Monthly Fern Finale—Moonworts!

"A very singular and very pretty plant ... [leaflets] are rounded and hollowed, and thence its name came of Moonwort" Sir John Hill, 1770. (image from Atlas der Alenflora 1882).

About 2075 years ago, during the first century BCE, Roman philosopher Cicero wrote of introductions—their importance and how they should be constructed (1):

"one's opening remarks, though they should always be carefully framed and pointed and epigrammatic and suitably expressed, must at the same time be appropriate to the case in hand; for the opening passage contains the first impression ... and this ought to charm and attract the [reader] straight away."

After looking up "epigrammatic" (relating to a short saying or poem that expresses an idea in a clever, funny way), I sat down to construct an introduction "appropriate to the case in hand" — Moonworts.

First the epigram, from Botrychium lunaria by Giles Watson.

Hear the latch click in the gloom,

Thus gain admittance to the room.
By fern and stealth, no guile nor wealth
Can buy a lock to hinder me. 

Now the charm, of which there's no shortage!

After an unknown number of years underground, Botrychium simplex grows a leaf (J. Hollinger).

Moonworts (Botrychium species) are attractive little ferns, and it's unfortunate they're rarely seen. They live mostly underground in the intimate company of Glomus—a fungus that forms mutually beneficial subterranean relationships with nearly 80% of vascular plants. Typically Glomus supplies nutrients to the plant, and the plant supplies Glomus with carbohydrates via photosynthesis. But photosynthesis requires sunlight, so how can a Moonwort make carbohydrates if it lives underground? Maybe it's a parasite rather than a partner. This is just one of Moonworts' mysteries.

When conditions are right (another mystery), or perhaps when the stars align, a Moonwort sends up a single leaf. Though distinctive it's difficult to spot, being small, short-lived, and often hidden in vegetation or duff. But lucky is the finder of a Moonwort! If collected by the light of a full moon, the fertile part can be used to pick locks, unshoe horses, and turn mercury to silver.

Moonwort leaf with a sterile leaflike trophophore and a fertile (magical) sporophore; closeup shows sporangia have opened and released spores (Britton & Brown 1913).

Mingan's Moonwort releasing spores; soon it will wither and be gone (R&N Crawford).

The first known scientific description of Moonwort appeared in 1542, in a revolutionary herbal by physician and botanist Leonhart Fuchs: De Historia Stirpium Commentarii Insignes, Notable Commentaries on the History of Plants. It contained 500 high quality and largely accurate illustrations to help with identification—a novel approach which Fuchs felt obliged to explain: "a picture expresses things more surely and fixes them more deeply in the mind than the bare words of the text."

Lunaria minor, from Fuchs's 1542 herbal. BHL

Leonhart Fuchs c. 1543 (source).

Descriptions in Fuchs's herbal were brief and often "borrowed" from earlier works, an accepted practice. Lunaria minor was said to have a round stem, with a single leaf divided into seven parts and with a stem atop which were seeds (fern reproduction was assumed to involve seeds, though none had been found).

Lunaria minor (the name) and Moonwort seeds would persist for several centuries. Then in 1753, pioneering plant taxonomist Carl Linnaeus put Moonworts in the genus Osmunda (but he too referred to seeds; spores weren't accepted until the mid 1800s). In 1845, Czech botanist Carl Presl moved Moonworts to the genus Botrychium, where they mostly reside today (2), and recognized 17 species. But in the first "modern" treatment of Moonworts, Jens Clausen (1938) reduced this to just six, all of which occurred in both Europe and North America.

We look back on Clausen's classification as much too simplistic. But nearly 50 years would pass before someone took enough interest in North American Moonworts to do something about it—specifically Warren and Florence Wagner, who upped the number to 22. Study and discovery have accelerated since. Currently 38 Moonworts are known for North America (Farrar 2024), with several more species in the pipeline.

A recently-described North American Moonwort—Botrychium farrarii (Legler & Popovich 2024). Note variation.

With so many species and such small plants, Moonwort identification is notoriously difficult (3). Characters are often minute (10x magnification helpful). Differences can be subtle, relative, and variable. No wonder we're regularly referred to experts for confident id. And the experts may resort to molecular techniques (e.g. DNA, enzymes) for verification.

So what are we to do in our South Dakota plant guide, aimed at enthusiasts as well as professionals? We shall follow the advice of Leonhart Fuchs, still sound after all these years. High quality photos will accompany relatively brief descriptions. Discussion of lookalikes will note similar species, offer possibly useful differences, and most likely refer the user to technical manuals and experts.

Fortunately, we do NOT have to identify a Moonwort to species to enjoy it! Just finding one is exciting, and examining it closely can be magical. For example ...

Botrychium matricariifolium was named for its twice-divided trophophores, reminiscent of the leaves of matricary (chamomile). It appears to be rare in South Dakota, found at a few sites in the Black Hills.

Matricary Moonwort is a relatively large moonwort—to 25 cm tall (MWI).

Up close, the trophophore has a lacy elegance (MWI). 

The branched sporophore has many bead-like sporangia, each one containing thousands of spores! (MWI)

Prairie Moonwort, Botrychium campestre, may be our smallest Moonwort. In South Dakota it occurs in grasslands, true to its name. It too appears to be rare, but one never knows with Moonworts! It may be hiding in the grass, or lurking underground for years, waiting for the stars to align.

Prairie Moonwort usually is less than 4 cm tall (NPS).

Botrychium simplex, Least Moonwort, has been found at widely scattered sites, from grasslands and sandhills in eastern South Dakota to a picnic area in the Black Hills. It's both extremely variable and quite similar to at least four other species in the state, making id extra difficult.

Variation in Botrychium simplex—yikes! (compiled from this source).

I'm including the next photo because I loved the comment on the field form—"Not expected out here!" That's a Moonwort for you. They seem to do just fine beyond the limits of "typical habitat". But typical habitat may just be where we typically look for them. Obviously we still have a lot to learn!

Least Moonwort (center) along a seepy creek in sagebrush steppe in Nevada! (mreala)

And so the Monthly Fern series comes to a close. Thank you for reading, happy holidays to all, and best wishes in the year to come!

Notes

(1) In De Oratore, Cicero was actually addressing speaking, but his advice for introductions seems applicable to writing.

(2) Some former Botrychium species are now in separate genera, though not everyone agrees. In South Dakota we have Botrypus virginianus (Rattlesnake Fern) and Sceptridium multifidum (Leathery Grapefern).

(3) In contrast to seed plants and true ferns, where identification relies heavily on reproductive parts (flowers, cones, spore-filled sporangia), Moonwort identification relies almost entirely on the leaf-like trophophore. What it's shape and size? Is it divided? how many times? For leaflets—specifically the lowest pair—determine shape, margins, and how they attach to the midrib. Compare them to those above ... and more.  Small size and variability compound the challenge.

Sources

Farrar, DR, and others? Moonwort Systematics, Ada Hayden Herbarium, Iowa State University. Accessed December 2025. A great resource, with descriptions and photos for many Botrychium species.

Farrar, DR, Gilman, AV, and Moran, RC. 2017. Ophioglossales, in New Manual of Vascular Plants of Northeastern United States and Adjacent Canada. NYBG Press (apparently not yet published—another moonwort mystery).

Farrar, DR, and Johnson, C. 2024. Botrychium subgenus Botrychium: Moonwort biology basics. American Fern Journal 114:10-21. https://doi.org/10.1640/0002-8444-114.1.10

Hill, J. 1770. The useful family herbal: or, An account of all those English plants, which are remarkable for their virtues ...  BHL

Legler, BS, and Popovich, SJ. 2024. Botrychium farrarii (Ophioglossaceae), a new diploid Moonwort species from the Bighorn Mountains of Wyoming, U.S.A. American Fern Journal 114:32–48. PDF

Geohopping across Nevada

Burners at incipient plate boundary in western Nevada. Are they waving California goodbye? (original unknown)

Many times I've crossed Nevada in the company of Frank DeCourten and Norma Biggar (hereafter called D & B). Actually I've never met either one, but I know their Roadside Geology of Nevada well. That's where I learned of the state's traumatic history—torn apart, reassembled, buried in ash and welded rock, and now being torn apart again. These stories can be hard to grasp, but I've read and reread the lengthy introduction enough to be awestruck by landscapes that many travelers find dull.

Sturdily bound, with high quality paper—my copy has survived lots of use.

Maps, diagrams and photos are abundant!

In the eight years since D & B published their book, I've often parked off the highway at their suggestion to study and photograph a geologic feature. I think of this as geohopping to geostops, rather than my usual geotripping to geosights (and later blogging about it). Now it's time to give the geostops their due.

One of my favorite stretches of highway between Laramie, Wyoming (home) and the California Central Coast (home of relatives) is US 6 across Nevada. Traffic is light, towns are few, and the geology truly is dramatic!

Geo highlights along US Highway 6, May 2025.

For example, about thirty million years ago, widespread cataclysmic destruction associated with the Great Ignimbrite Flareup (GIF) created Hell right here on Earth. Supervolcanoes erupted repeatedly across today's Nevada depositing ash thousands of feet deep, much of it welded into rock by the searing heat ("ignimbrite" means "fire cloud rock"). Trying to recreate that terrifying Flareup in my mind is one of the joys of driving across Nevada.

But it's impossible to properly imagine the GIF, in part because "no volcanic eruptions ever witnessed by humans come close to rivaling these prehistoric paroxysms." And the geologic record suggests it may be one of the largest ever. Consider this: in Nevada at least 230 supervolcanoes ejected an estimated 17,000 cubic miles of lava! Here's another way to think about it: at least 30 of these eruptions each equaled 600 Mt. St. Helens eruptions!

Blue Jay Maintenance Station on left, remnants of cataclysmic destruction behind.

About 90 miles southwest of Ely, I stopped at Palisade Mesa in the southern Pancake Range. Parking is available at a small rest area next to the Blue Jay Maintenance Station. Volcanic rocks of the GIF are nicely exposed on the steep slope to the east.

Rock pancakes stacked oldest to youngest, from bottom to top.

Palisade Mesa is one of multiple gently-tilted stacks of volcanic rock that give the Pancake Range its name. The escarpment at Blue Jay shows at least four episodes of eruption, all from the immense Central Nevada caldera complex. The pale bottom (oldest) layer is a lightly-welded tuff from an ash flow c. 31 million years ago. Next is a thin black band of glassy vitrophyre—"a flow of glowing ash that became densely welded."

Vitrophyre—beautiful memento of incandescent destruction. James St. John.

The massive brown layer above the vitrophyre is a younger tuff, about 30 million years old. Being a fan of columnar jointing, it was my favorite. The summit is a 2.75 million-year-old tuff that's sufficiently welded to provide an erosion-resistant cap.

I 💖 columnar jointing—created by contraction with cooling.

The view south beckoned.

Palisade Mesa obviously deserved a longer visit, perhaps a hike along the base and up the valley to the south. But not this time. Instead I continued west.

Those who cross the middle of Nevada (e.g. east to west) soon become aware of its extensive deformation even if they have no idea what happened. For example: When I left the Pancake Range I crossed Hot Creek Valley, then the Hot Creek Range, then Stone Cabin Valley, then the Monitor Range, and then Ralston Valley before stopping in Tonopah near the crest of the San Antonio Mountains. This is typical Nevada topography—valleys and mountain ranges one after another, all trending roughly north–south. The great pioneering geologist Clarence Dutton called them “an army of caterpillars marching north from Mexico".

Left of center, caterpillars are marching across the Basin and Range Province (NPS).

The cause of this curious pattern is east-west continental stretching, which started something like 18 million years ago and continues today. Some parts of Nevada and adjacent Utah and California have nearly doubled in width! In the process normal faulting has dropped basins, leaving adjacent land standing high, as mountain ranges.

In Tonopah, I stopped for gas and groceries as I often do. Here Hwy 6 merges with heavily-traveled Hwy 95, but at Coaldale Junction they diverge, and once again I had the highway mostly to myself. This is where I stumbled upon Radio Goldfield several years ago, broadcasting very local news and interesting country-ish, old-timey, new-to-me music. It's still going strong.

At the advice of D & B, I kept an eye out for a diatomite quarry on the left, near the junction with NV Hwy 264. The white patches were obvious. This diatomite is thought to be the same age as late eruptions of the GIF, but the setting was entirely different—a shallow freshwater lake where diatoms (microalgae) basked in the sun. Now they're diatomaceous earth, a soft crumbly rock that's 80–90% silica. Among its many uses are metal polish, toothpaste, cat litter, dynamite, thermal insulation, and bonsai soil amendments.

I would have enjoyed examining the diatomaceous earth, but wasn't clear on ownership.

Diatomaceous earth up close; scanning electron micrograph by Dawid Siodłak.

After continuing west across Montgomery Pass, I dropped into Queen Valley for the final geostop of the day, parking in a large pullout not far from California. Across the valley was the north end of the White Mountains; the snowy Sierra Nevada was visible in the far distance. It was a lovely peaceful place, or so it seemed that day. But nearby were clear signs of geologic trauma.

White Mountains rise steeply above floor of Queen Valley.

Normal faulting evidenced by triangular facets (arrows).

Across the valley at the base of the White Mountains is a normal fault just 3 million years old. This is the Queen Valley fault—a tiny piece of the immense Walker Lane. I had entered a profound but vague tectonic boundary, where the Basin and Range Province meets the great Sierra Nevada.

At Walker Lane (yellow), very different tectonic regions meet. SAFZ is San Andreas Fault Zone, a critical part of this story (Carlson et al. 2013).

Walker Lane is young—just 10 million years old at the south end, and only a few million at the north. The combination of Basin and Range extension and transverse movement of the Sierra Nevada has created a complex zone of faults that's poorly understood. Even so, Walker Lane generates a great deal of excitement among geologists. Perhaps a new plate boundary is forming! Maybe California will drift away!

Like the better known San Andreas Fault to the west, Walker Lane is contributing to the slow, incessant, contrary motions of the Pacific and North American tectonic plates, which are pulling a large part of California northward. Currently the San Andreas is responsible for about 80% of this movement but Walker Lane appears to be catching up.

Fauds & Henry (2008) predict that in another 7 to 8 million years or so, the northern part of the San Andreas will join Walker Lane, extending the Gulf of California north by hundreds of miles and turning California into a peninsula along a new plate boundary. 

If this tectonic shifting continues, as the authors think it will, California will become the island that was regularly reported by explorers hundreds of years ago! This was the "famous cartographic error that appeared on many European maps from the 16th to the 18th centuries" (David Rumsey Map Collection).

"Novissima et accuratissima totius Ameriae" by Nicolaes Visscher, 1690. Large island off the west coast of North America is California. DRMC

Peering even further into the future, we may well find that California Island has become an exotic terrane (quit snickering!). As such, it could travel far and wide before being stopped at some convergent plate boundary, thousands of miles from its origin at Walker Lane.

But Emmie ... our ephemeral lives mislead us. The Earth is far from stable.

Sources

agimark 2018. Splitting North America – The Walker Lane; Part 1 – The Tectonics; Volcano Hotspot blog. Accessed Dec 2025.

Carlson, CW, et al. 2013. Kinematics of the west-central Walker Lane ...  Geosphere 9: 1530–1551.

David Rumsey Map Collection, an unbelievably wonderful resource for fans of old maps. WARNING: it's very easy to spend a lot of time here. https://www.davidrumsey.com/

DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press.

Faulds, JE, and Henry, CD. 2008. Tectonic influences on the spatial and temporal evolution of the Walker Lane: An incipient transform fault along the evolving Pacific – North American plate boundary. Nevada Bureau of Mines and Geology, Arizona Geological Society Digest 22. The future of California is discussed on page 463. PDF

Wolterbeek, M. 2020 (Feb 18). How the burgeoning Walker Lane may split the American West; in Nevada Today, UNV Reno.

A Darkling Path through the Ferny Ferns

Be there dragons here?

It's November and The Monthly Fern series is winding down. Looking back, I realized that most of the ferns I chose are distinctive—they're aquatic or have dimorphic leaves or are primitive lycophytes or grow large enough to inveil a romantic tryst! So this month's post will feature the ferny ferns (my term)—the ones we immediately recognize as ferns. However, figuring out which specific kind isn't guaranteed. If only ferns had flowers—so showy and diverse! Instead we must rely on leaves (1).

As the days shorten it would seem that writing descriptions for our Guide to South Dakota Plants would be appealing, especially given my current subjects—ferns and their relatives. But they can be difficult, and at times inscrutable. Of course they aren't the ones to blame. We are—specifically we botanists who seek order in their labyrinthine world.

I try to make my plant descriptions user-friendly, as our intended audience is broad—professionals, academics, students, enthusiasts, and eager novices (2). Being online makes this much easier. There will be many photos so I can shorten the text and minimize technical terms. Even so, there remain features that must be explained, for example the lovely lacy leaves of the ferny ferns.

The much-divided leaves of ferny ferns are the basis for "fernlike"—for example, "Western Yarrow leaves are fernlike" (SAplants).

Fern descriptions typically start with the plant—height, form (erect, spreading, sprawling), behavior (colony-forming, clumped), and other fairly straight-forward things. Leaves are next. Position (basal, on the stem), color, dimensions, and overall shape are easy to describe. But then ... we're faced with the dreaded degree of dissection. How many times is the leaf divided? Are there true segments? Are the segments themselves divided and are these divided as well? Here the guides I've been using as examples diverge, perhaps out of confusion. Suddenly the way forward becomes unclear; the path darkens considerably.

Entering the darkling world of leaf division.

"Leaf Division" from Fern Structure (USDA Forest Service).

In my web wanderings, I found a figure showing degrees of leaf division (above). It seems clear, though one needs to know that "pinnate" means divided and "-fid" means "nearly". For example, "pinnatifid" means nearly once-divided—division doesn't quite reach the midrib of the leaf as it does in "pinnate".

I intend to use this figure, perhaps as a pop-up, but will replace "pinnate" with "division" thereby eliminating the need for translation. "fid" situations will be accommodated with "nearly", for example "nearly twice-divided".

My version—actually a common approach, not my invention. 

Declaration of degree of division is followed by description of the ultimate segments—their shape, size, hairiness, margins, and such. This can provide much-welcomed help with identification. 

Ready for a test? Using the photos below, describe leaf division in Male Fern, Dryopteris filix-mas, and characterize the ultimate segments.

Male Fern's clumped ascending leaves can be more than a meter long. Аимаина хикари

Once-, nearly twice-, or twice-divided? Note the toothed (but not spiny) margins of the ultimate segments (click on image to view). Nick Turland

The sources I use all say that leaves of Male Fern are nearly twice-divided (pinnate-pinnatifid). But you needn't feel bad if you chose a different answer—you are correct. Male Fern leaves are usually once-divided at the tip, often twice-divided near the base, and nearly twice-divided in between. But adjacent segments can differ as the photo shows. Some are true segments, with division reaching all the way to the midrib. Others don't quite make it.

With no obvious path through this shadowy world, let's change the subject.

It's not uncommon for fern identification to be difficult, as even experts acknowledge (e.g., Cobb et al. 2005):

"Many ferns are distinguished by the finer details of the blade and how it is divided, and descriptions of fern blades can seem difficult and frustrating to beginners" (italics mine; I too get frustrated, and take offense at being labeled a beginner).

This is where a truly user-friendly guide can help, with lookalikes and tips for identification.

Be discriminating in your choice of guides.

In South Dakota we have an especially fine (= difficult) example of lookalikes—Fragile Fern vs. Oregon Cliff Fern. They grow on the same types of sites and look oh-so-similar. Both have nearly twice-divided leaves (often twice-divided at the base) and their ultimate segments have rounded tips and toothed margins.

Fragile Fern, Cystopteris fragilis (MWI).

Oregon Cliff Fern, Woodsia oregana (MWI).

Several small but distinctive features can help with identification (10x magnification recommended). Ultimate segments of Fragile Fern are not glandular and usually hairless, and the margins are irregularly toothed. In contrast, Oregon Cliff Fern segments are glandular hairy (more so on the underside), and the margins are regularly toothed.

Fragile Fern, with irregularly toothed segments (MWI).

In Oregon Cliff Fern, segments are regularly toothed (MWI).

Those familiar with these ferns in the wild have another tip, and it's something that's easier to see. Our Cliff Ferns (Woodsia species) often have persistent dead leaf stalks. This isn't the case for Fragile Fern.

It's not unusual for a mature Cliff Fern to have more dead stalks than leaves (Andre Zharkikh).

You can relax now. No more tests. We're very close to the end, with reassuring light visible ahead. And if you found leaf division tedious and difficult, think how I must feel after attempting to explain it! Sometimes I have to remind myself that I love plants.

Notes

(1) Replacing fern terminology—frond, stipe, pinnae, e.g.—with the more familiar terms used for angiosperms—leaf, leaf stalk, leaflet—has become fairly common (for example, Flora of North America). Others adhere to tradition, explaining terms in a glossary or introduction (for example, Cobb et al. 2005).

(2) I'm not enough of an expert to write descriptions of South Dakota plants myself. Instead I rely on the knowledge of others, both in printed manuals and online. The majority of photos also are by others, available online through Creative Commons licenses.

(3) Some readers may be thinking, "Just find fertile leaves with sori!" (spore clusters). After all, we've been told repeatedly that sori are distinctive. But those of Fragile Fern and Oregon Cliff Fern are hard to distinguish at maturity. Fragile Fern does have distinctive pocket-like indusia, but only when young (see photo of leaf segments in post).

Sources

All fern art created with NightCafe AI Art Generator.

Cobb, B, et al. 2005. Peterson Field Guide to Ferns, 2nd Ed. Northeastern and Central North America. Provides excellent lookalike information and tips.

Minnesota Wildflowers, a guide to the flora of Minnesota. This was the first online guide I found, and remains the most user-friendly of those I've seen (there aren't all that many, online guides being relatively new). Fortunately South Dakota and Minnesota share many plant species, and this website will be our main source of photos.

USDA Forest Service. Ferns. Highly recommended.

The Monthly Fern—More Quirks of Quillworts

Jon Keeley with several of his beloved quillworts (date unknown).

Once again The Monthly Fern series is featuring the Prairie Quillwort and its relatives—genus Isoetes. One post was not enough for these fascinating plants! Not only are they the sole survivors of a plant group that dominated 300 million years ago (see last month's post), they use CAM photosynthesis (1) ... that's astonishing! In fact it's so unexpected that when Jon Keeley announced it 40+ years ago, he was written off as ignorant (Keeley 2014).

Here's the conundrum. CAM photosynthesis is thought to have evolved in flowering plants (angiosperms) in hot arid environments. Many succulents, including most cacti, are CAM plants. But quillworts are primitive spore-producing lycophytes predating flowering plants by c. 200 million years. And almost all are aquatic.

Isoetes and other lycophytes split from ferns and seed plants long ago (source; black, red labels added).

Lycophyte diversity by Kingfiser (click link for full names and more info).

When I was an undergraduate long ago, only one type of photosynthesis was known (or so we were taught). As a grad student a decade later, I learned there were three: C3 is the common type; C4 and CAM are restricted to certain groups and situations (2). Since then I've largely ignored photosynthesis. But when I read that quillworts are CAM plants, I was intrigued! It was time to learn more. (Information here is from Khan Academy's Biology Unit 8, Photosynthesis unless noted otherwise.)

Photosynthesis: 1st stage powered by sunlight; 2nd makes food & oxygen for us to consume.

Photosynthesis is complicated and very chemical, but the basic process is simple. There are two stages. In the first, energy from sunlight is captured and converted to chemical energy. In the second, this chemical energy is used to convert carbon dioxide and water into glucose and similar carbon-based compounds, releasing oxygen in the process.

These are the benefits we reap. We consume carbon-based compounds for energy and to build proteins, DNA, muscles and more. And we breathe oxygen. If photosynthesis were to stop, we would die—either starve or suffocate.

As wonderful as photosynthesis is, there's room for improvement. The widespread C3 type, used by 85% of plants, is surprisingly inefficient. Carbon dioxide is captured and a sugar molecule created only about 65–80% of the time. The problem lies with an important but indiscriminate enzyme—rubisco—which will happily bind oxygen instead of carbon dioxide if given the chance (more here). 

Rubisco is the "molecular equivalent of a good friend with a bad habit" (KA, modified slightly).

This inefficiency is significantly less in C4 and CAM photosynthesis. But there's another problem and it's a big one—water loss. Plants take in carbon dioxide from air via stomata (pores) on leaf surfaces. But water vapor is lost at the same time, especially on hot dry days. Many plants close their stomata at night to prevent water loss, and when it's hot, some close stomata during the day as well. But then there's no source of carbon.

This is where CAM plants excel. They open their stomata at night and collect carbon dioxide, storing it for use the next day when the sun is shining. That way they can photosynthesize without opening their stomata and losing water. So clever!!

CAM photosynthesis requires extra energy compared to the common C3 type, but apparently it's worth the cost. CAM is used by at least 16,000 species, c. 7% of all plants. Most are desert plants, including at least 99% of the 1700 species of cacti (source). And then there are the quillworts, nearly all of which are aquatic at least part of their lives. Why would they bother with energy-expensive CAM?

Isoetes melanopoda, Prairie Quillwort, uses CAM even though it's aquatic (©2015 Robbin Moran).

Prairie Quillworts photosynthesizing by the light of day, with CO2 they gathered before dawn (Andrey Zharkikh).

Like terrestrial CAM plants, aquatic quillworts gather and store carbon dioxide at night but for a different reason. Terrestrial CAM plants have no access to CO2 during the day because their stomata are closed to prevent water loss. Quillworts have no risk of water loss, but for them daytime uptake of CO2 is difficult. It diffuses poorly in water to begin with, and most of the other plants in the pond are better at sucking it up for photosynthesis (4).

By the end of the day, the amount of CO2 in pond water is quite low. But as soon as night falls and photosynthesis stops, it quickly rises. "This must be when quillworts open their stomata to collect CO2" you may be thinking—as I did. But then a memory floated to the surface. Stomata don't work underwater! Aquatic quillworts have none, or non-functional ones at most.

From what I've read, there's still much to be learned about carbon dioxide uptake in Isoetes. However we do know that it varies with species and habitat. The few quillworts that are fully terrestrial—never submerged in water—have functional stomata and use C3 photosynthesis. They never use CAM, nor can they be converted to CAM even by keeping them underwater for a long time.

Isoetes histrix, Land Quillwort, is terrestrial (but often reported as aquatic). Late season photo by Sam Thomas; added insert by Peter de Lange.

Those quillworts that live part of their lives submerged, for example in vernal pools, are impressively versatile. They utilize CAM until water is low enough to expose their leaf tips to air. Then the stomata start to become functional and C3 photosynthesis begins to take over, progressing down each leaf cell by cell keeping just above the water! (Keeley 2014)

Isoetes howellii in dried vernal pool. It was in Howell's Quillwort that Jon Keeley stumbled upon CAM photosynthesis. © 2004 Carol W. Witham.

The many quillworts that are entirely aquatic are more puzzling. They have no stomata and their leaves are covered with a waxy cuticle. And yet they thrive, especially where other plants can't.

Aquatic Isoetes lacustris, the Lake Quillwort (Alina Ambrosova).

Isoetes lacustris in its favorite environment—lake bottom with sparse vegetation (5). (Alina Ambrosova) 

Aquatic quillworts seem to be more common in oligotrophic waters, where nutrients are scarce and there's little competing vegetation. So how do they survive if other plants can't? Probably with their unusual roots (6).

These roots have a large central air cavity that accumulates carbon dioxide gathered from sediments. Next to the cavity is bundle of vascular tissue that delivers it to the plant above. Furthermore, being CAM plants they collect CO2 at night as well as during the day, thereby doubling their harvest. Sometimes they truly flourish, covering the lake bottom in a dense green underwater carpet! (Moran 2004)

And with that, I will close. As you may suspect, this was one of my more challenging posts. Just when I had everything figured out, another puzzle would present itself. But I'm not complaining. In fact that's what I enjoy most about getting to know plants—pondering and unraveling their many little mysteries. And I know that the next time I meet up with a quillwort, it will be far richer experience.

So primitive, so simple in form, and yet so alluring (Isoetes englemannii, Nathan Aaron).

Notes

(1) C2 carbon concentration is sometimes considered a type of photosyntheses.

(2) CAM refers to crassulacean acid metabolism. To be clear, there is no "crassulacean acid"; the name refers to acid metabolism in the family Crassulaceae, where CAM was discovered (source).

(3) The widespread occurrence of CAM likely is due to repeated convergent evolution. After sequencing the pineapple genome, Ming et al. (2015) concluded that CAM arose from relatively simple reconfiguration of C3 pathways. See also Wickell et al. 2021.

(4) Many aquatic plants collect CO2 via bicarbonate; it appears that quillworts are unable to do this (Keeley 2014).

(5) Is that an alga on the leaves of Isoetes lacustris? If so, it might affect light capture but not CO2 uptake, which is done by the roots.

(6) Isoetes lacustris roots look very much like the fossilized roots of Lepidodendron trees, its ancient relatives.

Sources (in addition to links in post)

Keeley, JE. 1981. Diurnal acid metabolism in vernal pool Isoetes. Madroño 28:167-171. BHL

Keeley, JE. 1998. CAM Photosynthesis in submerged aquatic plants. The Botanical Review 64:122–158. PDF.

Keeley, JE. 2014. Aquatic CAM photosynthesis: A brief history of its discovery. Aquatic Botany 118: 38–44. http://dx.doi.org/10.1016/j.aquabot.2014.05.010

Lane, N. 2010. Life Ascending: The Ten Great Inventions of Evolution. WW Norton & Co.

Moran, Robbin. 2004. "Some Quirks of Quillworts" in A Natural History of Ferns. Timber Press.

Wickell, D, et al. 2021. Underwater CAM photosynthesis elucidated by Isoetes genome. Nat Commun. 12:6348 (open access).