The Monthly Fern??—Prairie Spikemoss

Selaginella densa—moss, fern, fern ally, or none of the above? Coin is 19 mm across.

This month the South Dakota fern series features another oddity—a spikemoss, genus Selaginella. It's even more unusual than last month's Water Clover, for while water clovers are ferns, spikemosses are not, at least not anymore. So where in the greater scheme of plant classification do they belong?

The Prairie or Dense Spikemoss, Selaginella densa, is the more common of South Dakota's two spikemosses. It occurs in the Black Hills and scattered across the west half of the state. If you live in or have wandered across the Great Plains or Rocky Mountains, you may have seen it, for it grows on a wide range of sites—prairies, alpine meadows, dry rocky slopes, rock crevices, sandstone, quartzite or granite rock, and dry gravelly, clayey or sandy soil (Flora North America). Or maybe you overlooked it, as I used to do. After all, it looks very much like a moss (1).

Spikemosses were first classified—given a name and assigned to a plant group—by the great Swedish botanist Carl Linnaeus, founder of today's system of naming organisms. In his Species plantarum (1754) he placed them in the CRYPTOGAMIA MUSCI section—the mosses. That was a big mistake, but at the time it was a reasonable decision. Like mosses, Selaginella produces spores (2). But unlike mosses, it has vascular tissue—plumbing for transporting water and nutrients.

In the late 1890s another Swedish botanist was studying spikemosses, while preparing a Catalogue of the Flora of Montana and the Yellowstone National Park. Per Axel Rydberg had emigrated to the United States in 1882, hoping for a career as a mining engineer. But after a serious injury in an iron mine in Michigan, he moved to eastern Nebraska to teach mathematics. He also studied botany at the University of Nebraska—the beginning of a "lifelong devotion to plant studies" in the Great Plains and Rocky Mountains (source).

In 1895 and 1896, Rydberg was sent to Montana by the US Department of Agriculture to collect grasses and forage plants. The next summer he returned, with the first field expedition of the New York Botanical Garden. He made about 1800 collections representing 800 species—20,000 specimens in all (replicates were collected for exchange or sale to other institutions).

Rydberg's Catalogue included a large foldout map showing localities mentioned in the text. He noted that the eastern half of the state was "practically unexplored botanically." (BHL)

Going through his collections that winter, Rydberg saw that the flora of Montana was poorly known, even with his additions. "It was therefore considered advisable to extend the work and study all the material from the state that was accessible." He examined specimens from 16 institutions and private individuals, ranging from the Lewis & Clark collection (1803–1804) to the Montana Ladies' [Columbian] World's Fair Set (1893). By the time the Catalogue was published in 1900, Rydberg had added 776 species to the flora of the Rocky Mountain region, including 163 novelties—species new to science (Rydberg would become known as a notorious splitter).

Among the novelties was a low densely-tufted plant with very short stems covered in bristle-tipped leaves 3–5 mm long. Fertile stems were taller, to c. 4 cm, with spore-bearing leaves (sporophylls) neatly arranged in four ranks, forming terminal strobili (aka cones).

Prairie Spikemoss forms dense mats in this soil crust. Matt Lavin photo.

Selaginella densa's 4-angled strobili rise above very short sterile stems that look like clusters of bristle-tipped leaves. cinthyadasilva photo.

Rydberg knew the plant was a spikemoss, but the dense "moss-like" form was not something he had seen before. After careful study of seven specimens, he concluded it was a new species, calling it Selaginella densa. The holotype (basis for formal description) was a specimen collected in 1889 by Valery Havard, a French-born American military physician, explorer and botanist.

Havard identified his specimen (NYBG) as S. rupestris, which is widespread in the east half of the US.

In his Catalogue Rydberg followed the accepted classification of the day. He included Selaginella densa in the Pteridophytes—spore-bearing vascular plants, mainly ferns. He put it near the end of the section, with horsetails, clubmosses and other oddballs. These were the Fern Allies. Like ferns they bore spores, had vascular tissue, and reproduced via two distinct independent life stages. But otherwise they were decidedly unfernlike, and very different from each other. Just look below!

Horsetails and scouring rushes, genus Equisteum, have jointed stems with cylindrical sheaths tipped with teeth. These are thought to be highly modified leaves. Spores are born in terminal cones.

Unbranched species of Equisetum are called scouring rushes. Andre Zharkikh photo.

Whisk ferns, genus Psilotum, have linear shoots that fork in the upper half. Minuscule scale-like leaves subtend globose spore containers 2–3 mm across.

Sideways view of a whisk fern. Mary Keim photo.

Quillworts, genus Isoetes, are aquatic, with grass-like clusters of linear leaves. Spores are born in sac-like structures in enlarged leaf bases.

Bolander's Quillworts in a lake in the Wasatch Mountains, Utah. Andrey Zharkikh photo.

Clubmosses, family Lycopodiacee, are a more diverse group, with 7 genera and 27 species in North America. Some are suggestive of spikemosses; in fact spikemosses were put in the genus Lycopodium by Linnaeus.

These clubmosses, all formerly genus Lycopodium, are now 4 separate genera; from Ferns and Evergreens of New England, 1895 (BHL).

The Fern Allies group came into use in the early 1800s, as a catchall for diverse, puzzling, somewhat fernlike plants. But after about a century botanical experts began to object. Some Allies appeared to be more closely related to ferns, others not so much. Then less than a century later, the Allies got caught up in a revolution. Biologists were switching to a phylogenetic approach to classification. In a nutshell (a very tiny one), they now hope to classify organisms based on evolutionary relationships, i.e., so that all members of a group share a common ancestor. The Fern Allies do not, so they were reclassified (3).

The commonly accepted classification splits the Allies into two groups that diverged long ago, early in the evolution of vascular plants. One includes ferns, horsetails, whisk ferns and seed plants. The other group is much smaller, a collection of relatively primitive plants: quillworts, clubmosses and spikemosses. These are lycophytes (answer to question at top of post). For a longer summary, see The Ferns and their Allies at Cliffnotes. For a deep discussion, start with Pteridophyte taxonomy on Wikipedia.

Fern and lycophyte classification from the Pteridophyte Phylogeny Group. Black labels added, not sure how that guy in the corner snuck in.

Notes

(1) Not all spikemosses are as humble and mosslike as ours. Selaginella is a large genus with c. 800 species, mainly of the tropics and subtropics. In hospitable habitat, spikemosses can be quite showy—some are iridescent!

Selaginella uncinata, Blue Spikemoss, is native to moist shady sites in southern Chile and is widely cultivated; leaves are 3–4 mm long (Flora North America). GKA Dickson photo.

(2) Actually Linnaeus couldn't decide whether "fern dust" was pollen or seeds. The concept of spores would come later.

(3) It's been really hard to give up Fern Allies! It's such a handy label for those diverse kinda-fernlike species. Not surprisingly, the name hasn't gone away. Sometimes it appears under an alias, for example "Fern Relatives" in Ferns of Northeastern and Central North America (2005). More often it pops up in casual conversation, or is used by older botanists who haven't bothered to learn the new scheme. After resorting to "Fern Allies" in a message to pteridologist Robbin Moran, I committed to learning it (Robbin is much too kind to disapprove directly, but he did refer to "lycophytes" in his reply).

Sources (in addition to links in post)

Cobb, B, Farnsworth, E, Lowe, C. 2005. Ferns of Northeastern and Central North America. 2nd ed. Peterson Field Guide Series.

Linné, Cv. 1754. Species plantarum v2. BHL

Moran, Robbin. 2004. A Natural History of Ferns. Timber Press.

Rydberg, PA. 1900. Catalogue of the Flora of Montana and the Yellowstone National Park. Memoirs of the New York Botanical Garden. BHL

The Monthly Fern: Water Clover & its odd spores

"It is worth clarifying that these plants are not clovers." (Photo by Bill Dodd).

For July, South Dakota's fern-of-the-month is the Hairy Water Clover, Marsilea vestita. Among our ferns it's quite the oddball—in habitat, behavior, leaves, and especially the spores. They suggest that one of its ancestors was the evolutionary start of seed plants!

Water Clovers grow nearly worldwide. On the order of 45 to 65 species are recognized (experts disagree on number), of which 5 are native to North America. M. vestita, the Hairy Water Clover, is the only species in South Dakota (so far). It's known from many sites across the state, in shallow water and on mud. Plants are rooted (not free-floating) and tolerate seasonably dry conditions, for example persisting after a pond has dried up. In fact, wet followed by dry can aid in reproduction and dispersal (Montana Field Guide).

Marsilea vestita is rhizomatous and colonial; in wet habitat, leaflets usually are horizontal (MWI).

M. vestita in a field; in drier habitat, leaflets often are ascending (Mary Ellen (Mel) Harte photo).

Hairy Water Clover can grow as tall as 20 cm on moist soil, or to 40 cm in water (in order to reach the surface). Leaves are dimorphic—sterile and fertile—but neither is fern-like. Sterile leaves have blades with four rounded triangular leaflets, and look a lot like four-leaf clovers! No other plant can be confused with Marsilea (1). 

Sterile leaves of Hairy Water Clover have divided blades on long slender stalks (MWI).

The sterile leaves also are unusual in behavior. Water Clovers are the only ferns known to be nyctinastic—moving with the onset of darkness. During the day leaf segments are nearly horizontal. Then as the sun sets they bend upward, forming a packet of sorts (Montana Field Guide has an account of this and other interesting features of Water Clover).

Fertile "leaves" are located near the base of sterile leaf stalks. In shape and size they resemble beans or peppercorns (source of another common name, Pepperwort). Being unusual they of course have a special name—sporocarp (= spore body); oldtimers like Linnaeus called them capsules.

Marsilea vestita. The hairy sporocarps contain 2 kinds of spores (lower right). Britton & Brown 1913.

Young sporocarps are greenish, hairy and slightly soft. With maturity they dry out, darken and become very hard. In this state they can survive for many years; the record is said to be 100. The Marsilea sporocarp is an effective unit of dispersal, often by way of waterfowl digestive tracts.

Like almost all ferns, Water Clovers have sori—clusters of sporangia which contain the spores. But the arrangement is quite different. In a typical fern, sori are located on the underside of leaves. In Marsilea, they're neatly arranged inside the sporocarp.

A typical fern; sori are clusters of sporangia, which contain dust-sized spores (2). USDA Forest Service.

Marsilea sporocarp with sori; below it, a sorus with sporangia, which release spores (no source given).

A cross-section through a Marsilea sporocarp (above) reveals an orderly but complicated interior. Inside the container-like sori are 2 kinds of sporangia. This is where things get exciting. They produce 2 kinds of spores—male and female!

Most ferns, 99% in fact, release a single type of spore—tiny, 1-celled, asexual. But not Water Clovers. They're among the 60 fern species (out of c. 10,500 total) with male microspores and female megaspores. These are heterosporous ferns; interestingly, all are aquatic (more here).

When a Marsilea microspore bursts open, many sperm (aka spermatozoids) swim off in search of an egg to fertilize—not unlike sperm of typical ferns. It's the megaspores that are so unusual. Not only are they 10 times the size of a typical fern spore, they're complex, with specialized parts.

Marsilea megaspore, c. 0.8 mm long, with 2 cells (no source given).

Shortly after leaving its sporangium, the megaspore divides to become two joined cells. The upper cell will produce an egg, which gives off chemicals to attract sperm. If a sperm successfully wriggles through the opening and reaches the egg, fertilization takes place, leading to development of an embryo and then a baby fern.

In comparison, the megaspore's basal cell is huge, and rich in carbohydrates and fats. These will sustain the developing fernling in its first days, before it can photosynthesize. In this way, a megaspore is like a seed, which supplies nutrients for its young seedling. Perhaps an ancestor of Marsilea was the evolutionary beginning of seed plants (more here).

Some readers may be wondering where the gametophytes are—those tiny independent plantlets that are the sexual stage of ferns. Good question! Water Clovers do have gametophytes, but they are minute and NOT independent (another similarity to seed plants). For more about the life cycle of heterosporous ferns, see (3) in Notes.

Simplified?

Now we finally arrive at the long-promised answer to the burning question, "How many spores would fit in a typical [empty] can of soda?"

Fern spores are truly tiny. A handful looks like a pile of dust. To show just how small they are, Robbin Moran (2021) calculated the number of average-sized spores that would fit in a typical can of soda, which has a capacity of 355 milliliters. For spore volume, he used 125,000 µm3, assuming for simplicity that a spore is a cube. What do you think? How about a ballpark estimate?

Hmmm ... 777,000?

Maybe 10 million?

According to Robbin's calculations the answer is 4,440,000,000 (4.4 billion). Yikes, that's a lot! Yes, spores are tiny indeed (4).

Notes

(1) While Water Clovers are easily recognized, distinguishing the Hairy Water Clover from others in the genus is not easy, requiring sporocarps. If you intend to document an occurrence, be sure to collect both types of leaves. See Marsilea in Flora of North America for a species key and descriptions.

(2) It seems sporangia are readily mistaken for spores, as a search for images of "fern spores" suggests. Many of the images actually are sporangia clustered in sori.

(3) In the previous Monthly Fern, I made a big deal out of the 2-stage life cycle of ferns, which involves separate tiny green sexual plantlets—gametophytes—that give birth to baby ferns. Heterosporous ferns have gametophytes, but they are minute and not independent. They develop inside the persistent spore wall, where they give rise to either sperm-producing antheridia or egg-producing archegonia (gametophytes of typical ferns usually have both). For the female gametophyte, developing inside the spore wall provides additional protection for the embryo, but there's no access to sunlight to photosynthesize food for the fernling. That's why the nutrient-rich basal cell is so important.

Life cycle of Marsilea, a heterosporous fern (labels added). See Milne Publishing for details.

(4) I couldn't find Robbin Moran's article online. If you'd like to read more about fern spores, and all of Robbin's calculations and conclusions (e.g. 4.44 billion spores taken together has a surface area nearly equal to 8.5 ping-pong tables), send me an email address and I'll send you a PDF file.

Sources (in addition to links in post)

Hooker, WJ, and Greville, RK. 1831. Figures and descriptions of ferns, principally of such as have been altogether unnoticed by botanists, or as have not yet been correctly figured. Vol. 2. BHL

Milne Publishing. Marsilea. Accessed June 2025.

Montana Field Guide. Montana Natural Heritage Program. Hairy Water Fern—Marsilea vestita.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Moran, R. 2021. Fern Spores, Soda Cans, and Ping-Pong Tables. Fiddlehead Forum (May–Dec).

Pinson, J. About Ferns, American Fern Society.

PremaBotany (Prema Iswary). December 2018. Marsilea.

Monthly Fern: polypodies & fern sex (or do they?)

Left, Polypodium saximontanum, leaves to 25 cm long (Matt Berger); right, P. virginianum, leaves to 40 cm long.

For May, the South Dakota "Monthly Fern" series features polypodies—Polypodium saximontanum and P. virginianum. The genus name comes from the Greek "poly" meaning many and "podion" meaning little foot, referring to the bumps (old leaf bases) on the creeping stems. Polypodies occur worldwide, but are more common in the Northern Hemisphere. About 100 species are recognized, with 11 in North America. They grow mostly on rock (source).

Creeping stem (rhizome) of Common Polypody; bumps on right are old leaf bases. © 2007 Robbin Moran.

South Dakota's two polypodies have a notable distribution—one on each side of the state. P. saximontanum, Rocky Mountain Polypody, grows on granite outcrops in the Black Hills, in the far west. P. virginianum, Common Polypody, is restricted to a small area of Sioux quartzite, in the far east very close to Minnesota (where it's common). It's a good thing they live 350 miles far apart. They're very similar and would be difficult to distinguish if their ranges overlapped.

South Dakota polypodies: Rocky Mountain Polypody in Black Hills (green dot); Common Polypody in Minnehaha County (pink dot); SEINet search May 2025.

Rocky Mountain Polypody on granite, Black Hills, SD (JD McCoy).

Common Polypody on Sioux quartzite, Palisades State Park, SD.

Both species have evergreen leathery deeply-lobed leaves with straw-colored stems. Common Polypody leaves tend to be longer and wider than those of Rocky Mountain Polypody (see first photo). No other clear differences in vegetative characters were found in published descriptions.

As for reproductive structures—which are critical for fern id as we've been told repeatedly—the South Dakota polypodies again are very similar. Both have round sori (spore clusters) arranged in two rows on the underside of leaf lobes; indusia (covers) are absent. The spores are yellow, so much so that even though they're housed in brownish sporangia, they give the sori a yellowish cast.

Common Polypody, P. virginianum (MWI).

Rocky Mountain Polypody, P. saximontanum (Kelly Fuerstenberg).

It is possible to distinguish Common and Rocky Mountain polypodies based on their sori, but it isn't easy. Both have sporangiasters—tiny transparent jelly-like blobs separating the sporangia (1). In P. virginianum, most sporangiasters have gland-tipped hairs, while in P. saximontana, gland-tipped hairs are few or absent. Be forewarned—sporangiasters are said to be so small that one needs macro photos or a good hand lens to examine them.

Polypodium virginianum sori, with sporangiasters with gland-tipped hairs. © 2007 Robbin Moran (arrows added).

Sporangiasters with and without glandular hairs, Polypodium amorphum; very helpful photo by James Thomas.

While we're on the subject of "reproductive" structures, let's address a common misconception (fern buffs excepted). Strictly speaking sori, sporangia and spores are not reproductive structures, for ferns cannot reproduce themselves directly. Instead, their spores give rise to plants quite unlike the parent fern.

This leads us to the fern life cycle, the so-called bugbear of beginning botany students. But we can dispense with complicated details and off-putting terms and still understand and appreciate the curious life of ferns. They and their relatives the lycophytes (formerly "fern allies") are the only land plants that exist as two different free-living beings—kinda like a butterfly and its caterpillar (2).

I find it helpful to first think about flowering plants (angiosperms) with their familiar sex organs. Flowers have eggs in ovaries and sperm in pollen. When a cell of each joins in fertilization, the result is a seed. If conditions are right, the seed germinates and grows into a plant like its parent.

Angiosperm life cycle; note the single free-living being—the plant (modified from source).

But ferns are different. Instead of seeds, they produce millions of tiny asexual spores. If conditions are right, a spore germinates and grows into a minute green plant very different from its parent, even though they have the same DNA. I find this so cool to think about! Unfortunately there seems to be no user-friendly term for these little beings, only "gametophyte" or "prothallus".

Gametophyte of Polypodium vulgare (light microscope at x4 magnification); Viséan.

John Lindsay, a British surgeon working in Jamaica, was the first to describe fern gametophytes (1794), though he didn't call them that and didn't fully understand what they were. Hoping to figure out how ferns reproduce, he had sprinkled "dust" from a fern leaf (today's spores) on dirt in a flowerpot. "I placed the pot in a window of my room, watered it daily, and every day or two examined a small portion of the [dirt] by the microscope ... but observed no alteration till about about the 12th day after sowing." At that point the soil began to turn green "as if it were covered with some small moss".

Lindsay made a nice drawing showing the stages of fern development he observed (full illustration here).

Excerpt from Lindsay's drawing: top, germination; lower right, tiny scales; lower left, first fern leaves.

With the microscope Lindsay could see particles of dust germinating—"pushing out their little germ, like a small protuberance, the rudiment of the new fern" (8–11 above). After a few more weeks the "moss" had grown enough to be visible to the naked eye, looking like small scales (13). These grew to be roundish and bilobate, similar to liverworts (14). Finally a tiny leaf emerged from the scale (15), followed by larger ones (16) until there was a fern like the one that produced the dust. Understandably, Lindsay concluded the dust was fern seed.

It wasn't until the 1840s that botanists finally got rid of fern seed. It had become obvious that despite their alluring beauty, ferns are not sexual creatures. That honor belongs to their gametophytes.

A typical gametophyte, with antheridia and archegonia (source).

On the underside of a gametophyte are little bumps that come alive in the presence of water. Some release wriggling spiral filaments that swim away. Others open to receive a spiral filament if one happens by. These are sex organs: male antheridia release wriggling sperm, and female archegonia each contain an egg. If a sperm wriggles down the neck of an archegonium, it arrives at a large cell—an egg. Fertilization produces a zygote, which develops into a baby fern growing out of the gametophyte (15 in Lindsay's drawing above). If conditions are right, it will become a full-sized fern, thereby completing a life cycle.

Here's the life cycle of a fern, emphasizing the two independent free-living beings that make it so cool! (3)

Fern life cycle—green fern (aka sporophyte) and brown gametophyte (Sigel et al. 2018, much modified).

Once again I'm ending a post without addressing a promised topic. With ferns, it's too easy to go down a rabbit hole! So I will do another Monthly Fern for June—about "the burning question" of how many fern spores fit in a Coke can. What's your guess? Here's a hint: a typical soda can holds 0.355 liters (1.5 cups). And here's an average-sized spore:

Polypodium virginianum spore. Copyright © 2007 by Robbin Moran.

Notes

(1) Sporangiasters may help keep sporangia from drying out prematurely (source). Moran (2017) notes that "In immature sori they form a continuous, protective covering over the young sporangia, thus acting like an indusium."

(2) In thinking about ferns and gametophytes, butterflies and their caterpillars came to mind. In both cases, two forms are produced from the same DNA by using different genes. This is dramatic in butterflies and caterpillars, but not so much in ferns and their gametophytes. In fact, Sigel et al. (2018) found a nearly 90% overlap in genes expressed in Polypodium amorphum ferns and gametophytes. And there's an even bigger difference. Butterflies and caterpillars are both diploid (two sets of chromosomes); there is no independent haploid form that produces gametes—no equivalent of the fern gametophyte. So my comparison of butterflies and ferns was a stretch.

(3) Strictly speaking all land plants alternate between sporophyte and gametophyte life stages (spore- and gamete-producing). But only in ferns and lycophytes are both stages free‐living beings. In seed plants only the sporophyte is free-living; in mosses and liverworts, only the gametophyte is free-living. More here.

Sources in addition to links in post

Lindsay,  John. 1794. Account of the Germination and Raising of Ferns from the Seed. Trans. Linn. Soc, London 2:93–100. BHL.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Moran, RC. 2017. Division Polypodiopsida, Ferns in New Manual of Vascular Plants of Northeastern United States and Adjacent Canada. NYBG Press Digital Content (not available as of June 2025, pers. comm.)

Rothfels, C. 2022. Fiddleheads: Fern life cycles and identification. Online workshop for Jepson Herbarium (videos).

Sigel, EM, et al. 2018. Overlapping patterns of gene expression between gametophyte and sporophyte phases in the fern Polypodium amorphum. Front. Plant Sci. 9:1450. FREE

USDA Forest Service. Fern Reproduction.

The Monthly Fern: Bracken—dreadful or delightful?

"Shelter" by Colin.

Come my sweet and let us lie in

Some idyllic wooded glade

And let us stay til merry-made

Amid the Bracken.

For April, the South Dakota fern-a-month series features the world's most widespread fern—Pteridium aquilinum, Bracken (aka Pasture Brake, Eagle Fern, Helecho Macho, and more). Thought to be native to the Northern Hemisphere, it's now widely naturalized and known from all continents except Antarctica. In South Dakota, it grows in the Black Hills in the western part of the state (1).

Bracken is said to thrive in a variety of habitats—woodlands, fields, old pastures, thickets, disturbed soils, burned areas, and marshes. But sources vary on this. For example, some say it's intolerant of wet soil and shade; others say it can grow well in all but very alkaline soils. In any case, Bracken forms extensive colonies of robust plants to 1.5 m or more tall, from deep rhizomes to 20 feet long. The large triangular leaves (fronds) are twice or thrice pinnately compound (2- or 3-times divided into leaflets). When fully grown, the blades often bend to horizontal, shading much of the ground (source).

Leaf division is an important character in fern identification, but can be hard to understand and explain. However I will try. Below is a thrice pinnately compound Bracken leaf. It's divided into 9 large segments (one terminal), which are divided into many narrow segments, which are divided (or nearly so) into small ultimate segments. "Pinnately compound" means segments line up on each side of an axis (rachis). More here.

Bracken frond by Olegivvit (labels added).

As in most ferns, Bracken's spores are borne on the underside of leaves in clusters called sori (remember?). In Bracken, sori are continuous along leaflet margins. In youth, the sori are covered; with maturity, leaflet margins unroll, exposing mature sporangia (spore shooters).

In Bracken, young sori are protected under rolled leaflet margins, sometimes with tiny membranous indusia (flaps, click image to view); Zharkikh photo.

In this frond, leaflet margins have unrolled and lines of brown sori are visible; Zharkikh photo.

Brown "beads" are sporangia; each contains many minuscule spores ready to be "shot"; Zharkikh photo.

My first encounter with Bracken was in the Bear Lodge Mountains in the northwest Black Hills. That was at least 40 years ago, but the memory remains vivid. In a stand of tall quaking aspen, Bracken's horizontal fronds formed a lovely lacy ground cover. It was an idyllic setting, and still comes to mind when I think about Pteridium aquilinum.

That memory prompted me to search for a bit of English poetry about Bracken (it has close ties to moorlands). But it was rarely mentioned, and never in an idyllic setting (2). Perhaps poets know of Bracken's reputation. It's not a particularly nice fern, and there are many reasons to dislike it. The horrors that follow were provided by Robbin Moran, a man who who loves ferns!

Bracken grove with conifers and ferns and little else; Charlesblack photo.

The common objection to Bracken is that it's weedy—in fact a noxious invader. With its vigorous growth and colonial habit, it swamps (with litter) or shades out other species. Eradication is difficult due to its deep extensive rhizomes.

Other dangers lie hidden. Bracken is filled with nasty stuff, ostensibly for defense against insects and other herbivores. It contains at least two kinds of insect hormones, which cause uncontrolled molting and death in any insect that eats it. It also contains thiaminase, an enzyme that breaks down vitamin B1. This makes Bracken hazardous to livestock, which often find it palatable. Overconsumption will cause thiaminase-induced staggers (treatable with vitamin B1, thiamine).

And there's more. Bracken is rich in tannins and therefore bitter-tasting, which is good. For if consumed, for example in the absence of other forage, tannins inhibit enzymes critical to cellular metabolism. Bracken also produces a deadly chemical weapon, hydrogen cyanide, in response to tearing of leaf tissue, thereby deterring or killing the perpetrator.

Gosari, a popular Korean dish of Bracken fiddleheads; Hyeon-Jeong Suk photo.

Bracken fiddleheads (young shoots, also called croziers); Phil Gayton photo.

After reading of Bracken's many hazards, I was surprised to learn that people happily eat its fiddleheads. But this is dangerous too. Though cooking removes tannins and thiaminase, carcinogens remain, and increased rates of stomach and esophageal cancers have been reported where fiddleheads are popular, for example in Japan, Korea, and Britain.

The main carcinogen is the compound ptaquiloside (3). It occurs throughout Bracken plants, but is highly concentrated in young growth, in spring and early summer. Humans take up ptaquiloside mainly by eating fiddleheads, but there are other sources—airborne spores, milk and meat from affected animals, and contaminated ground and surface water where Bracken grows (source).

Now that I know of Bracken's nefarious ways, do I feel foolish about my early love affair? No, for it also offers delights, as the Radnoshire Wildlife Trust in Wales explains: "Bracken can be an important habitat in its own right.  It supports over 40 species of invertebrate, forming an important part of the diet for 27 of these while 11 are found only on bracken. It is an important breeding habitat for moorland birds ... and reptiles and mammals benefit from its shelter." It's also a great candidate for areas in reserves and gardens where nothing else will grow. Just keep an eye on it!

Finally, Bracken is beautiful, in fact so beautiful that its lovely lacy fronds make a woodland irresistible. Even Robbin Moran agrees. "The grove seems so peaceful and idyllic" he writes, though he knows that in the shadows there lurks a femme fatale.

"Afternoon light" by Colin.

PS Last month I promised to include the fern life cycle in April's Monthly Fern post. But being overwhelmed by Bracken's dark side, I've postponed "the bugbear of botany students" until May. You're off the hook for now! But this also means you must wait for an answer to the burning question: "How many average-sized fern spores does it take to fill a can of Coke?" Stay tuned.

Notes

(1) I'm surprised Bracken hasn't been reported from eastern deciduous forests in the far eastern counties of South Dakota. Looks like good habitat to me, and it grows in Minnesota not all that far away.

(2) In the absence of suitable poetry, I wrote my own (I promise I won't do it again). I did find a poet named Bracken, as well as Bracken, a literary magazine: "Bracken is green and lush, coarse and delicate, drinks from the earth, and spreads underground, more root than frond. Bracken is understory, invades, takes over, shades and protects. We seek poetry and art that will root, tender and tough, in us."

(3) At least one of Bracken's insect hormones also is a carcinogen (source).

Sources

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Royal Horticultural Society. Advice—Bracken.

Stone, J. A successful fern, or a case for control. Radnorshire Wildlife Trust (blog).

The Monthly Fern: Northern Holly Fern, a spore shooter

This month's South Dakota fern is Polystichum lonchitis, Northern Holly Fern (Andre Zharkikh).

It was the first paragraph on the first page of Robbin Moran's Natural History of Ferns that hooked me—specifically the words of a thief in Shakespeare's Henry IV:

"We steal as in a castle, cock-sure; we have the receipt of fern-seed, we walk invisible."

So intriguing! Or maybe not. Maybe you doubt that fern-seed can bestow invisibility (1). But think about it ... have you ever seen fern seeds? 

In Shakespeare's time no one had seen fern seeds because they were invisible. In our time we haven't seen fern seeds because they don't exist. And for those who answered "yes" to the question above—spores are not seeds (2).

Northern Holly Fern, well-armed with clusters of sporangia, aka spore shooters (Andre Zharkikh, bar added).

Northern Holly Fern is widespread in the Northern Hemisphere, but often uncommon where it occurs. It's considered an arctic-alpine, boreal, and montane species, which may seem odd for a South Dakota plant. However it grows specifically in the Black Hills (3), which are famous for plants seemingly out of place. Northern Holly Fern is likely a relic of the last Ice Age, when western South Dakota was much cooler (but not glaciated).

Circumboreal Polystichum lonchitis (Cremastra); Black Hills added (location approximate).

Like most ferns, Polystichum lonchitis has little clusters on the underside of its leaves. These are sori (singular sorus), and deep inside are spores. Shape and position of sori are helpful in identification. Often each sorus has a little cover, the indusium, and its shape (or absence) also is useful in identification. Sori of holly ferns are round, lined up between the leaflet midrib and edges, and have peltate (umbrella-like) indusia—tiny round membranous covers on short stalks.

Northern Holly Fern's round sori with peltate indusia (Andre Zharkikh).

Now a short tour of sori diversity, providing a glimpse the level of detail needed for identification. It's handy to have a 10X magnifier.

Left: Maidenhair Fern, Adiantum pedatum, has linear sori partly covered by rolled leaf edges (false indusia). Right: Bracken, Pteridium aquilinum, has continuous sori with indusia hidden under rolled leaf edges (MWI & MWI).

Top: young Fragile Fern, Cystopteris fragilis, with round sori; indusia will wither as spores mature. Bottom: Wood Fern, Dryopteris carthusiana, with round sori covered by kidney-shaped indusia (MWI & MWI).

Common Polypody, Polypodium virginianum, has plump yellow sori with no indusia (MWI).

However we still haven't seen any spores. The sori's brown or yellow dots are not spores, they are sporangia. Inside the sporangia, finally, are the spores—in abundance!

 

Fern leaf with sori containing sporangia containing spores (and some scattered about); USDA Forest Service.

Fern spores are tiny, usually 30–50 micrometers across (1 µ = 0.001 mm), which is narrower than a human hair (source). Or think of it this way—a fern frond just 60 cm long will produce something like 7,000,000 spores! (source) As fine as dust, spores can fly in the lightest breeze, especially if the sporangium kicks them out of the house.

Holly Fern spores, scale bar = 10 µ. © Robbin Moran 2012.

In some ferns, the sporangia simply split open and let their spores fall to the ground. Last month's Sensitive Fern is an example. But in Holly Ferns, in fact in many ferns, sporangia hurl their spores. I like to call them spore shooters, as does Robbin Moran. Others call them launchers or catapults. In any case, their spores can reach speeds of 10 m/sec! (Llorens et al. 2015)

The diagram below shows a spore-shooting sporangium in action. The ring of blue and red cells is the annulus; the cells are filled with water. As the outside dries the annulus curves back, opening the sporangium and cocking the catapult or shooter. Elastic pressure increases until the annulus suddenly collapses, sending spores flying. They get quite a boost in speed, but probably just as important, they're dislodged from the interior of the sporangium. Apparently my "house" analogy above was appropriate. These clingy spores need encouragement to go out into the world.

Modified from Llorens et al. 2015.

Some readers may still be bothered by my earlier statement that spores are NOT seeds. After all, won't a spore give rise to a full-sized fern, like the pea that becomes a pea plant? Well ... yes, but not directly. A true seed has a full set of chromosomes (diploid), and can give birth to a seedling. But a spore has only half a set (haploid). Before a fernling can arise, fertilization must take place.

This leads us to the fern life cycle, which even pteridomaniacs call "the bugbear of botany students". Maybe "fern sex" would be less scary. In any case, stay tuned. I will squeeze it in amidst the beauty and intrigue of the next Monthly Fern.

Northern Holly Fern. Thanks to Andre Zharkikh, who kindly shares his many plant photos "to show other people the beauty of nature".

Notes

(1) To provide invisibility, fern seed must be collected precisely at midnight on Midsummer's Night Eve, while it's falling from the plant.

(2) If you thought spores were seeds, you're in venerable company. Carl Linnaeus, father of modern taxonomy, did also. He first thought the fine dust shed by ferns was pollen. But he then admitted he knew too little about primitive plants to conclude "whether what I see is seed, or dust of the anthers." However years later, in 1751, he announced that the dust was fern seed (Moran 2004).

(3) Flora North America and USDA Plants, sources usually considered reliable, do not include South Dakota in the known range of Polystichum lonchitis (the latter shows it as reported but without documentation). It was first documented in the Black Hills in 1977, and has been found elsewhere in the northern Hills since (see map from SEINet herbarium search, zoom in to South Dakota).

Sources, in addition to links in post

Llorens, C, et al. 2015. The fern cavitation catapult: mechanism and design principles. J. R. Soc. Interface 13: 20150930.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Pinson, Jerald. About Ferns. American Fern Society.

Summers, A. 2005 (Dec). Spore Launchers; Ferns and fungi that explosively reproduce. Natural History.

USDA Forest Service. Ferns. Helpful information, photos, and diagrams for aspiring pteridomaniacs. And there are coloring pages! (have a range of greens ready)