Trends and concepts in fern classification

Abstract

Background and Aims Throughout the history of fern classification, familial and generic concepts have been highly labile. Many classifications
and evolutionary schemes have been proposed during the last two centuries, reflecting different interpretations of the available
evidence. Knowledge of fern structure and life histories has increased through time, providing more evidence on which to base
ideas of possible relationships, and classification has changed accordingly. This paper reviews previous classifications of
ferns and presents ideas on how to achieve a more stable consensus.

Full text

INVITED REVIEW
Trends and concepts in fern classification
Maarten J. M. Christenhusz1,* and Mark W. Chase1,2
1
Jodrell Laboratory, Royal Botanic Gardens, Kew, Richmond TW9 3DS, UK and
2
School of Plant Biology, The University of
Western Australia, Crawley, Western Australia 6009, Australia
* For correspondence. E-mail m.christenhusz@kew.org
Received: 15 September 2013 Returned for revision: 23 October 2013 Accepted: 28 November 2013
†Background and Aims Throughout the history of fern classification, familial and generic concepts have been highly
labile. Many classifications and evolutionary schemes have been proposed during the last two centuries, reflecting
different interpretations of the available evidence. Knowledge of fern structure and life histories has increased
through time, providing more evidence on which to base ideas of possible relationships, and classification has
changed accordingly. This paper reviews previous classifications of ferns and presents ideas on how to achieve a
more stable consensus.
†Scope An historical overview is provided from the first to the most recent fern classifications, from which conclu-
sions are drawn on past changes and future trends. The problematic concept of family in ferns is discussed, with a
particular focus on how this has changed over time. The history of molecular studies and the most recent findings
are also presented.
†Key Results Fern classification generally shows a trend from highly artificial, based on an interpretation of a few
extrinsic characters, via natural classifications derived from a multitude of intrinsic characters, towards more evolu-
tionary circumscriptions of groupsthat do not in general align well with the distribution of these previously used char-
acters. It also shows a progression from a few broad family concepts to systems that recognized many more narrowly
and highly controversially circumscribed families; currently, the number of families recognized is stabilizing some-
where between these extremes.Placement of many genera was uncertain until the arrival of molecular phylogenetics,
which has rapidly been improving our understanding of fern relationships. As a collective category, the so-called
‘fern allies’ (e.g. Lycopodiales, Psilotaceae, Equisetaceae) were unsurprisingly found to be polyphyletic, and the
term should be abandoned. Lycopodiaceae, Selaginellaceae and Isoe
¨taceae form a clade (the lycopods) that is
sister to all other vascular plants, whereas the whisk ferns (Psilotaceae), often included in the lycopods or believed
to be associated with the first vascular plants, are sister to Ophioglossaceae and thus belong to the fern clade. The
horsetails (Equisetaceae) are also members of the fern clade (sometimes inappropriately called ‘monilophytes’),
but, within that clade, their placement is still uncertain. Leptosporangiate ferns are better understood, although
deep relationships within this group are still unresolved. Earlier, almost all leptosporangiate ferns were placed in a
single family (Polypodiaceae or Dennstaedtiaceae), but these families have been redefined to narrower more
natural entities.
†Conclusions Concluding this paper, a classification is presented based on our current understanding of relationships
of fern and lycopod clades. Major changes in our understanding of these families are highlighted, illustrating issues of
classification in relation to convergent evolution and false homologies. Problems with the current classification and
groups that still need study are pointed out. A summary phylogenetic tree is also presented. A new classification in
which Aspleniaceae, Cyatheaceae, Polypodiaceae and Schizaeaceae are expanded in comparison with the most
recent classifications is presented, which is a modification of those proposed by Smith et al. (2006,2008) and
Christenhusz et al. (2011). These classifications are now finding a wider acceptance and use, and even though a
few amendments are made based on recently published results from molecular analyses, we have aimed for a
stable family and generic classification of ferns.
Key words: Bibliography, classification, convergence, cryptogams, Cyatheaceae, fern family concepts, fern allies,
ferns, homology, lycopods, monilophytes, Polypodiaceae, pteridophytes, history of botany.
INTRODUCTION
One can only understand current classifications of a particular
group of organisms after a thorough review of past systems.
Classification of ferns has been particularly unstable in the past,
and to understand why this has been so problematic we review
the excellent article by Tryon (1952), in which the history of clas-
sification was discussed up until that time. Since that date, many
other classifications have been proposed, some of which were dis-
cussed by Pichi-Sermolli (1973).We build upon these andprovide
an overview of our understanding of fern relationships and fern
classification from the early 1970s to the present, addressing the
influence of molecular phylogenetic studies in redefining genera
and families. We will highlight some of the major changes in clas-
sification and identify genera that need further study to come to a
better understanding of fern phylogeny. Many of these problemat-
ic genera were already suggested by Holttum (1973) to need
monographic study. We summarize current knowledge of fern
relationships in an annotated classification and point out where
foci for future studies should be directed.
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In commemoration of the 250th birthday of Carolus Linnaeus,
Pichi-Sermolli (1958) provided an overview of higher classifica-
tion of pteridophyta (understood to mean at that time ferns and
‘fern allies’). He stated that classifications until then had
reflected four trends; (1) accepting four major lineages usually
treated as classes: Psilopsida, Lycopsida, Sphenopsida and
Pteropsida, the last including ferns and seed plants (e.g.
Jeffrey, 1903;Arnold, 1948); (2) treating ferns and ‘fern allies’
as a group but excluding Psilopsida, treating the latter as
related to Bryophyta (Lam, 1948) or as an independent lineage
together with the fossil Psilophytopsida (Rothmaler, 1951); (3)
maintaining Pteridophyta in their classical sense, including all
vascular cryptogams (e.g. Campbell, 1940;Reimers, 1954);
and (4) recognizing the independent divisions Psilophyta,
Lycophyta, Sphenophyta, Pteridophyta and Spermatophyta
(Benson, 1957). Pichi-Sermolli effectively dismissed trends 1,
2 and 4, concluding that all should be treated in a single division
in the classical sense, placing them next to Spermatophyta. His
classification scheme is discussed further below, but we high-
light these four approaches here so they can be compared with
the more detailed discussion of trends below.
The concept of family has always been fluid in ferns and, up to
now, no clear family concept has been established. Families are
generally defined as one or more genera that share several mor-
phological characteristics, and many angiosperm families have
had a long history of recognition based on shared floral and
fruit features. However, in ferns, it has never been clear from
what part of the plant these features should be derived, and this
has led to a shifting number of families being recognized and dif-
fering circumscriptions for many of the more frequently recog-
nized families.
Characters and terminology of ferns
Most readers will have a general idea of what a fern is, but a
review of their typical characters and life histories will help
focus attention on their key characters (those typically used in
fern classification); this section of course can be skipped by pter-
idologists. There are several profound differences from flowering
plants and mosses that define ferns and allow different characters
to be used in the classification of this group. We therefore thought
it appropriate to include a short introduction to the terminology
of ferns and highlight the features important in their classifica-
tion.
Ferns are vascular plants that produce spores and undergo an
alternation of generations (with separategametophyte and sporo-
phyte generations that exist as free-living plants). Lycopods are
similar to ferns in this regard, but ferns are the sister group of
the seed plants (gymnosperms plus angiosperms), whereas the
lycopods are sister to all other vascular plants (ferns plus the
seed plants). Mosses, hornworts and liverworts (bryophytes)
are also spore-producing plants with alternation of generations,
but their sporophytes are reduced in size and dependent on the
dominant gametophyte stage.
Alternation of generations. The leafy fern plant we observe in the
fields, marshes and forests is called a sporophyte (Fig. 1B), a vas-
cular plant that through meiosis produces spores with half the
number of chromosomes found in the mother plant. When a
spore lands in a suitable place, it grows into a free-living
gametophyte (Fig. 1A), a haploid plant. Typically gametophytes
are photosynthetic, but there are mycoheterotrophic gameto-
phytes in some fern genera and families (Merckx et al.,
2012). Gametophytes, often called prothalli,producegametes
through mitosis: male gametes, formed in antheridia,swimto
the archegonia where female gametes are produced. The
mobile, free-swimming, flagellate male cells are the reason for
the water dependence of ferns; water is required for their move-
ment from one gametophyte to another. Gametophytes can be bi-
sexual or unisexual, and some can also reproduce vegetatively (a
few may have lost the sporophyte stage altogether). From the fer-
tilized female gamete, a new sporophyte develops. This life-
history pattern differs from that in mosses, in which sporophytes
are parasitic on the photosynthetic gametophyte, and from seed
plants. In the latter, male and female gametophytes develop
inside the pollen and ovule, respectively, with ovules being
retained on the parental sporophyte until after fertilization, when
at some point it is released as a seed with the new sporophyte
partly developed inside. Spore and gametophyte morphology
and the number of sperm flagellae have been used for classification
of ferns. Alternation of generations in ferns was first described by
Lindsay in 1794 (Fig. 2).
Sporophyte structure. A fern sporophyte consists of a stem, which
is often called a rhizome, even if it is above ground or forms a
20 m tall trunk, as in some tree ferns. The rhizome can have
various forms of stelar structures and orientation and scales, all
of which are commonly used characters for identification and
classification. From the rhizomes, leaves (also called fronds)
grow, which in ferns are megaphylls (in contrast to lycopods,
which have a similar life cycle but bear microphylls: simple
leaves with single unbranched veins). Leaves emerge as fiddle-
heads (Fig. 1C; circinnately coiled), although some groups
have different types of vernation; circinnate vernation is not ex-
clusive to ferns and also occurs, for example, in the carnivorous
sundews, Droseraceae, and in the gymnosperm genus Stangeria
T.Moore (Zamiaceae). The petiole, or stipe, is used as a diagnos-
tic character and in particular the number of vascular strands is
important. Leaf blades can be simple or highly divided, and
this has in the past been used for classification; however, in
most cases, leaf division does not apply above the rank of
species. Fertile leaves bear spore-producing sporangia, which
are typically organized in groups, called sori. Usually sori are
on the lower side of the leaf, and some groups have specialized
structures associated with their sporangia and sori.
Sporangia. Spores are formed in sporangia.Ineusporangiate
ferns, sporangia are formed from a group of cells, which is the
plesiomorphic state. Eusporangia are found in all other vascular
plants, except in the leptosporangiate ferns, where the sporan-
gium develops from a single cell into a structure with a stalk,
wall and spores. Leptosporangiate ferns form a clade that
includes the bulk of fern species, but eusporangiate ferns are
composed of several independent groups. In some families,
most notably Marattiales, eusporangia are fused together,
forming a complex synangium. The numbers of cells in the
stalk of a leptosporangium and the shape and orientation of the
annulus, the structure that splits to release spores, have been
used as diagnostic characters.
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Sori. Sporangia can be organized in various ways. Sometimes
they cover the entire lower lamina of the blade, especially
towards its apex, which is called an acrostichoid sorus
(Fig. 1F). In most taxa, sporangia are organized in discrete sori
(Fig. 1E). The typical leptosporangiate sorus consists of a stalk
to which sporangia are fixed; a sorus can be covered with a scale-
like structure called the indusium (Fig. 1E). The shape and organ-
ization of the indusium has been an important character for
higher classification of leptosporangiate ferns and is still
employed today, although usually in combination with other
A B
C D
E F
FIG. 1. Characters of ferns. (A) Gametophyte [Ptisana attenuata (Labill.) Murdock, Marattiaceae]. (B) Sporophyte [Ptisana sp. nov. (Kamau & Christenhusz 638,
EA, K), Marattiaceae]. (C) Circinnate vernation (Sphaeropteris excelsa (Endl.) R.M.Tryon, Cyatheaceae). (D) Sporocarps [Salvinia natans (L.) All., Salviniaceae].
(E) Discrete sori with indusia [Polystichum falcatum (L.f.) Diels non Fe
´e, Polypodiaceae]. (F) Acrostichoid sori [Acrostichum durvillei (Fe
´e) C.Presl, Pteridaceae].
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characters. It was clear, even before the advent of molecular sys-
tematics, that there is a great deal of convergence in sorus char-
acters, so on their own they have not reliably revealed a great deal
about relationships.
Indument. Fern leaves can be glabrous or densely covered in hairs
or scales. The shape and anatomy of these hairs are often used to
diagnose species, but this is rarely useful at higher levels of clas-
sification. Scales of the rhizomes have been demonstrated to be
conservative and phylogenetically informative, especially if
clathrate scales are present (these are translucent like a stained-
glass window).
Spores. Spore characters are useful in classification and phylo-
genetically informative, and they have also allowed calibration
of DNA-based phylogenetic trees with the ages of fossil
spores. Most remarkable are perhaps the heterosporous ferns.
Heterospory is a condition in which a single sporophyte produces
a smaller number of large spores, megaspores, that develop into
female gametophytes and, more typically, numerous micro-
spores that develop into male gametophytes. In ferns, this condi-
tion is only found in two groups of aquatics: Marsileaceae and
Salviniaceae; it is clearly an adaptation to their aquatic environ-
ments, where the female gametophyte develops inside the
megaspore and is thus protected from the surrounding water.
These heterosporous ferns were often placed among ‘fern
allies’, together with the unrelated heterosporous lycopods
(Selaginellaceae, Isoe
¨taceae), but heterospory has evolved inde-
pendently in these lineages (and again in seed plants, which are
also heterosporous) and is not necessarily associated with aquatic
habitats in those lineages. Spores in heterosporous ferns are
formed in special structures called sporocarps (Figs 1D, 4I).
All other ferns and Lycopodiaceae are homosporous, the plesio-
morphic condition.
With this explanation of terms, we hope to have clarified the
various characters used in fern classification, which should
make a historical overview easier to follow.
HISTORICAL OVERVIEW
Difficulties with artificial classification and ‘natural’ systems
As with all botanical studies, the nomenclatural history of fern
taxonomy starts with Linnaeus (1753,1754), who first attempted
FIG. 2 . The first illustration of the germinationof spores by Lindsay (1794). 1. Leaf with sori. 2 –5. Sporangium opening, showing annulus and spores. 6– 7. Spores.
8–11. Developing gametophyte. 15– 17. Developing sporophyte.
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to organize ferns based on the shape and position of sori on
leaves; this use of a single character produced an artificial but
easily used classification. Fern diversity and anatomy had been
addressed in earlier studies (e.g. Cesalpino, 1583;Grew, 1682;
Malpighi, 1675;Morison, 1699;Van Rheede tot Drakestein,
1678– 1703;Plumier, 1705;Petiver, 1712), but it is important
to note that at the time little was known about the life cycle of
ferns. Linnaeus’s sexual system, based on the number of
stamens and pistils (the so-called ‘sex organs’ in flowering
plants), functioned well for artificially classifying angiosperms,
but it was impossible to place ferns in a system based on numbers
of male and female parts. Ferns were therefore placed in
Cryptogamae, among mosses, algae, fungi and even some animals
(corals, sponges, etc). In his Cryptogamia filices Linnaeus (1753,
Sp. Pl. 2: 1085– 1100) recognized 16 genera and 174 species:
Equisetum L. (6), Onoclea L. (1), Ophioglossum L. (6), Osmunda
L. (17), Acrostichum L. (25), Pteris L. (19), Blechnum L. (2),
Hemionitis L. (2), Lonchitis L. (3), Asplenium L. (20),
Polypodium L. (58), Adiantum L. (15), Trichomanes L. (11),
Marsilea L. (2), Pilularia L. (1) and Isoe
¨tes L. (1). All but the
last are still classifed as ferns; Isoe
¨tes is a lycopod and does not
have a close relationship to the other genera. Linnaeus’s classifi-
cation was artificial, with species of clearly distant relationships
placed together. This was the case for Linnaeus’ classifications in
general, but it served the purpose of organizing the numerous
newly discovered species until a more ‘natural’ system would
be available. Species had to be assigned to a genus as a require-
ment in Linnaeus’s binomial system, which is the reason why so
many fern species were often placed seemingly randomly in a
genus. This seems illogical now, but it made sense at a time
when ferns were biologically so poorly understood. In many
cases, the author may not have known in which genus to place
a new species and thus a guess was made, which was subsequent-
ly corrected when genera became better known or the generic
concepts changed, a process that still continues.
Indusium characters (as ‘fructifications’) were traditionally
employed, and we still use these today (but always in combin-
ation with other characters). James E. Smith (1793) used charac-
ters of the indusium and recognized 20 genera, paying attention
in particular to ontogeny and the method of indusial rupture. It
was the first fern classification presented as a ‘natural system’,
albeit that much diversity in opinion existed even then on what
constituted a natural genus in ferns.
It was Lindsay (1794), a British surgeon stationed in Jamaica,
who gathered ‘dust’ from several weedy ferns, sowed them in a
flower pot on his window sill and observed in detail germination
of spores and formation of gametophytes, followed by develop-
ment of sporophytes (Fig. 2). This discovery sparked anatomical
study of ferns, aiding the progress of understanding their struc-
tural biology. Gradually, this improved understanding of the
anatomy and biology of ferns led to many changes in interpret-
ation of fern morphology and to what was hoped would be a
more natural classification for them.
Swartz (1801) published an early handbook on ferns, in which
he treated some 670 species in 30 genera. In his slightly later
Synopsis filicum,Swartz (1806; 38 genera, 720 species) followed
the characters of sori and indusia as established by J. E. Smith
(1793) and presented an elaborated version of Smith’s classifica-
tion. Despite knowing at the time that Swartz’s genera were arti-
ficially delimited, this classification was followed by subsequent
authors for 30 years, probably as a result of no further insights
into other characters that could be used in addition to or in place
of sori and indusia. They werealso, for instance, the basis ofa clas-
sification by Desvaux (1827), who, due to a more detailed study of
soral characters, increased the number of genera to 79 infive fam-
ilies (Marsile
´es, Lycopodie
´es, Osmonde
´es, Marattie
´es, Filice
´es),
improving somewhat on the intrinsic naturalness of Swartz’s clas-
sification. This informal use of families by Desvaux contrasted
with all other fern taxonomists, who did not apply the family
concept to ferns and preferred to use other categories. An impres-
sive 1666 species were presented by Desvaux (1827),ofwhich
many species were new, but they were mechanically placed in
genera solely following the classification of Swartz (1801).This
system was soon challenged because species too different in
other characters, and obviously not closely related, were often
placed in the same genus.
An essentially new principle of classification was employed
by Presl (1836,1845,1851), who also added to his system vege-
tative characters (especially venation, habit, and rhizome and
petiole anatomy) in addition to the traditional fertile characters.
Spore characters were also discussed and illustrated, but these
were not used to group taxa. In total, Presl recognized 176
genera. Independent of Presl, John Smith (1841,1843) compiled
a classification, which, although differing in generic delimita-
tion, essentially used similar characters to distinguish the 138
genera recognized. Both should be credited for establishing
modern pteridology in the sense that they eschewed simpler
methods of relying only upon soral and indusial characters to
define a genus and instead emphasized that key generic charac-
ters could be drawn from any part of a fern. This was certainly
an improvement in terms of putting together species that were
morphologically and anatomically similar (in many cases), but
it was much more difficult to use; to many of their contemporar-
ies, Presl’s and J. Smith’s shifting emphases on soral characters
to define one genus, petiole anatomy for anotherand venation for
a third made their systems look arbitrary and flew in the face of
the standard practice of the time.
However these classifications were criticized by Hooker, who
initially accepted many of Presl’s genera (Hooker and Bauer,
1842), but later changed his opinion and only recognized 63
genera based on classical characters of the sorus; nonetheless,
he clearly saw the value of these other characters and treated
many of Presl’s and J. Smith’s genera at subgeneric levels. It
appears that he attempted to provide a consensus between the
large artificially delimited but well-established genera of
Swartz (1801,1806) and the newer treatments based on intrinsic
characters that were considered ‘more natural’, the definition of
which meant different things to different authors and changed
over time, particularly once the concept of evolution was pro-
posed. Hooker emphasized that characters of ‘fruiting parts’
were more important than characters of ‘vegetative parts’ for
classifying all plants at the generic level. This worked better
with angiosperms, for which the concept of family was long
established; many families had a long history of recognition,
with a consensus of ideas about which genera were to be
included. This sort of consistency of circumscription of the
major angiosperm families is without parallel in ferns, and the
emphasis on ‘fruiting parts’ for ferns did not result in either a
consistent or a widely accepted system. Hooker’s authority, as
the director of the Royal Botanic Gardens, Kew, led to the
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re-application of the Swartzian system, which lasted for another
50 years as the most often used fern classification.
Although sporangium and sorus characters remained predom-
inant as the basic framework for fern classification thanks to
Hooker’s prominence, some interest in applying other characters
to classification continued; Fe
´e, who published the elaborate
series Me
´moires sur la famille des fouge
`res (Fe
´e, 1844– 1873),
which included Genera filicum (6th and 7th Me
´moire), empha-
sized an even wider range of characters for generic delimitation
than Presl and recognized approx. 188 genera. The first Me
´moire
dealt entirely with characters of ferns useful for classification and
focused on venation. It also reviewed other characters, seeking
new ones in fertile structures. He applied, for instance, characters
of the spores and number of cells in the annulus. The last charac-
ter was not taken up by subsequent pteridologists, but it was re-
applied nearly a century later in the classification of Copeland
(1947).
John Smith, who had become curator of the living fern collec-
tion at Kew, knew his plants intimately, and his Historia filicum
(Smith, 1875) presented his views on classification and incorpo-
rated the genera of Presl and Fe
´e, accounting for 212 genera.
Even though the classification of Fe
´e found followers in the
French Empire, his and the classifications of Smith and Presl
remained in the shadow of the more influential one of Hooker
until the end of the 19th century.
Evolution enters the scene: a diminished importance of sori and
sporangia as pre-eminent characters for classification
The classification of Christ (1897) emphasized the importance
of vegetative characters, and this was adopted and expanded
by Diels (1898– 1900) in his treatment for Die natu
¨rlichen
Pflanzenfamilien (German for ‘the natural plant families’).
Diels presented a phyletic classification, a discipline in its
early stages, and he applied the modern usage of family as a
formal category, previously often referred to at higher taxonomic
ranks (e.g. suborders and orders). The classification of Diels was
heavily criticized by most subsequent workers, even though, or
maybe because, it was the first classification to take Darwinian
evolution into consideration.
Christ’s (1897) evaluation of previous systematic works gives
us an insight into the perception of that work at the time. He
wrote, for instance, about Die Farnkra
¨uter of Schkuhr (1809)
that the ‘nomenclature is outdated but that it contains many
good images’, and of Fe
´e’s Me
´moires (1844–1873) that ‘it con-
tains many species that were later not recognised, butthat it con-
tains valuable material, in particular the images’. In contrast, he
cited Mettenius (1856) as the ‘founding work for a natural sys-
tematics’ of ferns, but that classification was only natural in a
pre-Darwinian sense and the work of Mettenius was largely
ignored outside the German language sphere. Christ clearly
attributed the shift from a classification based on the sorus to a
classification based on the entire anatomy of the ferns to
Mettenius, which is only partly true (Presl and Fe
´e had already
applied this concept earlier), but Christ and Mettenius shared a
nationality, which may explain the preference. Christ accepted
99 genera of ferns in 13 families and divided them into
Leptosporangiatae and Eusporangiatae, possibly the first use of
the character of sorus development as a primary character.
At the turn of the century, phyletic classifications of ferns were
also being compiled elsewhere, the first being a treatment by the
British botanist Bower (1889), who based his ideas on his elab-
orate studies on anatomy, development and morphology. In his
three-volume work, The ferns,Bower (1923– 1928) recognized
12 families, and in Polypodiaceae, the first major use of this
now ubiquitously applied name, he recognized six evolutionary
lineages. This phyletic classification was, however, problematic,
mainly due to an emphasis on the location of sori on the leaf
blade.
Copeland (1929), who attempted to arrange the East Asian
genera into a phyletic sequence, was the first to address this
problem and suggested that a larger Polypodiaceae would be
more natural. The tree-fern lineages were included, not making
Polypodiaceae natural in the Bowerian sense, but this wider
concept of Polypodiaceae resulted in the name being variously
applied ever since.
Christensen’s influential Index filicum (1906) listed 147
genera and followed almost exactly the treatment of Diels,
except for some name changes to conform with the rules and
an increase in number to account for new genera described
during the six years between these publications. In subsequent
supplements, the number of genera grew to 213. Christensen
(1938) advanced from a bibliographer to a researcher in his
own right, taking up the advances in Bowerian ‘phylogeny’, and –
like Bower – he recognized 12 families, but divided his
Polypodiaceae into 15 subfamilies, about which he stated that
these should maybe be treated at the family level. His approx. 247
genera were based on a variety of vegetative (mostly hairs and
scales) and soral characters, but several genera were pointed out
to be of uncertain delimitation or status. Even though his classi-
fication was highly regarded and widely followed, Christensen
himself regarded it as a tentative scheme.
Ching (1940) divided ‘Polypodiaceae’ into 33 families,
grouped into seven ‘lines of evolution’ corresponding more or
less to Copeland’s families (1929), in which the essential unnat-
uralness of ‘Polypodiaceae’ was accepted. Holttum (1947),at
that time the dominant personality in fern taxonomy, did not
agree with certain relationships expressed by Bower, and he pre-
sented a revised classification of Polypodiaceae in which five
families were recognized, but he admitted that some are difficult
to define morphologically.
Microscopic characters added to the mix
Copeland (1947) was one of the first systematists to propose
that ‘valid’ taxa should reflect ‘naturalness’ and ‘convenience’,
which by this time meant that taxa must correspond to a single
evolutionary lineage and be well circumscribed, supported by
characteristics and easy to define. He based his Genera filicum
(1947) on extensive fieldwork in tropical Asia, and altogether
he recognized 305 genera, many of which included just a
single species. He divided Trichomanes but treated Asplenium
in the broad sense, which was prescient. On the other hand,
Holttum (1913) stated that ‘Copeland’s limitation seems to be
his lack of understanding of Christensen’s discovery of the
great importance of the details of hairs and scales as evidence
of relationships’, which is understandable from a field botanist’s
point of view. Microscopic characters are often difficult to
observe in the field when all you have is a hand lens, especially
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when other characters such as habit are more obvious to distin-
guish groups. Copeland also provided a good overview of the
concepts of fern classification preceding his magnum opus.He
presented a similar number of taxa to Ching (1940), which
Christensen stated had much to do with the two having the
same generic concept. However, Ching still recognized 28
genera more than Copeland, so Christensen’s criticism was not
entirely justified.
Holttum (1947) revised the classification of Christensen
(1938), which resulted from a detailed study of Malayan ferns.
It had been customary to place all leptosporangiate ferns in a
broad Polypodiaceae, apart from a few easily recognizable fam-
ilies such as Osmundaceae (considered intermediate between
leptosporangiate and eusporangiate ferns), Cyatheaceae (the
tree ferns with a helicogyrate annulus), Gleicheniaceae (with a
strange leaf morphology) and Hymenophyllaceae (the filmy
ferns with leaves one cell layer thick and cup-shaped sori).
Christensen (1938) also followed this, but stated that the subfam-
ilies he treated in Polypodiaceae ‘were perhaps better dealt with
as families’, which Holttum (1947) did. He redefined Polypodiaceae,
including Dipteris Reinw., but excluding Grammitidaceae. He
also segregated Thelypteris Schmid. from Christensen’s
Dryopteridoideae into Thelypteridaceae, and the remaining
subfamilies except Gymnogrammoideae, Vittarioideae and
Onocleoideae were placed by Holttum in a large and diverse
Dennstaedtiaceae with 11 subfamilies. The other group of tree
ferns, Dicksoniaceae, were also segregated, even though they
were previously thought to be closely related to Dennstaedtia
Bernh. Gymnogrammoideae was renamed Adiantaceae, and
Holttum included Vittarioideae because they are undoubtedly
related. Onocleaceae remained unplaced. Many matters
remained unsolved in Holttum’s scheme, and he did not claim
that all major groupings had been solved. Although it was
easier to recognize a broad circumscription of Polypodiaceae
and only segregate a few clearly divergent families, such as
Osmundaceae, Cyatheaceae and Hymenophyllaceae, all
authors of the time believed that this was unsatisfactory and
most split Polypodiaceae into several families. Which characters
should be emphasized and which families should be recognized
was far from uniform, and substantial disagreements among
authors were the standard.
Starting from the beginning (again), fossils included
Tryon (1952) concluded his historical overview predicting
that ‘the conflict between utility and naturalness will remain an
issue’ in future classifications. He suggested that taxa based on
evolutionarily circumscribed groups would not necessarily
have characters that are useful for identification, and indeed for
some modern fern families this is a major issue today. Current
examples of continuing contentious issues are the numerous fam-
ilies in the tree-fern lineage, segregation of Lonchitis,Saccoloma
Kaulf. and Cystodium J.Sm. from Lindsaeaceae (Lehtonen et al.,
2010), merging of Hymenophyllopsidaceae with Cyathea Sm.
(Christenhusz, 2009b), family placement of Nephrolepis Schott
(Smith et al., 2006;Hennequin et al., 2010) and difficulties with
morphological circumscription of Pteridaceae, to name a few.
Reimers (1954) produced a thorough overview of Pteridophyta
and included fossil taxa in his classification, setting it apart from
earlier classifications that dealt only with extant or fossil taxa.
He first defined his concept of Pteridophyta as ‘chlorophyllous,
autotrophic land plants with antithetic (heterophasic) change of
generations, and both generations organically independent’. In
this way he included the lycopods with ferns in a single lineage.
Reimers provided a developmental scheme in which fern lineages
were set out on a time scale, all essentially originating from
Psilophytales (in which he included the fossils Rhynia R.Kidston
&W.H.LangandPsilophyton J.W.Dawson) in the early
Devonian (320 million years ago). Modern Psilotales are directly
derived from them in his scheme, which was the general idea at
the time and one that prevailed until this century. We will not go
into the classification of his fossil taxa here, although we admit
that to come to a good classification of ferns, fossil lineages will
have to be included. It is, however, not within the remit of this
paper to compare fossil classifications with those for extant ferns.
Reimers concluded that there were four extant lineages (as
classes) in Pteridophyta: Lycopsida (Lycopodiales, Selaginellales
and Isoe
¨tales), Psilotopsida (Psilotales), Articulatae (Equisetales)
and Filices (Ophioglossales, Marattiales, Filicales, Marsileales
and Salviniales). In Psilotopsida, two extant families were accepted,
Psilotaceae and Tmesipteridaceae, because the two extant genera
‘appear to be relict forms’. What is interesting isthat Reimers men-
tioned that the lack of roots in these genera cannot be seen as proof
for any relationship with Psilophytopsida (which he placed in a
different class, in spite of the linearity shown in the scheme).
Articulatae included a great number of fossil taxa, but within
this lineage Equisetum was placed in its own order because he
stated that ‘relationships between this and fossil Calamitales
are not yet clear’. The largest group, Filices, were divided into
four subclasses, one extinct (Primofilicales) and three with
extant members: Eusporangiatae and Leptosporangiatae with
Osmundidae as a ‘transition group’. Eusporangiatae included
the obvious non-leptosporangiate orders Ophioglossales and
Marattiales, the latter divided into five families, of which one
is the fossil Asterothecaceae (which includes the giant trees,
Psaronius B.Cotta), and which should still be accepted as
separate from extant Marattiaceae (Christenhusz, 2007).
Leptosporangiate ferns were divided into three orders,
Filicales, Marsileales and Salviniales, on the basis of hetero-
spory of the last two orders; Filicales included a dozen families,
of which the largest, Polypodiaceae, was divided into 14 subfam-
ilies. Reimers, however, stated that Polypodiaceae would un-
doubtedly be polyphyletic in this concept, noting that
Dennstaedtioideae closely resembled Dicksoniaceae, that the re-
lationship of Dipteridaceae to Polypodiaceae was unclear and
that the origin of the obviously primitive Woodsioideae and
Onocleoideae required further study. We now know that if the
heterosporous ferns and Parkeriaceae are included, this would
make a monophyletic group (and could in theory be treated as
a single family), and most subfamilies treated here correspond
to many of the currently accepted families or subfamilies.
Differences lie mostly in the placement of single problematic
genera, e.g. Nephrolepis in Davallioideae, Pteridium Gled. and
Stenochlaena J.Sm. in Pteridoideae, Diacalpe Blume and
Peranema D.Don in Woodsioideae (now part of Dryopteris
Adans., see Zhang and Zhang, 2012), Cyclosorus Link,
Lomariopsis Fe
´e and Thelypteris in Dryopteridoideae and
Cheiropleuria C.Presl in Polypodiaceae. In spite of obvious mis-
placement of some genera, this work actually sets a good
example for how the fossil record can be used to aid in placement
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of extant taxa, and it has therefore been unfort unate that this clas-
sification did not find wider acceptance.
Alston (1956), while preparing an account for the Flora of West
Tropical Africa, came to disagree with the classification of Holttum
(1947). He segregated Lindsaeaceae from Dennstaedtiaceae and
maintained Vittariaceae as separate from Adiantaceae [although this
had already been done by Reimers (1954) at the subfamilial level].
He also segregated Aspleniaceae, Aspidiaceae, Athyriaceae
and Blechnaceae from Holttum’s Dennstaedtiaceae, and he
described Athyriaceae and Lomariopsidaceae. The movement of
genera by various authors between Athyriaceae, Aspidiaceae
and Dennstaedtiaceae shows, however, that relationships among
these genera were far from understood.
Pichi-Sermolli (1958) subdivided Pteridophyta into six classes,
which were mixtures of living and fossil taxa with the two ex-
ceptions noted: Lycopsida, Sphenopsida, Noeggerathiopsida
(fossil taxa only), Psilotopsida, Psilophytopsida (fossil taxa
only) and Filicopsida. He used Filicopsida because the name
Pteropsida was then often used in a broader sense to include all
vascular plants. Within Lycopsida, he placed Isoe
¨taceae amid
Lepidodendridae. The two genera in Psilotales were placed in
their own families, and he stated ‘that these show no close affinity
with the other groups of Pteridophyta’ and should be placed ‘near
the Psilotophytopsida purely as a matter of convenience, since
their derivation from them is very questionable’, a return to arti-
ficial classification in the absense of evidence of relatedness.
Moving on to his Filicopsida, he divided this class into seven
subclasses, of which Primofilicidae are extinct, and all others
have living members. For his classes Ophioglossidae and
Maratiidae, by most earlier authors joined in a single group
‘Eusporangiatae’, Pichi-Sermolli agreed that these two lineages
differ greatly in morphology, and he dropped the terms
Eusporangiatae and Leptosporangiatae ‘because the origin of
the sporangia does not offer us a sharply taxonomical distinc-
tion’. In his opinion, these terms ‘should be used as descriptive
morphological terms, not as names of taxa’. Osmundidae were
maintained in a separate subclass because of their ‘antiquity and
their paleontological history’. The living heterosporous ferns
are placed in two subclasses: Marsileidae and Salviniidae.
Formerly placed together, the two were not considered to be
related and were treated by Nakai (1949) as classes, but ‘it is
clear that they are related to homosporous Filicidae’. Within
Filicidae, Pichi-Sermolli agreed that delimitation and classifica-
tion had been difficult ‘and pteridologists are very far from agree-
ing about their classification’. The number of families had at that
time increased due to splitting of the traditional Polypodiaceae
(e.g. Ching, 1940;Holttum, 1947;Alston, 1956;Pichi-Sermolli,
1957), and this led to the suggestion they should be organized in
orders, which Pichi-Sermolli (1958) did in a tentative scheme.
Cytology enters the picture
Prior to 1950, cytological aspects of ferns were practically
unknown, and what was known was mainly due to the pioneering
work of Manton (1950,1954,1958;Manton and Sledge, 1954)
and Brownlie (1957,1958); as cytological information became
more widely available, it began to be applied in classifications.
As more chromosome counts accumulated over the next
decades, the position of certain species and wider affinities of
genera, families and higher ranks could be postulated. By the
1970s, chromosomes of around 15 % of fern species had been
counted, and Walker (1973) provided a good review of how cyto-
logical information could help in unravelling previous problems.
Manton (1958) demonstrating, for instance, that Pteridaceae of
Copeland (1947) did not form a uniform lineage, with Pteris,
Cheilanthes Sw. and Adiantum having chromosome numbers
based on 29 or 30, and the other group (corresponding to
the ‘dennstaedtioid group’) showing a wide range of basic
numbers, even within a single genus. Chromosome numbers
continued to play a role in classification for several decades,
only to be replaced by molecular phylogenetics in the 1990s, pro-
viding important insights into relationships of species, genera
and sometimes families. Mehra (1961) finalized the application
of chromosome numbers and described phyletic lineages in
terms of their cytological evolution, but this information was
not included in later fern classifications. Cytological data on
ferns are well documented and may in the future provide good
additional insights into fern relationships and evolution.
The beginning of a new approach to fern classification, before DNA
In 1961, Wagner wrote an essay discussing the problems
related to fern classification. He correctly concluded that due
to the problems of assumed homology of morphological traits
and associated convergent evolution abundant in ferns, classifi-
cation had been unstable and the concept of a fern family had
become meaningless. A few years later, Wagner (1969)
addressed the question of how a system of relationships is con-
structed. He discussed non-evolutionary classifications and the
then-modern phenetic systems of numerical taxonomy.
Numerical taxonomy in which all information is included, he
believed, could lead to circumscription of evolutionary group-
ings, but he stated that he was concerned about parallelisms
and convergences and that in the case of ferns he did not have
enough data needed to make an objective classification. He
also made a significant and relevant statement: ‘In botany it
may be that purely phenetic systems, no matter how detailed,
will not produce results that coincide with evolutionary relation-
ships. This possibility very much needs to be tested’. Wagner
illustrated the subjectivity of phenetics in assuming homology
of similar forms, when in fact they have evolved independently.
As an example, he used evolution of heterospory, which, al-
though appearing highly similar, certainly evolved several
times. Of course this argument over phenetic and evolutionary
classifications – the latter in which certain characters are
weighted and exhibit a trend from primitive to derived – is still
ongoing, and therefore the classification given in this review
below is as much biased and subjective as previous ones, were it
not that it is basedon far more data thanwere available in the1960s.
Similarly, Nayar (1970) criticized the classifications proposed
in the 1940s and 1950s because they diverged substantially,
which he blamed on the differing importance various authors
gave to morphological characters. He proposed a new classifica-
tion based on the ‘significant additions to our knowledge relating
to several morphological criteria of the pteridophytes’, which he
enumerated, but neither justified nor cited sources for this
‘knowledge’. In spite of his argument that subjective classifica-
tions are bad practice, his classification was also based on sub-
jective interpretation of morphological characters. Even
though the genera were better known than before, this again
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resulted in movement of genera into different families and
changes between family and subfamily names. He did attempt
to place his families and subfamilies into an evolutionary
scheme, although also here there are some odd placements.
Plagiogyriaceae, for instance, are in his scheme derived from
Osmundaceae, which is a strange placement for a group that
has sporangia bearing remarkable similarities to those of
Cyathea (Hooker and Bauer, 1842). Blechnaceae, to which
Plagiogyria (Kunze) Mett. generally was thought to be related
due to superficial similarity, are the only unplaced lineage in
Nayar’s scheme in which there had been no attempt to justify pos-
tulated relationships.
Because Pichi-Sermolli (1970b)published a detailed cata-
logue of fern family names, Holttum (1971) felt obliged to
comment on it and pointed out that ‘in the case of family
names we are very far from being able to make a good taxonomic
judgement’. Holttum stated that it is difficult to decide which
family names would be the correct ones to use because ‘most
family names of ferns have had such different meanings’ and
‘that such names are only intelligible if we associate them with
the names of particular authors’. Holttum therefore regarded
‘all family names of ferns as informal and tentative’, a
comment that may still be relevant today. Against this back-
ground, genetic studies of ferns were proposed, helping us under-
stand processes of speciation and inheritance in ferns (e.g.
Klekowski, 1971); this sort of publication did not involve such
genetic studies, but rather hinted at the advantages of using a
fern system rather than an angiosperm system for genetic ana-
lyses (due to their alternation of generations producing haploid
gametophytes).
The view in 1972: fern classification is still a jungle for the
user, unless ...
In April 1972, a symposium was held in London bringing pter-
idologists from around the world together to discuss phylogeny
and classification of ferns. In the resulting conference volume,
Holttum (1973) stated that it would be impossible to come to a
new classification without additional good monographic
studies. He was of the opinion that ‘phylogenetic understanding
will develop as classification improves’, but that classification ‘is
still in a very imperfect state’. Nevertheless, Holttum proposed a
tentative scheme, showing possible relationships, setting his
understanding of fern inter-relationships apart from the
schemes of Nayar and Kaur (1971) and Bierhorst (1971). The
groups that Holttum suggested should be revised taxonomically
[e.g. Adiantum,Asplenium,Cheilanthes,Ctenitis (C.Ch.) C.Ch.,
Diplazium,Dryopteris and Tectaria Cav.] are often groups that
even today are still in need of taxonomic study, and some still
need an improved generic circumscription.
In the same volume, Pichi-Sermolli (1973) provided an histor-
ical review of fern classification, in which he showed how clas-
sifications had changed over time and how unstable they had
been. He concluded that no real progress in fern classification
based on morphology could be made without the collaboration
of palaeobotanists. He placed his hopes in numerical approaches
due to their ‘being more objective’, which ‘will placate the fight
between splitters and lumpers’. We now know that this was a
forlorn hope and has not turned out to be the case, and arguments
about splitting and lumping are still a major problem in
taxonomy of all groups despite large amounts of genetic
(DNA) data having been collected. Knowing the phylogenetic
tree does not, of course, tell us how to delineate families or
genera.
Mickel (1974) criticized the development of phyletic classifi-
cations over the preceding years, stating that circumscription was
often ‘ill-founded and the relationships were often based on
speculation rather than solid evidence’. Mickel stressed the
need for proper evaluation of homologous characters, and he dis-
cussed some of the major issues of homology, for instance the
different types of scales and development of stelar structures.
He also pointed out the usefulness of stomata morphology to
infer phyletic relationships. He then presented a freely drawn
phylogram, which is backed by a discussion of characters, but,
even though the evidence was sound, the choice of characters
used to develop this phyletic classification was still subjective.
Crabbe et al. (1975) discussed the rearrangement of pterido-
phytes at the British Museum of Natural History and
New York Botanical Garden with a large number of pteridolo-
gists and provided a new generic sequence for fern herbaria.
They provided a complete list of genera, which was helpful in
making sense of the numerous names proposed and used in herb-
aria. Even though many groupings do not seem natural, it
reflected knowledge of fern classification at the time. They did
suggest an alphabetical system as an alternative, which is easy
for filing, but they pointed out the limitation in implementation
of generic changes without major reorganization of the entire
herbarium in that case. Placement of the heterosporous ferns
was still uncertain, and they were therefore placed at the end of
the sequence. Some genera were placed where we would not
expect them based on their morphology (e.g. Plagiogyria
among eusporangiate ferns). Despite its ambitions, this sequence
never found much of a following in herbaria.
Development of fern classification and further study of the
structures of ferns were summarized in Tryon and Tryon
(1982) and Kramer and Green (1990).Kramer and Green
(1990) was the most influential classification until the arrival
of molecular phylogenetics, although it should be stated that at
that time Polypodiaceae was generally taken in a broad sense, in-
cluding most leptosporangiate lineages, and within that it was
preferred to place genera alphabetically,a practice that is still fol-
lowed in some books and herbaria. It may make it easy to find a
genus quickly (if you happen to know in which genus a species is
placed), but it can be difficult and laborious to update the herbar-
ium when generic changes take place or to find a species when it
has previously been placed in more than one genus.
Molecular systematics (DNA) and fern classification
The advent of molecular phylogenetics has rapidly changed
our understanding of fern relationships through phylogenetic
analyses of DNA sequence data. Studies of plastid DNA data
slowly started emerging in the mid 1990s. Plastid data became
mostly used initially in seed plants, and maternal, biparental
(Sears, 1980;Corriveau and Coleman, 1988;Harris and
Ingram, 1991) and paternal (Neale and Sederoff, 1988) inherit-
ance of plastids is known in seed plants, but ferns still had not
been thoroughly studied. Gastony and Yatskievych (1992)
studied a natural hybrid of Pellaea Link with known parentage,
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which demonstrated that plastid DNA is maternally inherited in
this cheilanthoid fern.
In an early phylogenetic study, Hasebe et al. (1993) sequenced
four ferns to infer the relationships of ferns to other land plants.
Sequencing was still a laborious process, but, due to the develop-
ment and improvement of in vitro DNA amplification [ polymer-
ase chain reaction (PCR)], a much larger number of taxa could be
studied, and rbcL data for ferns quickly accumulated. Hasebe
et al. (1994) developed effective primers to sequence rbcL in
ferns; their paper focused on leptosporangiate lineages, and it
showed monophyly of heterosporous ferns within the leptospor-
angiate clade. They identified all main lineages in ferns and
showed that Dennstaedtiaceae were not monophyletic when lind-
saeoids were included; they also found Vittariaceae resolved
close to Adiantum. Possible changes in classification were sug-
gested, but not formally proposed. Wolf et al. (1995a) sequenced
45 species of dennstaedtioid ferns, which produced similar
results to the work of Hasebe et al. (1994), but here too no
formal recircumscriptions of groupings were proposed because
a much wider sampling of taxa was needed to assess monophyly
of such a diverse group of plants.
A large pteridophyte symposium was held at the Royal Botanic
Gardens, Kew, in memory of Holttum in 1995. This resulted in a
volume called Pteridology in perspective (Camus et al.,1995)in
which a wide range of topics in fern research were addressed, in-
cluding several early molecular phylogenetic studies, family,
genus and species concepts, speciation and systematics. In this
volume, Pahnke et al. (1995) discussed the utility of the internal
transcribed spacer (ITS) in phylogenetics of ferns. Wolf (1995b),
who earlier discussed the usage of plastid sequences in fern phylo-
genetics (1995a), evaluated the use of a combination of genes in
molecular phylogenetic studies. Comparisons of morphology
with molecular phylogenetic results for tree ferns (Conant et al.,
1995) and filmy ferns (Dubuisson, 1995) were also provided, in-
creasing our understanding of these lineages. A discussion of
the use of phylogenetic hypotheses for formal scientific classifica-
tions was presented by Hennipman (1995), and he stated that
‘modern higher classifications of ferns are a jungle for the user’,
showing the great need to simplify fern classification.
Hennipman discussed criteria used in the past for family and
generic classifications but did not provide a clear-cut way on
how to define a family in ferns; he emphasized lineages that he
considered monophyletic and which ones were in his opinion
clearly paraphyletic. He divided ferns into four orders,
Marattiales, Ophioglossales, Psilotales and Filicales, and his
order Filicales, which corresponds to the leptosporangiate
ferns, is divided into 14 families (Aspleniaceae, Cyatheaceae,
Davalliaceae, Dennstaedtiaceae, Dryopteridaceae, Gleiche-
niaceae, Hymenophyllaceae, Lindsaeaceae, Marsileaceae,
Osmundaceae, Polypodiaceae, Pteridaceae, Schizaeaceae and
Thelypteridaceae). The classification of Hennipman formed
the backbone on which more recent classifications have been
based because this simplified organization of the ferns was
much more accessible than previous schemes. Of course with
the molecular analysis of previously unsequenced genera, this
simplified classification was destabilized and new families
were again added.
The number of taxa studied increased when Hasebe et al.
(1995) analysed rbcL sequences of 107 species. Their results
showed that Grammitidaceae are embedded in Polypodiaceae,
and Davalliaceae are the closest relative of the Polypodiaceae
clade. Tectaria was found to be closer to Oleandraceae rather
than to Dryopteridaceae, with which it was usually associated.
Again, Dennstaedtiaceae were found to be polyphyletic,
but this time with three lineages emerging; Lonchitis and
Orthiopteris Copel. fell in a clade separate from Lindsaea
Dryand. Vittariaceae were found deeply embedded in
Pteridaceae and strongly supported as sister to Adiantum.
Tree ferns (including Metaxyaceae, Loxsomataceae and
Plagiogyriaceae) were monophyletic and diverged early from the
rest of the leptosporangiate taxa, just after the divergence of hetero-
sporous ferns. No clear placement was found for Psilotaceae, but
in one of their analyses (neighbor joining), Psilotum Sw. and
Tmesipteris Bernh. were sister to Ophioglossaceae, a portent of
what would be later found.
Although the techniques were still laborious, data for the
major groups began to accumulate rapidly, and certain problem-
atic fern groups were logically targeted using rbcL sequences,
e.g. cheilanthoid ferns (Gastony and Yatskievych, 1992;
Gastony and Rollo, 1995), dennstaedtioid ferns (Wolf, 1995a)
and vittarioid ferns (Crane et al., 1995), helping to better under-
stand relationships of genera in these groups.
Despite the molecular revolution in fern phylogenetics, morph-
ology was not forgotten, and Smith (1995) summarized non-
molecular ideas of fern relationships. He hypothesized that the
eusporangiate families Ophioglossaceae and Marattiaceae consti-
tuted isolated lineages and that Osmundaceae, Schizaeaceae, tree
ferns and a few other small families represent early branches off
the main lineage leading to other leptosporangiates. This was
based on fossil and living plant morphology and became the
most broadlyaccepted classification. Morphologically most leptos-
porangiate families were recognizable, but inter-relationships
among these werestill unknown and widely disputed. Fern systema-
tics had been diverted from producing phyletic schemes based on
morphological characters, and thus these ideas were never robustly
tested. Smith (1995) posed 16 challenging questions and invited
readers to answer as many as possible. Many of these questions
related to placement of genera with difficult to interpret morph-
ology – Ceratopteris Brongn., Hymenophyllopsis K.I.Goebel,
Monachosorum Kunze, Plagiogyria,Pleurosoriopsis Fomin,
Psilotum,Saccoloma Kaulf. – or to relationships of entire larger
groups – Dennstaedtiaceae, Grammitidaceae, Hymeno-phylla-
ceae, Lindsaeaceae, Schizaeaceae, Thelypteridaceae, Vittariaceae
and heterosporous ferns. Delimitation of Dryopteridaceae and
other higher leptosporangiate fern families also needed sorting
out because at the time their relationships were hotly debated.
Hasebe et al. (1994) had already resolved some of these issues in
their phylogenetic analysis of rbcL,showingAdiantum to be
sister to Vittariaceae, Monachosorum sister to Dennstaedtiaceae,
Lindsaeaceae sister to all higher leptosporangiates separate from
other dennstaedtioids, Plagiogyria and Metaxya C.Presl in the tree-
fern lineage, and the heterosporous ferns forming a lineage in lep-
tosporangiate ferns (see also Fig. 3).Thebaseoftheirtreedidnot
have good support, but it was after all a first attempt at using
DNA sequences to examine broader relationships within ferns;
many relationships had to be left for resolution in subsequent
studies.
Smith’s review (1995) soon led to analysis of combined mo-
lecular and morphological data sets. Pryer et al. (1995) presented
the first cladistic analysis based on morphology combined with
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Polypodium
Grammitis
Campyloneurum
Microsorum
Lepisorus
Pyrrosia
Platycerium
Selliguea
Drynaria
Loxogramme
Davallia
Oleandra
Triplophyllum
Tectaria
Arthropteris
Nephrolepis
Dracoglossum
Lomariopsis
Cyclopeltis
Ctenitis
Polystichum
Dryopteris
Arachniodes
Polybotrya
Elaphoglossum
Bolbitis
Pleocnemia
Leucostegia
Hypodematium
Didymochlaena
Blechnum
Woodwardia
Onoclea
Diplazium
Athyrium
Woodsia
Phegopteris
Thelypteris
Asplenium
Hemidictyum
Diplaziopsis
Rhachidosorus
Cystopteris
Gymnocarpium
Vittaria
Adiantum
Cheilanthes
Hemionitis
Pteris
Pityrogramma
Acrostichum
Ceratopteris
Coniogramme
Cryptogramma
Pteridium
Dennstaedtia
Monachosorum
Odontosoria
Lindsaea
Lonchitis
Cystodium
Saccoloma
Metaxya
Lophosoria
Dicksonia
Cibotium
Cyathea
Plagiogyria
Loxsoma
Culcita
Thyrsopteris
Marsilea
Salvinia
Azolla
Schizaea
Anemia
Lygodium
Stromatopteris
Glechenia
Dipteris
Matonia
Hymenophyllum
Trichomanes
Osmunda
Danaea
Marattia
Ophioglossum
Psilotum
Equisetum
Gymnosperms
Angiosperms
L
y
copods
Equisetaceae
Psilotaceae
Ophioglossaceae
Marattiaceae
Osmundaceae
Hymenophyllaceae
Gleicheniales
Schizaeaceae
Salviniales
Cyatheaceae
Saccolomataceae
Cystodiaceae
Lonchitidaceae
Lindsaeaceae
Dennstaedtiaceae
Pteridaceae
Cystopteridoideae
Rhachidosoroideae
Diplaziopsidoideae
Asplenioideae
Thelypteridoideae
Woodsioideae
Athyrioideae
Blechnoideae
Didymochlaenoideae
Hypodematioideae
Dryopteridoideae
Lomariopsidoideae
Tectarioideae
Davallioideae
Polypodioideae
Oleandroideae
Polypodiaceae
Aspleniaceae
FIG. 3 . Summary phylogenetic tree showing relationshipsof a representative selection of fern genera based on molecular (DNA) data, modified from Schuettpelz and
Pryer (2007),Lehtonen (2011),Rothfels et al. (2012) and Schneider et al. (2013).
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rbcL data for the same 50 taxa. They also found Ceratopteris and
Vittaria Sm. among pteridoid ferns and Plagiogyria in the tree-
fern lineage. Dennstaedtiaceae formed two unresolved lineages,
but with Lindsaea not grouping with either of these.
Heterosporous ferns and Schizaeaceae formed two independent
lineages also at unresolved positions within leptosporangiate
ferns.
Plastid 16S ribosomal DNA sequences were then applied to
address the placement of ‘fern allies’. Manhart (1995) found
Psilotum as sister to Tmesipteris, together forming a well-
supported sister group to Ophioglossaceae. This relationship
seemed odd at first, but it is now supported by morphological
characters (Schneider, 2013). When the problematic lycopod
Selaginella P.Beauv. was removed from their matrix (which
perhaps had been causing some sort of spurious attraction of
Equisetum to it), Equisetum also fell within the fern lineage but
without strong support. Hasebe et al. (1995) further addressed
the problems posed by Smith (1995) in an expanded rbcL phylo-
genetic analysis, which supported the sister relationship of
Psilotaceae and Ophioglossaceae and showed Dennstaedtiaceae,
Polypodiaceae, Pteridaceae and Dryopteridaceae in their trad-
itional senses to be polyphyletic.
Placement of Hymenophyllopsis, a peculiar genus from the
tepuis in the Guayana Shield, has always been problematic,
and it was therefore often placed in its own family,
Hymenophyllopsidaceae. The sorus structure is unique in
ferns, although superficially similar to filmy ferns. Wolf et al.
(1999) had shown that these are related to the tree fern genus
Cyathea, with which they share scale and spore morphology.
More recent studies have shown that Hymenophyllopsis is em-
bedded in Cyathea (Korall et al., 2006,2007), and thus it
should not even be treated as a genus, let alone as a family or
order. Christenhusz (2009b)and others now treat it as part of
Cyathea.
Lophosoria quadripinnata (J.F.Gmel.) C.Ch., a common
montane species in the Neotropics, was placed in its own family
by Pichi-Sermolli (1970b), who removed it from the mosly
extinct Protocyatheaceae, where it had been placed by Bower
(1923– 1928) and Reimers (1954). The relationship of this trunk-
less species to the tree ferns was never disputed, but Wolf et al.
(1999) showed it to be most closely related to Dicksoniaceae.
Placement of Equisetum was evaluated again by Pryer et al.
(2001a), who showed with strong support that it belongs to the
fern lineage. In the process, they refuted the prevailing theory
that horsetails and ferns form a transitional grade between lyco-
pods and seed plants. As a replacement name for the ferns includ-
ing Equisetaceae, Kenrick and Crane (1997) had proposed the
infradivision Moniliformopses, an invalid name under the
current nomenclatural code (McNeill et al., 2012). The term
‘monilophytes’, loosely based on this infradivision, became
established as a result of the paper of Pryer et al. (2004),in
which phylogenetic relationships of ferns were inferred. Why
this new term not based on a generic name is useful in the
context of a cladistic study is unclear to us. Nevertheless, it
was quickly picked up by the community and rendered into the
vernacular as ‘monilophyte’ (e.g. Pryer et al., 2001a,2004;
Schneider et al., 2004;Schneider, 2013). ‘Moniliform’ means
‘bead-shaped’, but it was never stated to what part of a fern, or
to which fern, this refers, although it was suggested that this
might refer to the stele structure of some extinct Devonian
taxa. It seems that a new colloquial name has been coined for
no particular reason other than providing a scientific-sounding
synonym for the word ‘fern’; in fact, it is often used in paren-
theses after the word fern, so ‘monilophyte’ is superfluous. It is
an alternative to the word ‘pteridophyte’, which traditionally
included all ferns plus lycopods; thus, an alternative, similar
sounding word was coined, without taking notice of its etymol-
ogy: ‘bead-plant’ being as uninformative as ‘wolf-plant’ or
‘lycophyte’, which should correctly be called clubmoss (in the
English vernacular) or lycopod (based on Lycopodium and refer-
ring to the vernacular ‘wolf-claw’ of many languages).
Psilotaceae were not included in the study of Kenrick and
Crane (1997), but Pryer et al. (2004) included these to show
they also belong to the fern clade. Critical taxa were included
in this data set and additional plastid (rbcL,atpB and rps4) and
nuclear (18S rDNA) regions were added to those previously
used. As expected, Osmundaceae were sister to the leptospor-
angiate ferns. Hymenophyllaceae – which had recently been
shown to consist of two monophyletic groups (Pryer et al.,
2001b) – was found to be sister to the gleichenioid ferns, and
tree ferns and heterosporous ferns were shown to belong to
‘core leptosporangiates’.
In a high-profile paper, Schneider et al. (2004) studied the age
of fern diversification and concluded that the higher leptospor-
angiates, to which the majority of extant ferns belong, diversified
in the Cretaceous, most probably in response to diversification of
the angiosperms. The hypothesis is that, because of the increase
in diversity of angiosperms, numerous new niches appeared in
which ferns could diversify. Schneider et al. (2004) also intro-
duced the term ‘eupolypods’, defined as the polypods excluding
‘basal’ polypods (e.g. Dennstaedtiaceae and Lindsaeaceae) and
pteridoids (Pteridaceae). The eupolypods were found to be repre-
sented by two clades (numbered I and II), on which we will elab-
orate below. This showed that many fern lineages are of similar
age to or younger than many angiosperm families, refuting the
usage of clade age as a useful character for defining a fern family.
Classification appeared to be more stable after publication of
the fern classification by Smith et al. (2006), which summarized
molecular findings to that date and provided synapomorphies for
the accepted families. Schuettpelz et al. (2006) added plastid
atpA to their earlier fern data set to gain better support at
deeper nodes. They continued their work on fern phylogeny
and performed a global analysis to serve as the basis for addres-
sing large-scale evolutionary questions (Schuettpelz and Pryer,
2007). They assembled and analysed the most inclusive molecu-
lar data set to date with three plastidregions (rbcL,atpB and atpA)
including 400 leptosporangiate fern species. Their findings were
of great value for the placement of genera that were previously
unavailable for molecular study and were dependent on the gen-
erous contributions of many fern collectors.
Meanwhile, a number of families were studied in greater
phylogenetic detail, resolving many taxonomic problems (e.g.
Korall et al., 2006,2007;Nagalingum et al.,2006,2007;
Christenhusz et al., 2008;Hennequin et al., 2008;Rothfels
et al., 2008). New findings were incorporated in the updated clas-
sificationof Smith et al. (2008), although this did not substantially
differ from his classification published two years earlier.
Schneider (2007) tested the congruence of molecular and mor-
phological data with the idea of incorporating fossil taxa in
phylogenetic studies, which is currently being further developed
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(Corvez et al., 2011). Morphological data have since been com-
bined with molecular data in analyses with some important
insights (e.g. Schneider, 2007;Lehnert et al., 2009;Lehtonen
et al., 2010;Sundue, 2010;Sundue et al., 2010).
In a new edition of A. Engler’s Syllabus of Plant Families,
Fischer (2009) adopted findings of recent molecular analyses
and treated Lycophytina as separate from Euphyllophytina.
Within the latter, he accepted super-class ‘Moniliformopses’ of
Kenrick and Crane (1997), comprising all ferns, Equisetaceae
and Psilotaceae. A synopsis was provided on classification of
this group, and it included all fossil taxa, which in many cases
were placed in families consisting of exclusively fossil taxa.
We did not check all fossil families, but we noticed that nomen-
clatural priority of family names was not always taken into
account. For the extant taxa, it simplyfollowed the classification
of Smith et al. (2006), although here and there with some reser-
vations and occasionally following some subclassifications
that had already been shown to be incorrect. The acceptance of
subfamily Stenochlaenoideae as separate from Blechnoideae,
for instance, is inappropriate if Woodwardia Sm. is included in
either of them.
The classifications of Smith (2006, 2008) initiated a discus-
sion about morphology being incompatible with the molecular
results, and indeed some groups such as Hypodematiaceae,
Lomariopsidaceae, Pteridaceae and Woodsiaceae (sensu
Smith) were difficult to define morphologically. Therefore, an
analysis with morphological data alone was carried out
(Schneider et al., 2009), and this recovered the same deep phylo-
genetic relationships (recognizing the four major clades in ferns:
Equisetidae, Ophioglossidae, Marattiidae and Polypodiidae)
found in previous studies of DNA sequence data. It would
appear that molecular data and morphology are compatible, pro-
vided they are treated in similar ways.
To provide a guide for herbaria to organize their specimens in a
phylogenetic way, a linear classification for all vascular plants
was composed of which here the lycopods and ferns are of con-
sequence (Christenhusz et al., 2011). This linear sequence was
not intended as a new classification and followed in many ways
Smith et al. (2006,2008), but major changes were made in the
treatment of some ‘eupolypods II’, with the disintegration of
Smith’s polyphyletic Woodsiaceae and with descriptions of
Diplaziopsidaceae, Rhachidosoraceae and Hemidictyaceae
(Christenhusz and Schneider, 2011;Christenhusz et al., 2011).
Also, placement of Equisetaceae as sister to all ferns seems to
be better than as sister to Marattiidae (as in Smith et al., 2006).
The position of Equisetum is poorly supported but always
among the deeper nodes of the fern clade, and therefore it
could end up in various places, depending on the method of
analysis or data set used (Lehtonen, 2011). Placement as sister
to all other ferns is also in agreement with palaeontological
studies that show its early appearance in the fossil record
(Taylor et al., 2009).
The classifications of Smith et al. (2006,2008) and
Christenhusz et al. (2011) mostly reduced the number of
genera, resulting in an expansion of several (e.g. Asplenium,
Blechnum,Cyathea and Hymenophyllum Sm.), but also resulted
in the acceptance of narrower generic concepts in other groups
(e.g. Hymenophyllaceae, Polypodiaceae and Pteridaceae).
These classifications are, however, remarkable in that they
combine morphological and molecular data to come to a
consensus classification, aiming to define monophyletic families
and genera with synapomorphies.
Some of the notable discrepancies are due to the traditionally
broader generic concepts used by workers in the Neotropics com-
pared with those in tropical Asia. Now we can combine the data
on these taxa and come to a consensus classification resulting in
evolutionarily defined natural genera and families. With the
wealth of data currently available (e.g. Eiserhardt et al., 2011;
Lehtonen, 2011;Lehtonen et al., 2012;Rothfels et al., 2012;
Zhang and Zhang, 2012;Liu et al., 2013;Schneider, 2013;
Schneider et al., 2013) it is now possible to compare generic con-
cepts across continents, applying a global concept of family and
genus.
DISCUSSION AND SOME HIGHLIGHED GROUPS
Concepts of higher classification of ferns and lycopods have
changed considerably throughout history, and we therefore
discuss a few groups with regard to their previous and current de-
limitation.
Fern allies have previously been loosely defined as vascular
spore-bearing plants that are not ferns. They have variously
included the lycopods (Huperzia Bernh., Lycopodiella Holub,
Lycopodium,Isoe
¨tes and Selaginella), horsetails (Equisetum,
Fig. 4A), whisk ferns (Psilotum and Tmesipteris, Fig. 4C),
Ophioglossaceae (Fig. 4B), water ferns (Marsileaceae, Fig. 4I,
Salviniaceae and Ceratopteris) and sometimes even members
of Schizaeaceae and Gleicheniaceae. Of course this was never
considered a natural group but just included genera that accord-
ing to subsequent authors could not be placed among the ferns
proper for various reasons. Because of its vague circumscription
and now evident non-monophyly, the term ‘fern ally’ should be
avoided. In a similar way, the term ‘eusporangiate fern’ is to be
avoided because this includes ferns that are not leptosporangiate;
it just describes the plesiomorphic state (eusporangia are also
present in seed plants and lycopods) and the taxa included do
not form a clade.
The water ferns (Salviniales, Fig. 4I) were previously placed
among ‘fern allies’ because they are heterosporous like
Selaginella and Isoe
¨tes. They have, however, been found to be
sister to the polypods, so they are deeply embedded in the homo-
sporous fern clade. Heterospory in general evolved several times
independently in lycopods, ferns and seed plants, but this evolu-
tion is not necessarily the result of adaptation to an aquatic life
form, although it appears as a plausible association for hetero-
sporous ferns.
Even though it is not a new concept (Linnaeus, 1753, already
included Equisetum in his ‘Cryptogamia filices’), the term ‘mon-
ilophyte’ was coined (Pryer et al., 2001a) for the fern clade in-
cluding Equisetaceae and Psilotaceae, in contrast to the
‘lycophytes’, both of which are linguistically erroneous and su-
perfluous terms. This clade has been traditionally called pterido-
phytes, but it has been suggested that ‘pteridophyte’ cannot be
used because in some classifications it has included the lycopods.
However, the origin of the term ‘monilophyte’ is unclear and has
never been published as a formal taxon and, moreover, its ety-
mology is obscure. ‘Sphenophytes’ (a term commonly used for
extant horsetails and their fossil relatives) share a common an-
cestor with ferns, even though their exact placement within the
fern lineage is still uncertain. It is therefore preferable to use
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the term ‘fern’, which as noted above has in the past often
included Equisetaceae and Psilotaceae, although alternatively
the early branching lineages of the fern clade could be treated
as independent lineages (‘sphenophytes’, ‘psilotophytes’, ‘mar-
attiophytes’), although this does not reflect their membership in a
clade with the rest of the ferns.
A B C
D E F
G
J K L M N
H I
FIG. 4 . Sori of major fern lineages. (A) Equisetales, Equisetum telmateia Ehrh. (B) Ophioglossales, Botrychium virginianum (L.) Sw.(C) Psilotales, Psilotum nudum
(L.) P.Beauv. (D) Marattiales, Marattia cicutifolia Kaulf. (E) Osmundales, Todea barbara T.Moore. (F) Hymenophyllales, Trichomanes cupressoides Desv. (G)
Gleicheniales, Dicranopteris linearis (Burm.f.) Underw. (H) Schizaeales, Lygodium volubile Sw. (I) Salviniales, Marsilea drummondii A.Braun. (J) Cyatheales,
Alsophila dealbata (G.Forst.) C.Presl. (K) Polypodiales, Dennstaedtia punctilobula (Michx.) T.Moore. (L) Pteris usambarensis Hieron. (M) Eupolypods,
Asplenium caudatum G.Forst. (N) Dryopteris sieboldii (T.Moore) Kunze.
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To illustrate the changing ideas of how ferns should be classi-
fied, we focus below on a number of groups and genera and de-
scribe the variation in opinions that have existed about how
best to handle their taxonomy.
Leptosporangiate ferns are a natural group, forming the crown
group of ferns including the vast majority of species. In the past,
most of these have at one time been included in a single family,
Polypodiaceae (or variants such as Filices or Dennstaedtiaceae,
variously including Osmundaceae, tree ferns and gleichenioid
ferns), resulting in great variability in application and circum-
scription of this family. The concept changed from including
the majority of the leptosporangiate ferns to the most recent
concept of those having scaly creeping rhizomes with abaxial
(rarely marginal), rounded to elliptic, elongate or acrostichoid,
exindusiate sori. The grammitid ferns were traditionally not
included, but molecular studies have shown that this group is
deeply embedded and therefore also part of Polypodiaceae,
expanding the description above to include taxa occasionally
having green spores and a petiole not always cleanly abscising
(as is the case with most other polypods; Smith et al., 2008).
Generic classification of grammitid ferns has been unstable
during the last 20 years, and they are currently divided in a
great number of genera, even though they form a subclade
within Polypodiaceae. It would be better for general users if
these are treated as a single genus (with approx. 700 species,
making it a large genus, the size of Asplenium, but smaller
than Elaphoglossum or Thelypteris) and treat most currently
recognized genera at the subgeneric level. Polypodiaceae as a
family should probably be expanded again to include all families
placed in ‘eupolypods I’, avoiding the need to recognize a great
number of monogeneric families; this approach is taken in the
classification presented below.
Placement and number of genera in the filmy fern family
Hymenophyllaceae (Fig. 4F) have long been debated. Traditionally
two genera have been recognized (Hymenophyllum and
Trichomanes) on the basis of differences in soral morphology,
but Iwatsuki (1977) pointed out that the morphological distinc-
tion is not clear-cut. The morphological diversity in the family
has resulted in recognition of a number of additional genera;
however, several molecular studies (e.g. Pryer et al.,2001b)
have shown that only two lineages are present, more or less corres-
ponding to the two traditionally recognized genera. Ebihara et al.
(2006) proposed on the basis of his analysis to maintain
Hymenophyllum in a broad sense, but meanwhile accepted eight
genera in the Trichomanes clade, which in our opinion may not
be warranted. As is the case for grammitid ferns, it seems a
better solution for general users to treat these as subgenera of
Trichomanes, accepting only two genera in Hymenophyllaceae.
Tree ferns (Fig. 4J) are sister to ‘polypods’ sensu Smith et al.
(2006) or Polypodiales sensu Christenhusz et al. (2011; see
Fig. 3). The tree-fern clade is usually divided into eight small
families, with Cyatheaceae being the largest (Korall et al.,
2007;Lehtonen, 2011). Tree ferns are all minimally genetically
divergent, which may be a result of the much longer generation
time of these long-lived plants ( palms are a similar example
from the angiosperms; e.g. Asmussen and Chase, 2001). It is
therefore possible to merge all families of Cyatheales in a single
family, which has generally not been done because there are few
universally diagnostic characters. The bestcharacter is the helico-
gyroid annulus of the sporangium, which links all these taxa
together and which has been used as the synapomorphy for the
order Cyatheales. This leads to a discussion of what defines an
order relative to a family. One character used is the age of diver-
gence between the individual lineages, which in the case of
Cyatheales is Late Jurassic (Schneider et al., 2004), similar to
the age of many angiosperm families. Cyatheales are too highly
divided at the family level, and the lineages should still be
revised taxonomically on thebasis of synapomorphies and mono-
phyly. In the classification below, the established families are
treated at the subfamilial level, allowing movement of genera
between them without altering their family placement. Not all
genera in Cyatheaceae sensu lato are tree like; they include trunk-
less Plagiogyria,Metaxya and Loxsoma R.Br. They also include
Hymenophyllopsis (Lellinger, 1984), a group of small ferns
from the Guayana Highlands in South America. This genus was
difficult to place on a morphological basis and was thus placed
in its own family or even its own order. It is now known that
these are embedded in Cyathea (Korall et al., 2007), and combi-
nations in Cyathea have been made for all eight species
(Christenhusz, 2009b).
Lonchitis has also been variously treated in the past, the genus
previously including many species, but now reduced to two (most
of the remaining species were moved to Blotiella Tryon,
Dennstaedtiaceae). Placement of Lonchitis has been contentious
in recent times. Earlier it was placed in Polypodiaceae subfamily
Dennstaedtioideae or Dennstaedtiaceae, but Smith (2006) placed
it in Lindsaeaceae. This makes the last difficult to define morpho-
logically; hence, it was placed in its own family Lonchitidaceae
(Schomburgk, 1848;Christenhusz, 2009a;Lehtonen et al., 2010;
Christenhusz et al., 2011), a family placed near Saccolomataceae
and Cystodiaceae among the early branching polypods, even
though the deep nodes of the polypods are not well supported in
most studies (Lehtonen et al.,2012).
Lomaria Willd. has, since its publication, been widely
accepted, so much so that Smith (1875) stated that it is a
natural genus containing a considerable number of species of
great uniformity of habit. Hooker (in Hooker and Bauer, 1842)
agreed with this to a certain extent, but he pointed out that a
number of species, including L. spicant (L.) Desv. ‘vacillate
between Lomaria and Blechnum ..., but such or similar grada-
tions are common to other genera of ferns, and if made an
excuse for abolishing a long-established one, the number of
genera would undergo a great reduction’. Hooker further dis-
cussed inclusion of Plagiogyria in Lomaria, and he pointed
out that there is an ‘affinity of those species with Cyatheaceae’
(to which we now know Plagiogyria is related), especially in
characters of the ‘capsules’ (the annulus of the sporangium is
oblique, e.g. helicogyrate), ‘yet the habit and sori are so entirely
in accordance with Lomaria that’ the genus Plagiogyria is diffi-
cult to identify. He further discussed differences in the number of
genera accepted in this group according to other authors.
Mettenius (1857), who excluded Plagiogyria, stated that
several species depart from the pteridoid character and complete-
ly merge into Blechnum, such that Blechnum and Lomaria are no
longer separate, whereas Presl (1851) kept them separate and
divided them into a multitude of genera based on minor
characters. Some of these genera are still in use today, but
recent phylogenetic studies (Lehtonen, 2011) have shown
Salpichlaena J.Sm. and Stenochlaena (which had been separated
from other genera on the basis of their climbing habit) to be
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embedded in Blechnum (specifically related to B. serrulatum
Rich.), which should also include Brainea J.Sm., Doodia
R.Br., Pteridoblechnum Hennipm. and Sadleria Kaulf.
Lomaria was based on the Australian L. nuda (Labill.) Willd.,
and Blechnum is based on the Neotropical B. occidentale L.;
the European species that has characters of the two was placed
in Spicanta C.Presl, an intermediate genus created to solve the
problem of intergrading morphology. The current consensus is
to apply a broad concept of Blechnum, but further studies on
the evolution of Blechnaceae are needed to assess which, if
any, of the segregate genera should be maintained.
Acrostichum was a genus applied initially to all ferns with sori
that were acrostichoid (Fig. 1F), i.e. distributed as a solid mass
across the back of the frond rather than organized in separate, dis-
crete structures. We know today that an acrostichoid fertile
lamina can be formed in a variety of ways, and thus it is not
useful for classification, although it is definitely a distinctive
structure. Therefore, ontogenyof this sorus type is more import-
ant for the placement of these ferns than its product. Initially
genera such as Blechnum,Ceratopteris,Danaea Sm., several
species of Asplenium,Bolbitis Schott, Tectaria, etc. were
included, but it was soon realized that these confluent sori were
covered by indusia at least at the younger stages, or, as in the
case of Danaea, they were fused sporangia with pores on the
apex (Christenhusz, 2010). It even included genera as different
from each other as Actiniopteris Link, Anetium Splitg.,
Anogramma Link, Cheiropleuria,Hemionitis,Platycerium
Desv., Polybotrya Willd., Schizaea Sm. and Todea Bernh. It
was Schott (1834) who separated Elaphoglossum Schott from
Acrostichum, which was followed by Moore (1857) who pro-
vided numerous new combinations. This process was completed
by Christensen (1906), after which Elaphoglossum was widely
accepted and Acrostichum (with A. aureum L. as the type
species) achieved its modern sense and was restricted to a few
species of mangrove ferns.
The name Thelypteris was coined by Schmidel (1763) based
on Acrostichum thelypteris L., but the name Thelypteris had
already been published in the same year by Adanson (his name
is a synonym of Pteris L.), making Schmidel’s name illegitimate.
This caused confusion, and the name was not accepted by subse-
quent authors. Pichi-Sermolli (1953) discussed this nomenclature,
and Thelypteris of Schmidel was subsequently conserved
(Holttum, 1968), resulting in the transferral of numerous species,
mainly from Aspidium Sw., Dryopteris and Nephrodium Rich.,
to Thelypteris. It was soon recognized as a natural group and
broadly accepted.Thelypteridaceae as a family were published [in-
cluding related genera such as Cyclosorus,Macrothelypteris
(H.Ito) Ching, Meniscium Schreb., Phegopteris (C.Presl) Fe
´e,
Oreopteris Holub, etc.] by Pichi-Sermolli (1970a), and molecular
studies soon showed five subgroups in this family, resulting in five
genera being recognized as comprising it, although it may be better
to accept only three at the generic level (see classification below).
Eupolypods form two subclades often called eupolypods I and
II. The first includes the families Davalliaceae, Dryopteridaceae,
Hypodematiaceae, Lomariopsidaceae, Nephrolepidaceae,
Oleandraceae, Polypodiaceae and Tectariaceae (sensu Christenhusz
et al., 2011). These families are morphologically divergent, and
it is difficult to see them as a single taxonomic entity, especially
if Dryopteridaceae and Polypodiaceae sensu stricto are com-
pared; nevertheless, they share characters of soral structure,
which are microscopic, and in the vascular structure of petioles
(generally an arch of vascular strands in a U-shape, except in
Hypodematiaceae). The second lineage (eupolypods II) includes
Aspleniaceae sensu stricto, Athyriaceae, Blechnaceae, Cysto-
pteridaceae, Diplaziopsidaceae, Hemidictyaceae, Onocleaceae,
Rhachidosoraceae, Thelypteridaceae and Woodsiaceae (sensu
Christenhusz et al., 2011), which share the character of a laterally
attached indusium (except in some Thelypteridaceae) and two
vascular bundles in their petioles. Even though synapomorphies
are few and not visible to the naked eye, it is possible to treat these
as two broad families instead of the current 18 (or 20) based on
their shared characters. Within these two lineages, a plethora
of families has recently been proposed (including the recent
segregation of Arthropteridaceae from Tectariaceae; Liu et al.,
2013), and many genera have changed family (e.g.
Christenhusz et al., 2013) when new phylogenetic analyses
have become available. Such a broad familial circumscription,
if accepted, would make the taxonomy of these groups more
stable, allowing further changes at subfamilial levels without
affecting the rank of family, which is the one above the genus
most frequently used by non-specialists.
A summaryof published studies showing relationshipsof major
groups of ferns as well as some problematic genera discussed
above is given in Fig. 3. This tree is further discussed below.
CONCLUSIONS
As often observed in taxonomy of many organisms, there has
been a trend from an initial artificial classification towards
more natural ones over time, although the definition of the
latter has changed several times. Artificial classifications are
useful for identification purposes, which were often the only in-
tention of early authors (such as Linneaus with his sexual system
of plant classification), and these were not intended to reflect
phylogenetic relationships between taxa; of course, phylogenet-
ic classification was not a conceptual framework available to tax-
onomists at that time. These artificial and early ‘natural’
groupings often resulted in species that are superficially similar
being placed together, which does not take convergent evolution
into account; some such cases in ferns required the application of
DNA phylogenetics to demonstrate that they were not reflective
of relationships. On the other hand, natural classifications may
separate species that are highly similar, particularly in terms of
‘key characters’ that were judged (often subjectively) to be im-
portant, in completely unrelated families, which makes these
families difficult to tell apart despite their distant relationship.
Fern classification is particularly full of such problems, and
now that we can use DNA studies to obtain information about
where problematic species and genera should be placed, we
often find that we lose the capacity to diagnose many families
with clear sets of morphological characteristics. As has been
observed previously, the more natural a classification becomes,
the more difficult it typically is to use, especially for researchers
not intimately familiar with the details of fern taxonomy. This is
of course not unique to ferns, and it can also be observed in recent
classifications of some angiosperm families such as Asparagaceae,
Liliaceae and Xanthorrhoeaceae in monocots and Acanthaceae,
Gesneriaceae, Lamiaceae and Plantaginaceae among eudicots in
APG III (2009). There are few characters that can be easily used
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to distinguish these families, and their history has been as compli-
cated and controversial as those for most fern families.
A second common trend has been the use of minute characters
to circumscribe narrowly circumscribed genera, and some
authors have used phylogenetic trees as the basis for placing
every pair of sister species into separate genera. Following
such episodes, another trend typically emerges, which expands
generic limits and subsumes such small genera into larger
ones, often redefining the original generic concept to fit this
greater number of species with more divergent morphology.
Even though this waxing and waning of generic limits mayeven-
tually produce a consensus taxonomy, it destabilizes nomencla-
ture and confuses the users. Ultimately, such instability hinders
research on ferns by non-taxonomic researchers, thereby retard-
ing progress on understanding their biology, conservation,
ecology and population dynamics. Of course, in an ideal
world, complete revisions at a generic level should be a pre-
requisite before familial revisions, but monographic study is
time consuming, material for study is not always available,
species sampling is often incomplete and funding for such re-
search is scant, resulting in many groups not having been
studied in their entirety. Nevertheless, when type species of
genera are sequenced and analysed, phylogenetic positions of
genera can be inferred and, even though a genus may be found
to be polyphyletic, the type species and its associated original
generic concept will remain, merely resulting in the transfer of
species to other genera, but not genera to different families.
Ferns have been notorious in the past for having an unstable
higher level classification, but molecular phylogenetic techni-
ques have helped to better understand relationships between
genera, and this has resulted in the three most recent classifications
(Smith et al., 2006,2008;Christenhusz et al.,2011) being highly
similar, especially when compared with most earlier classifications.
There are still likely to be changes in the treatment of families recog-
nized in future classifications. Numerous small families in the eupo -
lypods could be merged with a larger family to which theyare sister,
such as Davalliaceae with Polypodiaceae, Nephrolepidaceae with
Lomariopsidaceae, Hypodematiaceae with Dryopteridaceae, and
Hemidictyaceae with Aspleniaceae. We would actually argue that
to stabilize fern familial classification, the two clades informally
called eupolypods I and II should be treated at the family level,
Polypodiaceae and Aspleniaceae, respectively, in which the corre-
sponding families of Christenhusz et al. (2011) would be treated at
the subfamilial level. This may now seem like it would be further
destabilizing the fern classification, but it will in the longer term sta-
bilize matters, especially when taking into account the poor knowl-
edge of several currently recognized large families, such as
Woodsiaceae and Tectariaceae. Within a broader family concept,
the genera may be moved between subfamilies but will remain in
the same larger family, which is the only higher taxonomic level fre-
quently used by non-taxonomists.
Current consensus classification of extant Lycopodiophyta
(lycopods) and Polypodiophyta (ferns)
Numbers in parentheses after families correspond to estimated
numbers of genera and species (genera/species). Currently
accepted genera are listed; for a full synonymy of genera see
Christenhusz et al. (2011). Estimated species numbers for
some of the larger genera are also provided. Sori of all major
lineages are illustrated in Fig. 4.
[LYCOPODS; 3 families, 5 genera, approx. 1300 species]
LYCOPODIIDAE BEK.
Lycopodiales DC.
Lycopodiaceae P.Beauv.
1
(3/approx. 400)
(Huperzia Bernh., Lycopodiella Holub,
Lycopodium L.)
Selaginellales Prantl
Selaginellaceae Willk. (1/approx. 750)
(Selaginella P.Beauv.)
Isoe
¨tales Prantl
Isoe
¨taceae Reichenb. (1/approx. 140)
(Isoe
¨tes L.)
[FERNS; 21 families, approx. 212 genera, approx. 10 535
species]
EQUISETIDAE WARM.
Equisetales DC.
Equisetaceae Michx. (1/20)
(Equisetum L.)
OPHIOGLOSSIDAE KLINGE
Ophioglossales Link
Ophioglossaceae Martinov
2
(4/approx. 80)
(Botrychium Sw., Helminthostachys Kaulf.,
Mankyua B.Y.Sun, M.H.Kim & C.H.Kim,
Ophioglossum L.)
Psilotales Prantl
Psilotaceae J.W.Griff. & Henfr. (2/12)
(Psilotum Sw., Tmesipteris Bernh.)
MARATTIIDAE KLINGE
Marattiales Link
Marattiaceae Kaulf.
3
(6/approx. 130)
subfamily Danaeoideae J.Williams
(Danaea Sm.).
subfamily Marattioideae C.Presl
(Angiopteris Hoffm., Christensenia Maxon,
Eupodium J.Sm., Marattia Sw.,
Ptisana Murdock)
1
The number of genera in Lycopodiaceae is not yet certain, but molecular
phylogenetic studies (Wikstro
¨m and Kenrick, 1997) have clearly identified three
clades, corresponding to the genera Huperzia,Lycopodiella and Lycopodium.A
treatment of three generawill result in the loss of some well-established names
such as Diphasiastrum Holub as a synonym of Lycopodium and Phylloglossum
Kunze in Huperzia. Use of these names has resulted in the recognition of a great
number of genera in Lycopodiaceae, and as a result some authors havetreated the
three broader clades as subfamilies (e.g. Øllgaard, 2012). We see no benefit in
destabilizing the taxonomy of these genera to accommodate these additional
groups at the generic level,and these should therefore be treated at the subgeneric
level if some form of formal classification of these groups is desired.
2
Delimitation of genera in Ophioglossaceae is generally accepted, although
many segregate genera have been proposed, subdividing Botrychium and
Ophioglossum. The latter includes epiphytic Cheiroglossa C.Presl and
Ophioderma (Blume) Endl., which can be maintained as separate, but, since they
form a clade with core Ophioglossum, we have included them there.
3
Marattiaceae can be divided into two subfamilies, although one will be
monogeneric, which calls into question the need for subfamilies, especially when
there are only six genera recognized in total. Danaea is nevertheless sufficiently
genetically and morphologically distinct from other extant genera in the family
that it could be recognized at the subfamilial level. Archangiopteris Christ &
Giesenh. is embedded in Angiopteris, and, to prevent Angiopteris from being
synonymized with Marattia, the last has been divided into three genera
(Murdock, 2008).
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POLYPODIIDAE CRONQUIST,TAKHT.&ZIMMERM.
Osmundales Link
Osmundaceae Martinov
4
(4/approx. 25)
(Leptopteris C.Presl, Osmundastrum C.Presl,
Osmunda L., Todea Willd.)
Hymenophyllales A.B.Frank
Hymenophyllaceae Mart.
5
(2/approx. 650)
(Hymenophyllum Sm., Trichomanes L.)
Gleicheniales Schimp.
Gleicheniaceae C.Presl (6/approx. 165)
[Dicranopteris Bernh., Diplopterygium (Diels)
Nakai, Gleichenella Ching,
Gleichenia Sm., Sticherus C.Presl, Stromatopteris
Mett.]
Dipteridaceae Seward & E.Dale (2/9)
(Cheiropleuria C.Presl, Dipteris Reinw.)
Matoniaceae C.Presl (2/4)
(Matonia R.Br., Phanerosorus Copel.)
Schizaeales Schimp.
Schizaeaceae Kaulf. (4/approx. 190)
subfamily Lygodioideae Christenh.
6
(Lygodium Sw.)
subfamily Schizaeoideae Lindl.
(Actinostachys Wallich, Schizaea Sm.)
subfamily Anemioideae C.Presl
(Anemia Sw.)
Salviniales Bartl.
Marsileaceae Mirb. (3/approx. 65)
(Marsilea L., Pilularia L., Regnellidium Lindman)
Salviniaceae Martinov (2/approx. 20)
(Azolla Lam., Salvinia Se
´g.)
Cyatheales A.B.Frank
Cyatheaceae Kaulf. (approx. 14/approx. 700)
subfamily Thyrsopteridoideae B.K.Nayar
(Thyrsopteris Kunze)
subfamily Loxsomatoideae Christenh.
7
(Loxsoma R.Br., Loxsomopsis Christ)
subfamily Culcitoideae Christenh.
8
(Culcita C.Presl)
subfamily Plagiogyrioideae Christenh.
9
[Plagiogyria (Kunze) Mett.]
subfamily Cibotioideae Nayer
(Cibotium Kaulf.)
subfamily Cyatheoideae Endl.
10
(Alsophila R.Br., Cyathea Sm., Gymnosphaera
Blume, Sphaeropteris Bernh.)
subfamily Dicksonioideae Link
[Calochlaena (Maxon) R.A.White & M.D.Turner,
Dicksonia L’He
´r., Lophosoria C.Presl]
subfamily Metaxyoideae B.K.Nayar
(Metaxya C.Presl)
Polypodiales Link
Cystodiaceae J.R.Croft (1/1)
(Cystodium J.Sm.)
Lonchitidaceae C.Presl (1/2)
(Lonchitis L.)
Lindsaeaceae C.Presl (6/approx. 220)
[Lindsaea Dryander, Nesolindsaea Lehtonen &
Christenh., Odontosoria Fe
´e,
Osmolindsaea (K.U.Kramer) Lehtonen &
Christenh., Sphenomeris Maxon,
Tapeinidium (C.Presl) C.Chr.]
Saccolomataceae Doweld
11
(2/approx. 12)
(Orthiopteris Copel., Saccoloma Kaulf.)
Dennstaedtiaceae Lotsy (10/approx. 240)
[Blotiella Tryon, Dennstaedtia Bernh., Histiopteris
(J.Agardh) J.Sm.,
Hypolepis Bernh., Leptolepia Prantl, Microlepia
C.Presl, Monachosorum Kunze,
Oenotrichia Copel., Paesia St.-Hil., Pteridium Gled.]
4
Phylogenetic analysis of Osmundaceae clearly placed the cinnamon fern as
sister to all other Osmundaceae, which should thus be treated as Osmundastrum
cinnamomeum (L.) C.Presl (Yatabe et al., 1999). Todea and Leptopteris form a
clade and could be merged as a single genus, although to maintain stability we
have tentatively accepted Leptopteris here.
5
The filmy ferns are composed of two subclades, corresponding to the
traditional genera Hymenophyllum and Trichomanes, which we accept here. The
subdivision of Trichomanes into eight genera as done by Ebihara et al. (2006) is
possible, and the genera are stable and backed up by morphological characters.
However, we think that non-specialist users may find these genera difficult to
apply, and these are thus probably better treated at the subgeneric level within
Trichomanes.
6
Subfamily Lygodioideae Christenh.,subfam. nov. is based on full and direct
reference to the Latin diagnosis of Lygodium by Swartz (1801: 106). The genus
Lygodium is the type of this subfamily.
7
Subfamily Loxsomatoideae (C.Presl) Christenh., stat. nov. is based on full
and direct reference to the Latin description and type of Loxsomataceae in Presl
(1847: 31). The two genera in Loxsomatoideae aresimilar, and their separation is
mainly based on geographical isolation. They may be better treated as a single
genus.
8
Subfamily Culcitoideae (Pic.Serm.) Christenh., stat. nov. is based on full
and direct reference to the Latin description and type of Culcitaceae in
Pichi-Sermolli (1970a: 207).
9
Subfamily Plagiogyrioideae (Doweld) Christenh.,st at. nov. is based on full
and direct referenceto the Latin description and type of Plagiogyriidae in Doweld
(2001: XII).
10
Even though Cyatheoideae arein need of further study, there appears to be a
current consensus of four genera (Korall, 2006, 2007; Christenhusz et al., 2011),
although Gymnosphaera is not yet well defined and, because many combinations
are not yet made, it can be argued that the subfamily is best treated as including a
single large genus. Hymenophyllopsis, previously placed in its own family, and
Cnemidaria C.Presl are embedded in Cyatheaand should be treated as part of that
genus in any classification of the subfamily.
11
We still cannotmake an informed decision about whether or not to maintain
Orthiopteris as separate from Saccoloma. Phylogenetic study and monographic
revision of Saccolomataceae are needed, although the two genera are
undoubtedly closely related.
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Pteridaceae E.D.M.Kirschn. (approx. 44/approx. 1150)
subfamily Cryptogrammoideae S.Linds.
(ConiogrammeFe
´e, Cryptogramma R.Br., Llavea Lag.)
subfamily Ceratopteridoideae R.M.Tryon
(Acrostichum L., Ceratopteris Brongn.)
subfamily Pteridoideae C.Chr.
12
[Actiniopteris Link, Anogramma Link,
Aspleniopsis Mett.,
Austrogramme E.Fourn., Cerosora (Baker) Domin,
Cosentinia Todaro,
Jamesonia Hook. & Grev., Nephopteris Lell.,
Onychium Kaulf.,
Pityrogramma Link, Pteris L., Pterozonium Fe
´e,
Syngramma J.Sm., Taenitis Willd.]
subfamily Cheilanthoideae W.C.Shieh
13
[Allosorus Bernh., Aspidotis (Nutt.) Copel.,
Bommeria E.Fourn.,
Calciphilopteris Yesilyurt & H.Schneid.,
Cheilanthes Sw., Doryopteris J.Sm., Hemionitis L.,
MildellaTrevis., Myriopteris Fe
´e, Notholaena R.Br.,
Pellaea Link, Pentagramma Yatsk., Windham &
E.Wollenw., Pteridella Mett. ex Kuhn]
subfamily Vittarioideae Link
[Adiantum L. (approx. 200), Ananthacorus
Underw. & Maxon, Anetium Splitg.,
Anthrophyum Kaulf., Haplopteris C.Presl,
Hecistopteris J.Sm.,
Monogramma Comm., Polytaenium Desv.,
Radiovittaria (Benedict) E.H.Crane,
Scoliosorus T.Moore, Vittaria Sm.]
Aspleniaceae Newman ¼eupolypods II
14
(approx.
22/approx. 2780)
subfamily Cystopteridoideae Ching & Z.R.Wang
(Acystopteris Nakai, Cystoathyrium Ching,
Cystopteris Bernh., Gymnocarpium Newman)
subfamily Rhachidosoroideae M.L.Wang &
Y.T.Hsieh
(Rachidosorus Ching)
subfamily Diplaziopsidoideae Christenh.
15
(Diplaziopsis C.Chr., Homalosorus Small)
subfamily Asplenioideae Link
[Asplenium L. (approx. 700), Hemidictyum
C.Presl
16
,Hymenasplenium Hayata]
subfamily Thelypteridoideae C.F.Reed
17
[Macrothelypteris (H.Ito) Ching, Phegopteris
(C.Presl) Fe
´e,
Thelypteris Schmid. (approx. 1100)]
subfamily Woodsioideae Schmakov
(Woodsia R.Br.)
subfamily Athyrioideae B.K.Nayar
[Athyrium Roth (approx. 200), Cornopteris Nakai,
Deparia Hook. & Grev.,
Diplazium Sw. (approx. 350)]
subfamily Blechnoideae Hook.
[Blechnum L. (approx. 250), Onoclea L.
18
,
Stenochlaena J.Sm., Woodwardia Sm.]
19
12
Severalissues remain in the variable subfamily Pteridoideae. Anogramma is
polyphyletic with regard to Pityrogramma,Cerosora and Cosentinia (Schneider
et al., 2013). Perhaps these arebetter treated under a single genus or maybe as two
genera, merging Cerosora and Anogramma chaerophylla (Desv.) Link with
Pityrogramma and Cosentiniawith core Anogramma. The status of Aspleniopsis,
Austrogramma and Syngramma has not yet been studied, and some of these may
not remain after phylogenetic revision of this group. Pteris may be polyphyletic,
and it is possible that up to three genera should be accepted or some genera may
haveto be merged with it to achieve monophyly. Pteris in the broad sense includes
Afropteris Alston, Anopteris Prantl, Campteria C.Presl, Litobrochia C.Presl,
Neurocallis Fe
´e and Platyzoma R.Br. (Schuettpelz and Pryer, 2007;Christenhusz
et al., 2011).
13
The genera in this subfamily are in great need of recircumscription. Most
previously recognized genera, such as Cheilanthes,Doryopterisand Pellaea,are
not monophyletic, although about 14 clades can be recognized (Eiserhardt et al.,
2011). Bommeria,Calciphilopteris and Pentagramma appear to be
monophyletic. Pellaea,Astrolepis D.M.Benham & Windham, Paraceterach
Copel., ParagymnopterisK.H.Shing and Platyloma J.Sm. form a single clade and
should be united, but which species actually belong hereis still debated. It is also
unclear if Argyrochosma (J.Sm.) Windham should be excluded. Pellaea doniana
(J.Sm.) Hook., type of the genus Pteridella, does not group with core Pellaea, but
falls in a clade that includes Hemionitis and some Doryopteris.Aspidotis needs to
be redefined and should most probably include some Cheilanthes spp. and the
recently described genus Gaga Pryer, F.W.Li & Windham. Cassebeera Kaulf.,
Ormopteris J.Sm. and core Doryopteris form a clade (Eiserhardt et al., 2011) and
could be merged. Aleuritopteris Fe
´e and Sinopteris C.Chr. & Ching form a clade
with Allosorus, the last having priority (Christenhusz, 2012), and all species
should be moved to it. Cheilanthes,Doryopteris,Pellaea and Notholaena will
have to be redefined in morphological terms.Even though work is still needed on
this group, on the basis of Eiserhardt et al. (2011) we can conclude that this
subfamily probably consists of the following 14 clades, representing these
genera: Allosorus (including Aleuritopteris,Leptolepidium K.H.Shing &
S.K.Wu and Sinopteris), Aspidotis (including Gaga), Bommeria,
Calciphilopteris,Cheilanthes sensu stricto,Doryopteris (including Adiantopsis,
Choristosoria,Ormopteris and Trachypteris), Hemionitis,Mildella,Myriopteris
(the complex surrounding Cheilanthestomentosa Link and C. allosuroides Mett.,
including Cheilosoria), Notholaena (including Cheiloplecton Fe
´e), Pellaea
(including Argyrochosma,Astrolepis,Paraceterach,Paragymnopteris and
Platyloma), Pentagramma,Pteridella and an undescribed genus based on
Cheilanthes skinneri (Hook.) T.Moore. Whether it is better to accommodate
additional species in already redefined genera (as suggested above) or place the
entire Cheilanthoideae in the single genus Hemionitis is a subject of debate, but in
either case many new combinations will be needed.
14
Eupolypods II are here treated as one large family, Aspleniaceae. If this is
done, however, few synapomorphies remain, especially with inclusion of
Thelypteridaceae, but all members share the two vascular bundles in the petiole
and (except Thelypteridoideae) the laterally attached indusium. This treatment
would reduce the number of monogeneric families, and one large family that is
difficult to define morphologically is preferableto many smaller families that are
equally problematic to definemorphologically and differ only in relatively trivial
characters that are typically difficult to observe.
15
Subfamily Diplaziopsidoideae (X.C.Zhang & Christenh.) Christenh., stat.
nov. is based on full reference to the description and type of Diplaziopsidaceae
X.C.Zhang & Christenh. in Christenhusz et al. (2011), but excluding
Hemidictyum.
16
Hemidictyaceae are here included in Asplenioideae, which are here treated
in the broader sense. The two share many characters and form a clade.
17
The consensus on this subfamily has been that it includes five genera, but
problems in this group remain (Smith and Cranfill, 2002). Phegopteris and
Pseudophegopteris form a clade and are here combined under Phegopteris.
Cyclosorus is sometimes divided into a number of genera, some of which have
nomenclatural priority over it (notably Meniscium and Stegnogramma Blume).
The genus in the broad sense is monophyletic and includes the type species of
Thelypteris. It is preferable to treat this as a single large genus under the last, and,
when the taxonomy of this group is better understood, division into subgenera is a
better option than dividing the genus and moving species around, needing new
combinations and continuing to destabilize the taxonomy of this group.
18
The tiny family Onocleaceae used to be divided into four genera, but, with
only five species, this is too complicated. They form a well-supported sister
(Gastony and Ungerer, 1997) to Blechnoideae and should be treated as a single
genus Onoclea (including Matteuccia Tod., Onocleopsis F.Ballard and
Pentarhizidium Hayata)in that subfamily with which it shares several characters.
19
Blechnoideae comprise three major clades, one corresponding to Onoclea
sensu lato, a second corresponding to Woodwardia, sister to all other species that
can be treated as the single genus Blechnum. However, the subclade sister to the
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Polypodiaceae J.Presl & C.Presl ¼eupolypods I
20
(approx. 76/approx. 4070)
subfamily Didymochlaenoideae Christenh.
21
(Didymochlaena Desv.)
subfamily Hypodematioideae Christenh.
22
(Hypodematium Kunze, Leucostegia C.Presl)
subfamily Dryopteridoideae Link
23
[Adenoderris J.Sm., Arachniodes Blume (approx.
110), Arthrobotrya J.Sm., Bolbitis Schott, Coveniella
Tindale, Ctenitis (C.Chr.) C.Chr, Cyclodium C.Presl,
Cyrtomidictyum Ching, Dryopolystichum Copel.,
Dryopsis Holttum & P.J. Edwards, Dryopteris
Adanson (approx. 375), Elaphoglossum Schott
(approx. 750), Lastreopsis Ching, Lomagramma
J.Sm., Maxonia C.Chr., Megalastrum Holttum,
Mickelia R.C.Moran, Labiak & Sundue, Olfersia
Raddi, Phanerophlebia C.Presl, Pleocnemia C.Presl,
Polybotrya Humb. & Bonpl., Polystichopsis (J.Sm.)
Holttum, Polystichum Roth (approx. 370),
Pseudotectaria Tardieu, Rumohra Raddi, Stenolepia
Alderw., Stigmatopteris C.Chr., Teratophyllum Mett.]
subfamily Lomariopsidoideae Crabbe, Jermy &
Mickel
24
(Cyclopeltis J.Sm., Dracoglossum Christenh.,
Lomariopsis Fe
´e, Nephrolepis Schott)
subfamily Tectarioideae B.K.Nayar
25
[Aenigmopteris Holttum, Arthropteris J.Sm.,
Hypoderris R.Br.,
Pteridrys C.Chr. & Ching, TectariaCav. (approx. 250),
Triplophyllum Holttum]
subfamily Oleandroideae Crabbe, Jermy & Mickel
(Oleandra Cav.)
subfamily Davallioideae Hook.
26
[Davallia Sm., Davallodes (Copel.) Copel.]
subfamily Polypodioideae B.K.Nayar
tribe Loxogrammeae R.M.Tryon & A.F.Tryon
[Dictymia J.Sm., Loxogramme (Blume) C.Presl]
tribe Drynarieae Chandra
27
[Drynaria (Bory) J.Sm., Selliguea Bory (approx.
145)]
tribe Platycerieae Christenh., trib. nov.
28
(Platycerium Desv., Pyrrosia Mirb.)
tribe Microsoreae V.N.Tu
[Dendroconche Copel., Goniophlebium (Blume)
C.Presl, Lecanopteris Reinw.,
Lemmaphyllum C.Presl, Lepidomicrosorium Ching
& K.H.Shing, Lepisorus (J.Sm.) Ching, Leptochilus
Kaulf., Microsorum Link, Neocheiropteris Christ,
Neolepisorus Ching, Paragramma (Blume) T.Moore,
Phymatosorus Pic.Serm., Podosorus Holttum,
Thylacopteris Kunze, Tricholepidium Ching]
29
rest of Blechnum sensu lato contains the vining taxa Stenochlaena,Salpichlaena
J.Sm. and a few non-vining Blechnum species with long-creeping rhizomes,
which may have to be accepted at the generic level pending further studies.
Brainea,Doodia,Pteridoblechnum and Sadleria belong to Blechnum sensu lato
(Cranfill, 2001;Lehtonen, 2011).
20
As with eupolypods II, eupolypods I are here also subsumed intothe single
family Polypodiaceae sensu lato, reducing issues with placement of certain
genera, especially in Dryopteridaceae and Tectariaceae and the polymorphic
families Lomariopsidaceae and Hypodematiaceae. Despite their morphological
diversity, theyshare some soral characters and usuallyhave three vascular strands
in their petioles (except Hypodematioideae). This does somewhat destabilize the
current consensus classification, however, and many family names familiar to
users are reduced to the subfamilial level;however, it is a better option than adding
many more monogeneric families, such as Arthropteridaceae or
Didymochlaenaceae, in order to maintainbetter known names. Such proliferation
of families makes the sysem difficult to use and remember, and is frustrating for
students, who continually come across families they were never taught.
21
Didymochlaena does not form a clade with Hypodematium and
Leucostegia, with which it was previouslyassociated. It also shares no
morphological characterswith any other, which is why it is here placed as the sole
genus in a separate subfamily, Didymochlaenoideae Christenh., subfam. nov.
Characters: terrestrial ferns with erect, thick scaly rhizomes. The scales long and
narrow, almost hair-like. Leaves bipinnate, long-petiolate. Petioles with several
vascular bundles arranged in a half-circle. Pinnae articulated, all more or less
similar in size and shape, somewhat rectangular in outline. Veins free, forked,
with thickened endings before reaching the margin. Sori terminating a vein,
indusiate, often somewhat sunken in the blade, forming bumps on the upper side.
Indusia elongate, centrally attached along a line, opening on either side.
Sporangia long-stalked. Spores monolete, ellipsoidal to globose, tuberculate and
echinate. Type: Didymochlaena Desv.
22
Subfamily Hypodematioideae (Ching) Christenh., comb. & stat. nov. is
based on full reference to the Latin description and type of Hypodematiaceae in
Ching (1975: 96). It includes the genera Hypodematium and Leucostegia.
23
Cyrtogonellum Ching and Cyrtomium C.Presl are embedded in
Polystichum.Dryopsis is perhaps better not united with Dryopteris (as done by
Zhang, 2012) because Dryopteris may then not be monophyletic. Maxonia,
Olfersia and Polybotrya could also be merged, although these genera are well
established and monophyletic. The Ctenitis assembly including Rumohra,
Lastreopsis,Megalastrum and Pseudotectaria needs more study. The two
subfamilies proposed by Christenhusz et al. (2011) have not been supported in
more recent analyses and are therefore merged here.
24
Dracoglossumis placed here following recent findings of Christenhuszet al.
(2013).Nephrolepis probably also belongs here (as stated by Smith et al., 2006).
The unsampled Thysanosoria Gepp from New Guinea is here tentatively merged
with Lomariopsis on morphological grounds: Lomariopsis dimorphophylla
(Gepp) Christenh., comb. nov. Basionym: Thysanosoria dimorphophylla Gepp,
Contr. Phytogeog. Fl. Arfak Mts. 193. 1917. Lomariopsis pteridiformis (Ces.)
Christenh., comb. nov. Basionym: Gymnogramma pteridiformis Ces.,
Rendiconti della RealeAccademia delle Scienze Fisiche e Matematiche di Napoli
16: 30. 1877.
25
A number of genera previously placed in the Tectaria group by Holttum
(1991) appear to belong to either the Ctenitis assembly (Dryopteridoideae) or
Lomariopsidoideae. Further studies should show the relationships within this
subfamily. Psammiosorus C.Chr. belongs to Arthropteris (e.g. Lehtonen, 2011;
Liu et al., 2013).
26
Some controversyremains about the generic classification of Davallioideae,
due to the work of Kato and Tsutsumi (2009),Tsutsumi and Kato (2006) and
Tsutsumi et al. (2008). Even though clades can be recognized as genera in the
Davallia alliance, the type of the genus, D. canariensis (L.) Sm., has only recently
been included in a study (Liu & Schneider, 2013), supporting the concept of two
genera. Davallia is better treated in its broad sense including Araiostegiella
M.Kato & Tsutsumi and Humata Cav. because characters proposed to define
these genera do not hold up to closer scrutiny, which does not warrant splitting
them from Davallia.Araiostegia Copel. is tentatively included in Davallodes, but
it may warrant recognition.
27
Currently being studied, tribe Drynarieae probably only consist of two
genera (Drynaria and Selliguea), but how these are to be defined is still being
investigated (H. Schneider, pers. comm.).
28
Tribe Platycerieae (B.K.Nayar) Christenh., stat. nov., based on the Latin
description and type of subfamily Platycerioideae in Nayar (1970: 233). They
include the genera Platycerium and Pyrrosia.
29
Tribe Microsoreae need major study to determine generic delimitation,
especially Microsorum.Colysis C.Presl and Kontumia S.K.Wu & P.K. Lo
ˆc
belong to Leptochilus (Kim et al., 2013). Leptochilus heterophylla (S.K.Wu &
P.K.Lo
ˆc) Christenh., comb. nov. Basionym: Kontumia heterophylla S.K.Wu &
P.K.Lo
ˆc, Novon 15: 245. 2005. Many small genera are still commonly applied,
and larger genera are poorly defined, but with phylogenetic analyses it should be
possible to apply a broader generic concept and redefine these as larger more
natural entities.
Christenhusz &Chase —Trends and concepts in fern classification20
by guest on January 22, 2016http://aob.oxfordjournals.org/Downloaded from

tribe Polypodieae Hook. & Lindl. ex Duby
30
[Campyloneurum C.Presl, Grammitis Sw. (approx.
700), Microgramma C.Presl,
Pecluma M.G.Price, Phlebodium (R.Br.) J.Sm.,
Pleopeltis Humb. & Bonpl.,
Pleurosoriopsis Fomin, Polypodium L. (approx.
70), Serpocaulon A.R.Sm.,
Syngramma C.Presl]
ACKNOWLEDGEMENTS
We are grateful to Mike Fayfor insp iration and for suggesting that
we write this review. Samuli Lehtonen and Harald Schneider have
kindly discussed their ideas and knowledge about relationships of
ferns. We greatly appreciated the constructive comments by the
two anonymous reviewers, whose thorough reviews improved
our manuscript substantially.
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