DIVERSITY OF LIFE

DESCRIPTION OF THE PHYLUM PTERIDOPHYTA (SCHIMPER 1879)

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Pteridophyta (te-ri-DA-fa-ta) os made from two Greek roots that mean winged (pteryz -πτέρυξ); and plant (phyto -φυτό).  The reference is to the wing-like appearance of the compound leaves (fronds) that are characteristic of most ferns.  There are several names used for the ferns, among them are: Polypodiophyta, Filicinophyta, and Moniliformopses.  Haeckel (1866) introduced the name for ferns but he did not apply it to a Division (~= Phylum) level taxon (Smith 1955).  According to Smith (1955), the use of Pteridophyta as a division-level taxon was by Schimper (1879).

INTRODUCTION TO THE PTERIDOPHYTA

Ferns are quite successful plants.  They grow as perennial herbs, trees, epiphytes, and floating plants (Figures 1-34).  They have exploited almost all terrestrial and freshwater environments, and dominate in some of them.  Similarly, ferns have dominated terrestrial plant communities to varying degrees since their appearance in the Devonian.  The ferns are megaphyllous plants whose leaves (fronds) usually emerge by circinate vernation.  The leaves also usually are compound and are among the most complex leaves of any in the kingdom of green plants.  Their axes vary in complexity with steles of almost all types possible: protosteles, actinosteles, plectosteles, ectophloic siphonosteles, amphiphloic siphonosteles (solenosteles), dictyosteles, and eusteles (Figure 1).  

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The cladoxylids and coenopterids were the groups of plants, which together are called the preferns.  They showed the spectrum of steps required to form a webbed branch system that we recognize as a megaphyll.  Indeed, the terminal fertile appendage of Cladoxylon (see Figure 3) looked very much like a spore-bearing megaphyll.  The cladoxylids  were monopodial with small microphylls and spore-bearing frond-like branching systems.  Thus, they resembled the Trimerophytophyta from which they likely emerged.  All extinct, these organisms flourished during the Devonian but died out by its end.  Pearson (1995) believed that the cladoxylids gave rise to the Progymnospermophyta and, thus, to the seed plants. Stewart and Rothwell (1993) demonstrate potential affinities between the cladoxylids and all major groups now considered to be within the Pteridophyta as well as the seed ferns.  However, they end their discussion by saying, "...the Cladoxylids...can be added to our list of plant groups that represent unsuccessful evolutionary 'experiments' that ended in extinction" (Stewart and Rothwell 1993, p. 217).

Cladoxylon (Figure 3) had two types of leaf-like branching systems that were covered by microphylls.  These photosynthetic appendages were small and had open branching.  However, the fertile appendages were flattened into a single plane of dichotomously-branched axes, each of which terminated in a small sporangium.

Pseudosporochnus (Figure 4) grew to be very large and resembled present-day palms or tree ferns.  The lateral branches appeared frond-like with sterile and fertile appendages emerging as dichotomously branched systems.  Unlike Cladoxylon, the fertile appendages of Pseudosporochnus were not flattened.  These plants had a fossil history that ranged through much of the Devonian period.

Calamophyton (Figure 5) had strong monopodial growth with dichotomizing ultimate branches. The microphylls were round in cross section and spirally arranged on the stems. Sporangia occurrred on clusters of recurved stems, which bore striking resemblance to the sporangiophores of the Equisetales.  There were known from the middle Devonian.

Wattieza (Figure 6), whose stumps were know as Eospermatopteris, was one of the earliest trees and formed forests in the Gilboa, New York area during the middle Devonian.  One fossil described by Stein et al. (2007) stood at least 6 m tall.  Most notably, the lateral branching systems behaved as megaphylls in that each system seems to have abscised as a unit, rather than in pieces.

The coenopterids flourished from the Devonian to the end of the Permian when they died out.  The coenopterids were monopodial with spore-bearing frond-like branching systems that may have been the earliest true megaphylls. 

Thomas and Spicer (1987) consider the coenopterids to represent a grade of evolution.  Very likely, they are a paraphyletic group of early ferns that had the earliest megaphylls.  Not surprisingly, members of the group vary from having creeping stems to shrubs to trees.  If they are indeed paraphyletic, each of the following taxa may be a representative of a separate class. 

Stauropteris (Figure 7) was a small shrubby plant from the upper Devonian to the upper Carboniferous.  The axes had alternating pairs of frond-like branches emerging at the nodes.  Elongate sporangia occur on some of the terminal branches. At least one species is heterosporous (Stewart and Rothwell (1993).

Rhacophyton (Figure 8) had large frond-like branching appendages that emerged in a spiral pattern from slender axes.  The sterile appendages had primary pinnae in 2 ranks, each of which had small, dichotomously branching stems around the pinna. The fertile fronds were even more complex.  Some of the primary pinnae were sterile.  The fertile primary pinnae had ball-like dichotomously-branched appendages, each of which terminated in elongate sporangia.  Because the stems were so slender, they likely could not support such large fronds as an upright axis, but must have grown as creeping stems (stolons?).  Some of the stems showed evidence of secondary growth leading Stewart and Rothwell (1993) to suggest that this genus and related taxa may have been associated with the line leading to the Progymnospermophyta.  These appeared in the upper Devonian.  Related taxa persisted through the Carboniferous.

Zygopteris (Figure 9) were creeping or rhizomatous plants from the upper Devonian to the Permian. The rhizomes were covered with frond-like dichotomizing branches, essentially megaphylls, which occurred in two ranks. The stele was H-shaped in mature stems and showed evidence of secondary growth. The sporangia were at the tips or on the abaxial surface of the ultimate branches.

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These are the plants that have megaphyllous leaves.  That is, the megaphyll is a branch system that has become planar and webbed.  Despite the name, size is not an adequate diagnostic character to use in distinguishing megaphylls from microphylls.  Some taxa like Lepididodendron, a microphyllous plant, has very large leaves.  On the other hand, the scale-like megaphylls of cedars are quite small.  The principle character that distinguishes a megaphyll is a leaf-gap in the stele.  This is an opening or gap made by the stele of a branch (called a leaf trace) as it emerges from the stele of main stem (Figure 10).

The steps leading to the formation of a megaphyll are given in Figure 11.  This is a portion of the Telome Theory as proposed by Zimmermann (1952 and 1959), who proposed that all of the main plant organs can be derived from simple Rhynia-like axes called mesomes (sterile axes) and telomes (fertile axes).  Tbe derivation of megaphylls in this scenario is that the dichotomously-branching axis develops an unequal branching form (Figure 11-A) called overtopping.  The lateral branch system then becomes planar (Figure 11-B) and webbing elaborates between the axes.  Thus, a megaphyll is not a structure that evolved de novo but was assembled from existing structures.  Tomescu (2008) argures that such a sequence for megaphyll evolution must have occurred multiple times thus calling into question the homology of early megaphyllous appendages.

FIGURE 11. EVOLUTION OF A MEGAPHYLL A. Overtopping B. Planation C. Webbing Image from Bold et al. (1987)

PSILOTOPSIDA

OPHIOGLOSSALES

Botrichium (see Figure 12) and Ophioglossum are extant ferns that typically produce a single frond each year.  The small upright stem usually is underground with very short internodes.  Each leaf has a sterile pinna and a fertile pinna.  The fertile pinnae are not webbed, but have clusters of large eusporangia that are homosporous.  The sterile pinna can be highly dissected (Botrychium) or entire (Ophioglossum).  The gametophytes of these organisms resemble the carrot-like saprobic gametophytes of Lycopodium.  Although cryptic, Botrychium virginianum (Rattlesnake Fern) plants enjoy a large distributional range that includes temperate to America, Scandinavia, the Himalayas, and parts of Australia.  In addition, the Rattlesnake Fern can be among the oldest in the habitats where they occur (forest floor of rich woods or thickets with acid soils and shade).  I once saw a Botrychium with 45 leaf scars eroding out of a road bank.  That was in an area where the oldest trees were no more than 35 or 40 years old.  Other members of the genus and the class are among the rarest plants in an area.

PSILOTALES

Structurally, the psilophytes would seem to be out of place.  They grow as dichotomizing branching systems that do not have leaves or roots.  Instead, they have a prostrate rhizomatous branching system with rhizoids.  The upright stems are photosynthetic and are covered by enations or microphylls.  The sporangia occur as eusporangiate synangia at the terminus of short lateral stems (Figure 13).  The gametophyte is small, inconspicuous, and saprobic.  Also, it is monoecious, producing both antheridia and archaegonia on the same thallus.

The overall structure of the sporophyte would seem to make them remnants of the earliest radiation of vascular plants.  Such is the classical view that associates the psilophytes with the Rhyniophyta (see the figure from Pearson 1995).  However, molecular evidence (see Tudge 2000; and Pryer et al. 2001) suggests that the psilophytes are reduced ferns.  That was the intuition of Bierhorst (1971) who, based on structural evidence, saw a gradation in structure from the psilophytes to the fern families Stromatopteridaceae, Gleichineaceae, and Schizaeaceae.  Indeed, he interpreted the dichotomizing branches of Psilotum (Figure 14) as a highly reduced frond and the leafy branches of Tmesipteris (Figure 15) as modified fronds.  Modern molecular cladistic analyses show that they are sisters to Botrychium + Ophioglossum (e.g. Pryer et al. 2001).  However, morphology-based analyses (e.g. Schneider et al. 2009) suggest that they should be sisters to Equisetopsida.

	FIGURE 13. LIFE HISTORY OF PSILOTUM.  The sporangium (1-2, a eusporangiate synangium) produces  spores.  They germinate to produce inconspicuous thalloid gametophytes (4), which produce both archaegonia (5) and antheridia (6).  Antheridia release flagellated sperm which fuses with the egg to form a zygote (7).  The embryonic sporophyte (8) emerges from the archaegonium.
Image taken from: http://home.manhattan.edu/~frances.cardillo/plants/vascular/whiskfr2.html

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EQUISETOPSIDA

The horsetails or scouring rushes are distinctive in two ways: they have a stem that is jointed and ribbed and a strobilus of sporangiophores.  Although represented today by a single genus, Equisetum (Figure 16), the horsetails have a very long history and diverse representation in the fossil record.  They were especially abundant from the Devonian to the end of the Paleozoic.   A common feature of the class is the production of jointed stems (thus Bold et al. 1987, refer to this group as the Arthrophyta).  Also, branches arise from beneath the leaves rather than the more typical adaxial emergence. The stele is difficult to interpret, but stems appear to grade from siphonostelic to eustelic.  A very distinctive feature of the equisetophytes is the type of complex strobili.  Cones like those of Equisetum (Figure 16)are made of sporangiophores (modified leaves), each with multiple homosporous sporangia.  Equisetum is homosporous and its gametophytes are saprophytic, monoecious, and cryptic (see Figure 17).

Hyenia (Figure 18), a Devonian age equisetophyte, grew as a creeping rhizome from which upright photosynthetic stems emerged.  Some of the terminal branches of Hyenia are loosely-clustered sporophylls whose structures suggest the evolution of the Equisetum-like cone by reduction of internodes and reduction of the sporophylls.  

Calamites (Figure 19) grew as trees with strong monopodial growth and whorled leaves (megaphylls) at the jointed nodes.  Indeed, Calamites showed strong secondary growth. They had compound strobili with heterosporous sporangia.  Gametophytes have not been found in the large extinct forms.  These plants appeared in the upper Devonian and persisted to the Permian. Calamites was one of the dominant plants in the great Coal Age forests during the Carboniferous period.

Pseudobornia (Figure 20) were large trees (up to 20m tall) with articulating stems. The dichotomizing branches grew up to 3m long.  These plants appeared appeared to be simpler that Calamites.  They did not show evidence of secondary growth (or, if so, it was limited), and their sporangia were homosporous.  They were restricted to the Upper Devonian and may have given rise to the Calamites line.

Sphenophyllum (Figure 21) were creeping plants with prostrate stems that had solid cores and were triangular in cross-section.  Like Calamites, though, Sphenophyllum had whorls of wedge-shaped leaves.  These plants appeared in the lower Devonian and persisted through the Permian, and may have survived into the early Triassic. 

	FIGURE 17. LIFE HISTORY OF EQUISETUM. The sporophyte (1) produces a terminal strobilus of sporangiophores (2).  Spore tetrads mature with attached elater tissue (3-4).   The gametophyte (5) is inconspicuous and monoecious.  It produces small antheridia (6), and archaegonia (7).  Following syngamy (8), the embryonic sporophyte (9) emerges from the archaegonium.
Image taken from: http://home.manhattan.edu/~frances.cardillo/plants/vascular/equilc.html

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MARATTIOPSIDA

The marattiopsids are massive ferns that seem to be sisters to the equisetopsids, and have a fossil history which goes back to the Carboniferous.  Everything about them is large.  Their leaves can be up to 7.5 meters long, and their sporangia likewise are large, eusporangiate, and usually fused into large synangia.  The gametophytes are large, thallose and often perennial causing them to resemble Marchantia. The stems are supported as a palm-like tree by persistent leaf bases and exhibit secondary growth by a polycyclic dictyostele. The fleshy stems and roots often have mucilage chambers in a thick cortex.

A common genus is Angiopteris, a name that means "angel wings" (Figure 22).  The rhizomes are very large and fleshy, some are edible.  One species of Angiopteris has become an invasive plant on the island of Jamaica.

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Most of the living Ferns are assigned to the class, Polypodiopsida.  This class is, by far, the most speciose and most diverse in form of all the living fern groups.  The most fundamental synapomorphic character is the leptosporangium.  This is a particular type of fern sporangium that develops from one or two superficial cells and can have as few as 16 to 32 spores per sporangium.  They have characteristic springy, gracile stalks with a sporangium on the top.  Typically, the sporangium has cells of different thicknesses such that the sporangium dehisces suddenly via a horizontal slit and flings the spores by the combined actions of the sudden opening and the recoil of the springy stalk.  In most taxa the leptosporangia are clustered in sori and usually associated with indusia, extensions of leaf tissue that may cover or surround sori (Figure 23).

FIGURE 23. LIFE CYCLE OF PTERIDIUM A. Fertile megaphyll of the sporophyte B. Fertile pinna with sorus along the margin of the leaf C. Leptosporangia emerging from the sorus and covered by a false indusium D. Cordate gametophyte E. Archaegonium, antheridium, syngamy to produce a zygote F. Emergence of an embryonic sporophyte

Osmunda (Figure 24) and their relatives have a very complete fossil history which goes back to the Permian. The plants have a short erect stem with persistent leaf bases. The leaves are large with dichotomous venation in the pinnae. Sporangia are more massive than the typical leptosporangiate condition.  Indeed, they appear to be intermediate between a leptosporangiate condition and a eusporangiate condition.  Still, the sporangium has a unistratose wall, but it opens by a longitudinal slit (most leptosporangia open by a horizontal slit). The sporangia never occur in a sorus. The gametophyte is large (up to 5 cm long) and photosynthetic.

The filmy ferns, like Trichomanes (Figure 25), occur mainly in the southern hemisphere and in the tropics. Most are small, with very thin leaves, usually unistratose.  Furthermore, the stems are equally delicate and usually protostelic.  Sori are marginal and are surrounded by a cup-shaped indusium. The Trichomanes species that occurs in Pennsylvania lives entirely as a gametophyte on seeps and protected areas.  They are small branched filaments that reproduce only asexually as gemmae. 

Lygodium (Figure 26) is a member of the Schizaeales, an order that has a fossil history which dates from the Jurassic. Mainly, members of this order are tropical, but Lygodium occurs as far north as Pennsylvania. The sporangium has a thick stalk and an annulus which forms an apical cap (a longitudinal slit in Lygodium). Sporangia may be covered by an indusium-like flap, but the sporangia do not occur in sori. The leaves are quite variable, but usually small. However, the leaves of Lygodium remain meristematic at the tip and continue to grow as vines, more than 30 meters long for each leaf.  Stems are less significant and range from protostelic to dictyostelic. The gametophytes vary from filamentous to carrot-like.

The water ferns are all heterosporous with their gametophytes rarely exceeding the bounds of the spore wall.  This is true both of the megaspore and the microspore.  The plants differ vegetatively though they are all aquatic or semi-aquatic.  Marsilea (Figure 27)  is rhizomatous with leaves which resemble four-leaf clovers. Their rhizomes have a solenostele. At the nodes, leaves and adventitious roots emerge.  At some of the nodes, fertile leaves called sporocarps emerge.  They resemble seeds and remain closed until scarified (either through physical abrasion or through chemical degradation) at which point the gelatinous leaf emerges with its sori filled with sporangia (Figure 28).  I have seen them become particularly abundant in the depressions left by sand traps in abandoned golf courses in the central part of the US.

FIGURE 28. LIFE HISTORY OF MARSILEA The water fern, Marsilea, looks like a four-leaf clover, but circinate vernation gives it away as a fern.  It is rhizomatous from which leaves emerge at the nodes.  The the base of some of the leaves, a sporocarp (a hardened folded leaf with sporangia inside) develops (Top a&b).  The sporocarp develops as a gelatinous ring which allows the sori to emerge into the water (Bottom a&b).  These are heterosporous.  Microspores develop into multiflagellate sperm and the megaspores develop into a megagametophyte which does not exceed the bounds of the spore wall.  The daughter sporophyte grows from the zygote in the archegonium in the megaspore and appears almost like a germinating seed. Image from Ditmer (1964)

The other types of water ferns are the floating ferns.  Azolla, the mosquito fern (Figure 29), floats on the surface of the water.  It resembles small sprigs of red cedar on the water.  When the sunlight is most intense, the plants protect themselves with a red pigments that turns small ponds in the southern US red in the middle of the day.  Although they float on the water surface, they have a noticeable layer of wax on their upper surface.  This serves to reduce desiccation and to help them remain afloat by being caught in the surface tension.  Very often Azolla has a symbiotic Nostoc associated with the plant, presumably providing the plant with usable nitrogen compounds.  I have seen them grow in such densities that they effectively seal off the water surface from mosquitoes.  However, when they are that abundant, they prevent the penetration of light and the pond becomes anoxic.

Cyathea (Figure 30), a common tree fern with a fossil history which goes back to the Jurassic, can be a dominant plant in some tropical forests, particularly the mountain forests.  One such dominant can be seen in El Yunque, the montane rainforest of Puerto Rico.  Cyathea arborea is a robust member of the forest understory and even forms the canopy on steep areas of the mountain.  The trunk is an upright rhizome that can grow 12 o more meters high with a tuft of large leaves at its growing tip.  Thus, from a distance, they resemble palms.  However, the large fiddleheads emerging from the crown label them for what they are.  The sori are rounded on veins and are sheathed by a globose indusium. Dehiscence of the sporangia occurs by a transverse slit, and the gametophyte is thalloid with a midrib.

Dennstaedtia (Figure 31), the hay-scented fern, is common on the edges of woods in the northeastern US.  They grow from vigorous rhizomes that can dive many cm deep into the soil and shoot quickly into clearings.  They share these characteristics with their relatives, Pteridium, the bracken ferns.  They also produce allelopathic compounds that tend to discourage the growth of seed plants.  Thus, they can, when established, have a major impact on the regrowth of a forest.  Some of them are of some economic importance because they are poisonous to sheep and cattle.

Adiantum (Figure 32), the maiden hair fern, is one of the most beautiful ferns.  They grow from a creeping rhizome with distinctive thrice cut compound leaves.  Fertile pinnae are narrower than the sterile ones because the marginal sori are surrounded by a false indusium formed by the margins of the leaves curling over the sori.

Dryopteris (Figure 33), the wood fern, is one of the most conspicuous fern genera in the Eastern Deciduous Forest.  They grow from a rhizome that remains upright and does not creep.  Thus, the leaves tend to emerge in one vase-like cluster.  Each sorus is associated with a bean-shaped indusium.  The species in this genus readily hybridize making the identification of some individuals quite a challenge.

Polypodium (Figure 34) is a common small evergreen plant in the US woodlands.  The common polypody grows as an understory plant in the Eastern Deciduous Forest.  The southern polypody, however, grows as an epiphyte on the branches of large trees like the Live Oak of the southern coastal forests of the US.  Polypodium has sori that are naked.  That is, they are not associated with an indusium, either true or false.

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SYSTEMATICS OF THE PTERIDOPHYTA

According to Bold et al. (1987) and Lelinger (1985),  the ferns have been a problem in phylogenetics for some time.  The classical relationships of the groups of ferns can be seen in Pearson (1995) and Rothwell (1999), both of which are similar to the view of Bold et al. (1987).  Pryer et al. (2001), however, through molecular phylogenetic analysis indicate that the ferns, if considered as a monophyletic group, must include the psilophytes and horsetails.  After that, Scheuttpelz et al. (2006) and Schuettpelz and Pryer (2007 and 2008) confirmed the relationship.  Smith et al. (2006) created a revised Linnaean taxonomy of extant ferns using the recently confirmed relationships.  Schneider et al. (2009) analyzed fern phylogeny by a cladistic analysis using morphological characters and found a similar patterns that included Psilotum and Equisetum within the ferns.  We used Kenrick and Crane (1997b) for support in the inclusion of the extinct taxa, particularly the pre-ferns.

INTRODUCTION TO THE DOMAIN EUKARYA

LITERATURE CITED

Bierhorst, D. W. 1971. Morphology of Vascular Plants. In: N. H. Giles and J. G. Torrey. The MacMillan Biology Series. The MacMillan Co. New York. 

Bold, H. C., C. J. Alexopoulos, and T. Delevoryas. 1987. Morphology of Plants and Fungi. 5th Edition. HarperCollins Publishers, Inc. New York. 

Galtier, J. and F. M. Hueber. 2001. How early ferns became trees. Proc. R. Soc. Lond. B. 268: 1955-1957.

Kenrick, P. and P. R. Crane. 1997b. The Origin and Early Diversification of Land Plants: A Cladistic Study. Smithsonian Institute Press. Washington, DC.

Lellinger, D. B. 1985. A Field Manual of the Ferns and Fern-Allies of the United States and Canada. Smithsonian Institution Press. Washington, D.C.

Pearson, L. C. 1995. The Diversity and Evolution of Plants. CRC Press. New York. 

Pryer, K. M., H. Schneider, A. R. Smith, R. Cranfill, P. G. Wolf, J. S. Hunt, and S. D. Sipes. 2001a. Horsetails and Ferns are a Monophyletic Group and the Closest Living Relatives to Seed Plants. Nature. 409:618-622. 

Pryer, K. M., E. Schuettpelz, P. G. Wolf, H. Schneider, A. R. Smith, R. Cranfill. 2004. Phylogeny and evolution of ferns (monilophytes) with a focus on the early leptosporangiate divergences. American Journal of Botany. 91(10): 1582-1598.

Rothwell, G. W. 1999. Fossils and ferns in the resolution of land plant phylogeny. Botanical Review 65:188-218.

Schneider, H., A. R. Smith, and K. M. Pryer. 2009. Is morphology really at odds with molecules in estimating fern phylogeny? Systematic Botany. 34(3): 455-475.

Schuettpelz, E., P. Korall, and K. M. Pryer. 2006. Plastid atpA data provide improved support for deep relationships among ferns. Taxon. 55(4): 897-906.

Schuettpelz, E. and K. M. Pryer. 2007. Fern phylogeny inferred from 400 leptosporangiate species and three plastid genes. Taxon. 56(4): 1037-1050.

Schuettpelz, E. and K. M. Pryer. 2008. Fern phylogeny. In: Ranker, T. A. and C. H. Haufler, eds. Biology and Evolution of Ferns and Lycophytes. Cambridge University Press. Cambridge. pp. 395-416.

Smith, A. R., K. M. Pryer, E. Schuettpelz, P. Korall, H. Schneider, and P. G. Wolf. 2006. A classification for extant ferns. Taxon. 55(3): 705-731.

Smith, G. M. 1955. Cryptogamic Botany. Vol II. Bryophytes and Pteridophytes. 2nd ed. McGraw-Hill Book Co., Inc. New York. 

Soria, A. and B. Meyer-Berthaud. 2004. Tree fern growth strategy in the late Devonian cladoxylopsid species Pietzschia levis from the study of its stem and root system/ American Journal of Botany. 91(1): 10-23.

Stein, W. E., F. Mannolini, L. A. Hernick, E. Landing, and C. M. Berry. 2007. Giant cladoxylopsid trees resolve the enigma of the Earth's earliest forest stumps at Gilboa. Nature. 446: 904-907.

Stewart, W. N. and G. W. Rothwell. 1993. Paleobotany and the Evolution of Plants. 2nd edition. Cambridge University Press. Cambridge.

Thomas, B. A. and R. A. Spicer. 1987. The Evolution and Palaeobiology of Land Plants. Diocorides Press. Ecology, Phytogeography, and Physiology Series. Vol 2. Portland, Oregon.

Tomescu, A. M. F. 2008. Megaphylls, microphylls and the evolution of leaf development. Trends in Plant Science. 14(1): 5-12.

Wikstrom, N. and K. M. Pryer. 2005. Incongruence between primary sequence data and the distribution of a mitochondrial atp1 group II intron among ferns and horsetails. Molecular Phylogenetics and Evolution. 36: 484-493.

By Jack R. Holt.  Last revised: 04/17/2013