s not the only time that ferns sucked carbon out of the atmosphere. Much more recently, about 49 million years ago, the climate was in a near runaway state and was being dangerously overheated by greenhouse gases. A tiny water fern named Azolla, took advantage of the conditions in an isolated Arctic Ocean - which was then much warmer than it is today - indeed it hosted crocodiles. When the Azolla died, they sank to the bottom of the ocean taking with them large blankets of carbon. This went on for about 800,000 years and reduced the carbon in the atmosphere from about 3,500 parts per million to about 650 parts per million. The entire globe cooled. This rapid drawdown of carbon corresponds with the onset of a slow global cooling of about 35 degrees F (20 C), that eventually led to the glacial-interglacial cycle the earth experiences today. Did the cooling afforded by the Azolla instigate this? That’s not known but it certainly played a role.

Azolla’s ability to suck up 6 tons of carbon and a ton of nitrogen per acre every year has led rice farmers to use it as a fertilizer and for geo-engineers to propose Azolla farms to absorb anthropogenic carbon. I’m afraid I don’t share much enthusiasm for biologically driven carbon sequestration projects like growing massive amounts of Azolla. In order to make an impact on atmospheric carbon, these projects would have to transform vast amounts of the global surface and destroy great sweeps of the biosphere. Though, diverse reforestation is a hopeful exception.

On a more local scale, ferns are also known to inhibit some, but not all tree species. Ferns thus ultimately control the make up of forests in which they thrive . If flowers, coal, natural gas, and a livable planet aren’t enough to make you think ferns are astonishing, note that they also are capable of cleaning up toxic messes. Ferns are not only tolerant of many growing conditions and compounds, but they have a high efficiency of contaminant removal and grow rapidly. They’ve been used to remove heavy metals, radionuclides, nutrients, hydrocarbons, and volatile compounds from soil and water.

Finally, ferns are sources of shelter for small animals and a few species are known to feed on them such as the wood mouse, bull finch and lesser short tailed bat. In tropical rainforests, the bird’s nest fern hosts up to half the total invertebrate biomass within the rainforest canopy - underpinning the food chain and cycling nutrients. And ferns are also important for the regeneration of disturbed sites following a natural or anthropogenic disaster. Ferns can quickly colonize denuded sites, even when other plants can’t. As the ferns live and grow they help to develop the soil column such that other plants are able to succeed.

Because ferns have been around for so long, they have evolved into a dizzying collection of forms. There are moonworts, ferns that rarely grow beyond 3 inches tall and have delightful half-moon shaped leaves. Moonworts were much beloved by the witches and alchemists of the past who would collect it at the full moon and use it to stop bleeding, to access the fairy world and to create gold. In sharp contrast are the tree ferns which can grow to over 80 feet tall. There’s even some climbing ferns. While I’d be delighted to see any of these exotic marvels, for my own part, I’m content to head outside to see if any more fiddle heads are poking their way out of the ground and to pay homage to this ancient, world changing, life defining, flower giving plant.

© 2022 Pru Foster. For permission to reproduce this article contact the author at theprulife@yahoo.com