Royal Fern Family (Osmundaceae)

The Royal fern (Osmunda regalis) is bipinnate, or twice-divided which means that the pinna that extend from the central stem or rachis are divided into individual pinnules. Royal ferns can grow to the size of small trees.

Common Name: Royal Fern Family – Named for its most prominent member. The Royal Fern is relatively large, up to ten feet tall, evoking the grandeur that royalty implies. In addition to the royal fern, there are two other prominent species in the family: Cinnamon Fern, named for the light brown color of its fertile frond; and Interrupted Fern, named for the fertile fronds that “interrupt” the branches or pinna along the stem or rachis.

Scientific Name: Osmundaceae – The etymology of osmund (aceae is the  standard suffix to indicate family in taxonomy, ‘belonging to’ is a synonym) is not established. Several references cite  Osmunder as the Saxon name for the Norse god Thor, which would suggest a similar rationale as god-like or royal. However, the Saxon name for Thor is Thunor or Thonar [1] which is the origin of Saxon-English Thursday (vice Osmunday).  Other suggestions include os + mund as god + protector in Saxon or os + mundus as bone + clean in Latin. A more imaginative account comes from Harpers Magazine for Young People published on 9 December 1879 that relates the story of Osmund of Loch Tyne who hid his wife and daughter from marauding Danes in medieval Great Britain on an island covered with the large royal ferns.  His daughter who was thus saved commemorated the event by naming the ferns after her father. Referring to this story as a “fanciful tale,” the writer proffers a second option  as  “derived from an old Saxon word signifying strength, the specific name indicating its royal or stately habit of growth.”

Potpourri: Fern-like plants were among the original land biota, appearing with liverworts and mosses in the Devonian Period some 390 million years ago. They were the first vascular plants in having vessels or ducts to carry water and minerals upward and sustaining sugars downward to extend and expand photosynthetic tissues toward solar energy, unlike their ground hugging cohorts. They reached prominence in number and size during the succeeding Carboniferous Period, sometimes called the “Age of Ferns,” forming the coal beds that powered the Industrial Revolution of the 19th Century that created the carbon dioxide problems of the present (ferns are not at fault). While the primeval ferns appear only in the fossil record, their ancestors are evident to this day wherever the combination of wetness and sunlight meet the necessary and sufficient ecological requirements for their growth. [2] The osmundaceous ferns are among the oldest of those that still prevail, their longevity a result of robust physiology and adaptability, even to meteoric cataclysms like the Cretaceous – Paleogene extinction that extirpated all dinosaurs except birds. The species named royal fern, Osmunda regalis, is the only vascular plant to have  been found on all seven continents.

The royal fern family is the sole member of the order Osmundales, which is the oldest and smallest of the seven orders in the subclass Polypodiidae, which includes most of the 10,560 species of extant ferns. They are also called leptosporangiates on account of their characteristic  one cell thick spore cases, which differ from other ferns and seed plants which have multiple cell layers (lepto means ‘peeled’ in Greek and is used to connote weak or thin i.e. easy to be peeled). For comparison, as of 2016 there were 374,000 species of plants comprised of 295,383 flowering angiosperms, 44,000 algae, and 12,700 mosses, the ferns and several other groups like conifer gymnosperms ― about 500 new plant species are being added annually. [3] Osmundales is the smallest fern order since there are only 20 species in 6 genera, which would equate to one fifth of one percent of all ferns. Paradoxically, they are at the same time the most common fern fossil, with over 100 species in 20 genera that date to the early Mesozoic about 220 million years ago, which makes them the oldest. [4]  The paradox of becoming small in number (or stature) and old in age over time is something that every living thing must grapple with, both in the lifetime of one individual and in the span of a species.

The anatomy of a fossilized fern specimen recently unearthed provides some rationale to the conundrum of why there are so many fossil species with only a few survivors. A two inch long segment of fern stem surrounded by frond bases and rootlets which was removed from 220 million year old mafic volcanoclastic rocks in Sweden. Due to its apparent submergence in hydrothermal brines and consequent rapid permineralization by calcite, the preservation was complete to the subcellular level of the nucleus and even its entrained nucleoli. The detail that this uniquely preserved fossil provided allowed for thorough characterization. The physical measurements of stem size, nuclear parameters, DNA and chromosome count were essentially identical to those of a living royal fern family member. The incontrovertible conclusion was that the genome had been static for over 180 million years. [5] Why? One reason is that the royal ferns, both ancestral and recent, have a tough, tree-like constitution.

Ferns grew to great heights in the Carboniferous and tall tree ferns persist to the present,  primarily in tropical and subtropical regions. Deciduous and coniferous trees can grow only with the support of the cellulose and lignin of their woody trunks. Arborescent ferns  evolved two non-wood mechanisms to support their weight. One method employs a hardened tissue called sclerenchyma that extends along the length of the stem and provides support against buckling. The reinforced fern “trunks” are so hard that they cannot be cut down even with a chainsaw. A second method to support a lofty frond expanse is to surround the stem with a mantle of dense roots that provide rigidity by interlocking like rebar used in concrete. With up to five times the girth of the bare stem, the matrix is an unbending sheath. Osmundaceae is an ancient lineage, one that originally consisted  of several genera of tree ferns. The root support structure that evolved in the Mesozoic to provide for greater height (and   limit browsing by herbivores) was retained in the  family members that survived as royal, cinnamon, and interrupted ferns.  A cross section inspection of one of these three fern species requires a heavy saw, revealing a small central stem surrounded by black leaf bases, a structure that is similar to its ancient relatives. The retention of this feature may well have been the key to survival beyond the extinction at the end of the Cretaceous,  and why they never changed. [6]

The fertile fronds of the interrupted fern are along the rachis, interrupting the pinna

There are several other features that distinguish the members of the royal fern family. Most notable are the distinctive sporangia, the structures in which spores are produced. Most ferns have spores in small round structures called sori that are visible as brownish dots usually on the underside of the fronds. The royal ferns have separate structures called fertile fronds  with modified pinnae to which the sporangia are attached singly in clusters.  When mature, each sporangium opens with a long slit to allow the spores to escape, turning brown when empty before eventually disintegrating. The spores are green in color as opposed to the more common brown to black and are also relatively short-lived. Like all other ferns, the spores fall to the ground where they germinate if conditions are suitable into either male or female gametes whose sub rosa sexual union produces a new sporophyte frond. The fertile fronds take on a variety of shapes that distinguish the three main species:  Royal fertile fronds are at the end of the pinnae like crowns; Interrupted fertile fronds are intermittent along the rachis, interrupting it; and Cinnamon fertile fronds are on a separate stalk, turning cinnamon colored when  mature. [7]

The fertile fronds of the cinnamon fern are on a separate stem that turns cinnamon brown when the green spores are released.

The use of plants as herbal remedies for human ailments is not without reason, although modern science in medication is usually a better course. Phytochemicals are produced by plants through evolutionary mutation to ward off predatory animals from microbes to moose. Plant toxins that repel soil microbes can and have been used to prevent human microbes from gaining purchase, with the proviso that dose matters and poisons can kill. Those plants that have been around for millennia like the Osmundaceae are a good place to look for natural cures. One rarely sees a royal, interrupted, or cinnamon fern with the holes of leaf-cutter insects or the dark spots of fungal invaders. Native Americans put their sapience to good use in identifying curative concoctions mostly through trial, error, and observation. The six nations of the Iroquois Confederacy of the Northeast (who called themselves Haudenosaunee for the long houses they built) used all three royal family ferns for a wide variety of conditions including various “women’s problems” and joint pains. The Cherokee of the middle Atlantic region used  the cinnamon fern for snake bite and an unspecified spring tonic while the southern Seminoles used royal ferns for so-called  chronic conditions one of which was insanity. [8]

The peoples of Eurasia were no less adept at herbalism which started similarly as tribal remedies and succeeded to an organized doctrine with the advent of the written record. By the sixteenth century, a consensus emerged in the form of a number of publications listing various plants and their purported curative properties. Among them was The Herball of John Gerard who listed “Water-Fern or Osmund the water-man” as “a great triangle stalke two cubits high” which is surely Osmunda regalis, the royal fern. The root is described as  “great and thicke” with a hard woody part named “the heart of Osmond the water-man.” One must conclude from this account that the Osmund folk tale concerning the origin of the genus Osmunda has some basis in fact, however romanticized. The “Osmund” root, and especially its knotty core, was prescribed to be pounded and mixed with liquor and used for  “those that are wounded, dry-beaten and bruised.” These “wound-drinkes” would “dissolve cluttered blood in any inward part of the body.” The tender sprigs of spring were equally beneficent, especially when made into healing plasters for the “aforesaid wounds, punctures, and suchlike.” [9]  Hyperbole aside, palliative properties of royal fern have been demonstrated in the inhibition of head and neck squamous cell carcinoma, one of the most common forms of cancer. [10]

Linnaean taxonomy of the 18th century based on extrinsic characteristics has been obsolesced  by phylogeny based on DNA in the 21st century. This revolution in biology has had limited effects on animal classification … but plants and especially fungi have been mercilessly rearranged. The ferns were originally assigned to the class Cryptogamia (Greek for “hidden marriage”)  with 16 genera and 174 species which included fungi, algae and bryophytes. Since the arrangement of plant sexual organs (stamens and pistils) was the original basis for plant classification, the flowerless cryptogams were cryptic. A default fern  classification system was based on the location and shape of the spore-bearing sori on the fronds (which are absent in Osmundaceae), the first of  numerous reassessments over the next two centuries. The fact that fern spores germinate to form sexual gametes that mate in wet soil was not recognized until a British surgeon in Jamaica named Lindsay studied the development of “fern dust” he had planted in a flowerpot in 1794. The guessing game came to an end at an international symposium of pteridologists (fern botanists) in 1972 where it was settled that it  would be impossible to establish a proper taxonomy without monophyletic studies, which are really only possible by DNA comparison. [11]

The cinnamon fern (Osmundastrum cinnamomeum) is pinnate-pinnatifid, the pinnules are not fully cut to the rachis

The Pteridophyte Phylogeny Group (PPG) was established to bring DNA order out of the Linnaean farrago. Even though this seminal 2016 classification lists 11,916 species, 337 genera, 51 families, 14 orders, and 2 classes of ferns and fern allies (the lycophytes, mostly club mosses), it carries the caveat of “not intended as the final word … but rather a summary statement of current hypotheses.” This caution relates to complexity in evolutionary change in general and to the peculiarities of ferns in particular. The PPG restricted monophyletic lineage (single ancestor) only at the genus level and above because of fern polyploid speciation, which is a mutation where the number of chromosomes is increased. Humans have two sets of 23 chromosomes (46 total)  which is referred to as 2n; polyploidy would make this 3n or more. It is estimated that more than a third of all fern species are correlated to a change in ploidy. Even within the strictures of DNA analysis, the royal fern family Osmundaceae retained its coherence and relevance as the most ancient of the extant ferns. However, the royal, cinnamon and interrupted ferns, which used to all be in the genus Osmunda (and are still listed there in most field guides and in Encyclopedia Britannica) are now, tentatively in three separate genera. [12]

The interrupted fern (Claytosmunda claytoniana) is almost indistinguishable from the cinnamon fern except for the interruption.

The royal fern as Osmundaceae  type species retained its position in the genus as Osmunda regalis with several  variations like spectabilis. It is bipinnate, which means that the pinna that grow from the stem are fully divided or cut into separated pinnules; this is sometimes called two-cut or twice-divided. The cinnamon fern became Osmundastrum cinnamomeum as the only species in a new genus. Its fronds are  pinnate-pinnatifid, which means that the pinnules are only partial and not separated. This is also the case with the interrupted fern which is almost indistinguishable from the cinnamon fern without the very distinct difference in their fertile fronds, one a cinnamon brown stalk and the other interspersed along the main stem. The interrupted fern, O. claytoniana, became Claytosmunda claytoniana paying double homage to the noted colonial botanist John Clayton of Virginia. This is probably not the end of the names of the members of the royal fern family, but it is the end of this particular discussion of them


1. Encyclopedia Britannica Micropǣdia “Thor” Volume IX, William Benton publisher, University of Chicago, 1974  p.967.

2.  Wilson, C. and Loomis, W. Botany, Fourth Edition, Holt, Rinehart. and Winston, New York, 1967, pp 499-532.

3. Christenhusz, M. and Byng, J. “The number of known plants species in the world and its annual increase”. Phytotaxa. 2016  Volume 261 (3) pp 201–217.

4. Bomfleur, B. “The fossil Osmundales (Royal Ferns)—a phylogenetic network analysis, revised taxonomy, and evolutionary classification of anatomically preserved trunks and rhizomes”. PeerJ. 11 July 2017 Volume 5: e3443

5. Bomfleur, B. et al, “Fossilized Nuclei and Chromosomes Reveal 180 Million Years of Genomic Stasis of Royal Ferns,” Science, Volume 343 21 March 2014, pp 1376-1377.

6. Moran, R. A Natural History of Ferns, Timber Press, Portland, Oregon, 2004, pp 140-146.

7. Cobb, B. Farnsworth, E., and Lowe, C. Ferns of Northeastern and Central North America, Houghton Mifflin Company, Boston, 2005. pp.170-177.

8. The ethnobotany database at

9. Gerard, John, The Herball, or Generall Historie of Plantes, John Norton, Publisher, London, 1597.

10. Schmidt M, et al “The influence of Osmunda regalis root extract on head and neck cancer cell proliferation, invasion and gene expression. BMC Complementary and Alternative Medicine. 4 December 2017, Volume 17(518).

11. Christenhusz, M.; Chase, M. (2014). “Trends and concepts in fern classification”. Annals of Botany 13 February 2014, Volume 113 (4): pp 571–594.

12. Pteridophyte Phylogeny Group. “A community-derived classification for extant lycophytes and ferns”. Journal of Systematics and Evolution. November 2016 Volume 54 (6) pp 563–603.