Pteridophyta (te-ri-DA-fa-ta) os made from two Greek roots
that mean winged (pteryz -πτέρυξ); and plant (phyto
-φυτό). The reference is to the wing-like appearance
of the compound leaves (fronds) that are characteristic of most ferns.
There are several names used for the ferns, among them are: Polypodiophyta,
Filicinophyta, and Moniliformopses. Haeckel (1866) introduced the
name for ferns but he did not apply it to a Division (~= Phylum) level taxon
(Smith 1955). According to Smith (1955), the use of Pteridophyta as a
division-level taxon was by Schimper (1879).
Ferns are quite successful plants. They grow as
perennial herbs, trees, epiphytes, and floating plants (Figures 1-34). They have
exploited almost all terrestrial and freshwater environments, and dominate in
some of them. Similarly, ferns have dominated terrestrial plant
communities to varying degrees since their appearance in the Devonian. The
ferns are megaphyllous plants whose leaves
(fronds) usually emerge by
vernation. The leaves also usually are compound and are among the most
complex leaves of any in the kingdom of green plants. Their axes vary in
complexity with steles of almost all types possible: protosteles, actinosteles,
plectosteles, ectophloic siphonosteles,
dictyosteles, and eusteles
FIGURE 1. TYPES OF STELES IN VASCULAR PLANTS
A. Simple protostele
B. Ectophloic siphonostele
B'. Amphiphloic siphonostele
Atactostele, the only stele type not found in the ferns
Figure 13-6 from Bold et al. (1987)
& 2 = Preferns
M = megaphyllous ferns
L = leptosporangiate ferns
FIGURE 2. MAJOR CLADES OF THE PTERIDOPHYTA. The structure of
this cladogram comes from Smith et al. (2006) but informed by Kenrick and Crane (1997),
Scheuttpelz and Pryer (2007 and 2008), and Schuettpelz et al. (2006).
The cladoxylids and coenopterids were the groups of plants,
which together are called the preferns. They showed the spectrum of steps required to form a webbed
branch system that we recognize as a megaphyll.
Indeed, the terminal fertile appendage of Cladoxylon (see Figure 3)
looked very much like a spore-bearing megaphyll. The cladoxylids were monopodial with small
spore-bearing frond-like branching systems. Thus, they resembled the Trimerophytophyta
from which they likely emerged. All extinct, these organisms flourished during the
Devonian but died out by its end. Pearson
(1995) believed that the cladoxylids gave rise to the Progymnospermophyta
and, thus, to the seed plants. Stewart and Rothwell (1993) demonstrate
potential affinities between the cladoxylids and all major groups now considered
to be within the Pteridophyta as well as the seed ferns. However, they end
their discussion by saying, "...the Cladoxylids...can be added to our list
of plant groups that represent unsuccessful evolutionary 'experiments' that
ended in extinction" (Stewart and Rothwell 1993, p. 217).
Cladoxylon (Figure 3) had two types of leaf-like
branching systems that were covered by microphylls. These photosynthetic
appendages were small and had open branching. However, the fertile
appendages were flattened into a single plane of dichotomously-branched axes,
each of which terminated in a small sporangium.
Pseudosporochnus (Figure 4) grew to be very large
and resembled present-day palms or tree ferns. The lateral branches
appeared frond-like with sterile and fertile appendages emerging as
dichotomously branched systems. Unlike Cladoxylon, the fertile
appendages of Pseudosporochnus were not flattened. These plants
had a fossil history that ranged through much of the Devonian period.
Calamophyton (Figure 5) had strong monopodial growth with dichotomizing
ultimate branches. The microphylls were round in cross section and spirally
arranged on the stems. Sporangia occurrred on clusters of recurved stems,
which bore striking resemblance to the sporangiophores
of the Equisetales. There were known from the middle Devonian.
Wattieza (Figure 6), whose stumps were know as Eospermatopteris,
was one of the earliest trees and formed forests in the Gilboa, New York area
during the middle Devonian. One fossil described by Stein et al. (2007)
stood at least 6 m tall. Most notably, the lateral branching systems
behaved as megaphylls in that each system seems to have abscised as a unit,
rather than in pieces.
The coenopterids flourished from the Devonian to the end
of the Permian when they died out. The coenopterids were monopodial with spore-bearing frond-like
branching systems that may have been the earliest true megaphylls.
Thomas and Spicer (1987) consider the coenopterids to
represent a grade of evolution. Very likely, they are a paraphyletic group
of early ferns that had the earliest megaphylls. Not surprisingly, members
of the group vary from having creeping stems to shrubs to trees. If they
are indeed paraphyletic, each of the following taxa may be a representative of a
Stauropteris (Figure 7) was a small shrubby plant
from the upper Devonian to the upper Carboniferous. The axes had alternating pairs of frond-like
branches emerging at the nodes. Elongate sporangia occur on some of
the terminal branches. At least one species is heterosporous (Stewart and
Rhacophyton (Figure 8) had large frond-like
branching appendages that emerged in a spiral pattern from slender axes.
The sterile appendages had primary pinnae in 2 ranks, each of which had small,
dichotomously branching stems around the pinna. The fertile fronds were even more
complex. Some of the primary pinnae were sterile. The fertile primary pinnae
ball-like dichotomously-branched appendages, each of which terminated in
elongate sporangia. Because the stems were so slender, they likely
could not support such large fronds as an upright axis, but must have grown as
creeping stems (stolons?). Some of the stems showed evidence of
secondary growth leading Stewart and Rothwell (1993) to suggest that this
genus and related taxa may have been associated with the line leading to the
Progymnospermophyta. These appeared in the upper Devonian. Related
taxa persisted through the Carboniferous.
Zygopteris (Figure 9) were creeping or rhizomatous
plants from the upper Devonian to the Permian. The
rhizomes were covered with frond-like dichotomizing branches, essentially
megaphylls, which occurred in
two ranks. The stele was H-shaped in mature stems and showed evidence of
secondary growth. The sporangia were at the tips or on the abaxial surface of
the ultimate branches.
CLADE M: THE EUPHYLLOPHYTES, THE MEGAPHYLLOUS FERNS
These are the plants that have megaphyllous leaves. That is, the
megaphyll is a branch system that has become planar and webbed. Despite
the name, size is not an adequate diagnostic character to use in distinguishing
megaphylls from microphylls. Some taxa like Lepididodendron, a
microphyllous plant, has very large leaves. On the other hand, the
scale-like megaphylls of cedars are quite small. The principle character
that distinguishes a megaphyll is a leaf-gap in the stele. This is an
opening or gap made by the stele of a branch (called a leaf trace) as it emerges
from the stele of main stem (Figure 10).
The steps leading to the formation of a megaphyll are given in Figure
11. This is a portion of the Telome Theory as proposed by Zimmermann (1952
and 1959), who proposed that all of the main plant organs can be derived from
simple Rhynia-like axes called mesomes (sterile axes) and telomes
(fertile axes). Tbe derivation of megaphylls in this scenario is that the
dichotomously-branching axis develops an unequal branching form (Figure 11-A)
called overtopping. The lateral branch system then becomes planar (Figure
11-B) and webbing elaborates between the axes. Thus, a megaphyll is not a
structure that evolved de novo but was assembled from existing
structures. Tomescu (2008) argures that such a sequence for megaphyll
evolution must have occurred multiple times thus calling into question the
homology of early megaphyllous appendages.
Botrichium (see Figure 12) and Ophioglossum are extant ferns
that typically produce a single frond each year. The small upright stem
usually is underground with very short internodes. Each leaf has a sterile
pinna and a fertile
pinna. The fertile
pinnae are not webbed, but have clusters of large eusporangia that are
homosporous. The sterile pinna can be highly dissected (Botrychium)
or entire (Ophioglossum). The gametophytes of these organisms
resemble the carrot-like saprobic gametophytes of Lycopodium.
Although cryptic, Botrychiumvirginianum (Rattlesnake Fern) plants
enjoy a large distributional range that includes temperate to America,
Scandinavia, the Himalayas, and parts of Australia. In addition, the
Rattlesnake Fern can be among the oldest in the habitats where they occur
(forest floor of rich woods or thickets with acid soils and shade). I once
saw a Botrychium with 45 leaf scars eroding out of a road bank.
That was in an area where the oldest trees were no more than 35 or 40 years
old. Other members of the genus and the class are among the rarest plants
in an area.
Structurally, the psilophytes would seem to be out of place.
They grow as dichotomizing branching
systems that do not have leaves or roots.
Instead, they have a prostrate rhizomatous
branching system with rhizoids. The upright
stems are photosynthetic and are covered by enations
or microphylls. The sporangia occur
as eusporangiate synangia at the terminus of short lateral
stems (Figure 13). The gametophyte is small, inconspicuous, and saprobic.
Also, it is monoecious, producing both antheridia and archaegonia on the same
The overall structure of the sporophyte would seem to make them remnants of the earliest radiation of vascular
plants. Such is the classical view that associates the psilophytes with
the Rhyniophyta (see the figure from
Pearson 1995). However, molecular evidence (see Tudge
2000; and Pryer et al. 2001) suggests
that the psilophytes are reduced ferns. That was the intuition of
Bierhorst (1971) who, based on structural evidence, saw a gradation in structure
from the psilophytes to the fern families Stromatopteridaceae, Gleichineaceae,
and Schizaeaceae. Indeed, he interpreted the dichotomizing branches of Psilotum
(Figure 14) as a highly reduced frond and the leafy branches of Tmesipteris
(Figure 15) as
modified fronds. Modern molecular cladistic analyses show that they are
sisters to Botrychium + Ophioglossum (e.g. Pryer et al. 2001).
However, morphology-based analyses (e.g. Schneider et al. 2009) suggest that
they should be sisters to Equisetopsida.
FIGURE 13. LIFE HISTORY OF PSILOTUM.
The sporangium (1-2, a eusporangiate synangium) produces spores.
They germinate to produce inconspicuous thalloid gametophytes (4), which produce
both archaegonia (5) and antheridia (6). Antheridia release flagellated
sperm which fuses with the egg to form a zygote (7). The embryonic
sporophyte (8) emerges from the archaegonium.
The horsetails or scouring rushes are distinctive in two
ways: they have a stem that is jointed and ribbed and a strobilus
of sporangiophores. Although represented today by a single genus, Equisetum
(Figure 16), the horsetails
have a very long history and diverse representation in the fossil
record. They were especially abundant from the Devonian to the end of the
Paleozoic. A common feature of the class is the production of jointed
stems (thus Bold et al. 1987, refer to this group as the Arthrophyta).
Also, branches arise from beneath the
leaves rather than the more typical adaxial emergence. The stele is difficult to interpret, but stems appear to grade from
siphonostelic to eustelic. A very
distinctive feature of the equisetophytes is the type of complex strobili. Cones like those of Equisetum
(Figure 16)are made of sporangiophores (modified leaves), each with multiple
sporangia. Equisetum is homosporous and its gametophytes are
saprophytic, monoecious, and cryptic (see Figure 17).
Hyenia (Figure 18), a Devonian age equisetophyte, grew as
a creeping rhizome from which upright photosynthetic stems emerged. Some
of the terminal branches of Hyenia are loosely-clustered sporophylls
whose structures suggest the evolution of the Equisetum-like cone by
reduction of internodes and reduction of the sporophylls.
Calamites (Figure 19) grew as trees with strong monopodial growth
leaves (megaphylls) at the jointed nodes.
Indeed, Calamites showed strong secondary growth. They
had compound strobili with
heterosporous sporangia. Gametophytes have not been found in the large extinct
forms.These plants appeared in the upper Devonian and persisted to
the Permian. Calamites was one of the dominant plants in the great Coal Age
forests during the Carboniferous period.
Pseudobornia (Figure 20) were large trees (up to
20m tall) with articulating stems. The dichotomizing branches grew up to 3m long.
These plants appeared appeared to be simpler that Calamites. They did not
show evidence of secondary growth (or, if so, it was limited), and their
sporangia were homosporous. They were restricted to the Upper Devonian and
may have given rise to the Calamites line.
Sphenophyllum (Figure 21) were creeping plants
with prostrate stems that had solid cores and were triangular in cross-section. Like Calamites,
though, Sphenophyllum had whorls of wedge-shaped leaves. These plants
appeared in the lower Devonian and persisted through the
Permian, and may have survived into the early Triassic.
FIGURE 17. LIFE HISTORY OF EQUISETUM.
The sporophyte (1) produces a terminal strobilus of sporangiophores
tetrads mature with attached elater tissue (3-4). The
gametophyte (5) is inconspicuous and monoecious. It produces small
antheridia (6), and archaegonia (7). Following syngamy (8), the embryonic
sporophyte (9) emerges from the archaegonium.
The marattiopsids are massive ferns that seem to be sisters
to the equisetopsids, and have a fossil history which goes back to the
Carboniferous. Everything about them is large. Their leaves can
be up to 7.5 meters long, and their sporangia likewise are large, eusporangiate,
and usually fused into large synangia.
The gametophytes are large,
thallose and often perennial causing them to resemble Marchantia. The stems are supported
as a palm-like tree by persistent leaf bases and exhibit secondary growth
by a polycyclic dictyostele. The fleshy stems and roots often have mucilage chambers in a
A common genus is Angiopteris, a name that means
wings" (Figure 22). The rhizomes are very
large and fleshy, some are edible. One species of Angiopteris has
become an invasive plant on the island of Jamaica.
CLADE L. POLYPODIOPSIDA, THE LEPTOSPORANGIATE FERNS
Most of the living Ferns are assigned to the class,
Polypodiopsida. This class is, by far, the most speciose and most diverse
in form of all the living fern groups. The most fundamental synapomorphic
character is the leptosporangium.
This is a particular type of fern sporangium that develops from one or two
superficial cells and can have as few as 16 to 32 spores per sporangium.
They have characteristic springy, gracile stalks with a sporangium on the
top. Typically, the sporangium has cells of different thicknesses such
that the sporangium dehisces suddenly via a horizontal slit and flings the
spores by the combined actions of the sudden opening and the recoil of the
springy stalk. In most taxa the leptosporangia are clustered in sori
and usually associated with indusia,
extensions of leaf tissue that may cover or surround sori (Figure 23).
FIGURE 23. LIFE CYCLE OF PTERIDIUM
A. Fertile megaphyll of the sporophyte
B. Fertile pinna with sorus along the margin of the leaf
C. Leptosporangia emerging from the sorus and covered by a false indusium
D. Cordate gametophyte
E. Archaegonium, antheridium, syngamy to produce a zygote
F. Emergence of an embryonic sporophyte
Osmunda (Figure 24) and their relatives have a very complete fossil history which goes back to the Permian. The
plants have a short erect stem with persistent leaf bases. The leaves are large with
dichotomous venation in the pinnae. Sporangia are more massive than the
typical leptosporangiate condition. Indeed, they appear to be intermediate between a
condition and a eusporangiate
condition. Still, the sporangium has a unistratose wall, but it opens by a
longitudinal slit (most leptosporangia open by a horizontal slit). The sporangia never occur in a
sorus. The gametophyte is large (up to 5
cm long) and photosynthetic.
The filmy ferns, like Trichomanes (Figure 25), occur mainly in the southern hemisphere and in the tropics. Most are
small, with very thin leaves, usually unistratose.
Furthermore, the stems are equally delicate and usually protostelic. Sori are marginal and are surrounded by
a cup-shaped indusium. The Trichomanes species that occurs in
Pennsylvania lives entirely as a gametophyte on seeps and protected areas.
They are small branched filaments that reproduce only asexually as gemmae.
Lygodium (Figure 26) is a member of the Schizaeales, an
order that has a fossil history which dates from the Jurassic. Mainly, members
of this order are tropical, but Lygodium
occurs as far north as Pennsylvania. The sporangium has a thick stalk and an annulus which forms an
apical cap (a longitudinal slit in Lygodium). Sporangia may be covered by an
indusium-like flap, but the sporangia do not occur in sori. The leaves are quite variable,
but usually small. However, the leaves of Lygodium remain meristematic at
the tip and continue to grow as vines, more than 30 meters long for each
leaf. Stems are less significant and range from protostelic to
dictyostelic. The gametophytes vary from filamentous to
The water ferns are all heterosporouswith their gametophytes rarely exceeding the bounds of the
spore wall. This is true both of the megaspore and the microspore.
The plants differ vegetatively though they are all aquatic or
semi-aquatic. Marsilea (Figure 27) is rhizomatous with leaves which resemble
four-leaf clovers. Their rhizomes have a solenostele.
At the nodes, leaves and adventitious roots emerge. At some of the nodes,
fertile leaves called sporocarps emerge. They resemble seeds and remain
closed until scarified (either through physical abrasion or through chemical degradation)
at which point the gelatinous leaf emerges with its sori filled with sporangia
(Figure 28). I
have seen them become particularly abundant in the depressions left by sand
traps in abandoned golf courses in the central part of the US.
FIGURE 28. LIFE HISTORY OF MARSILEA
The water fern, Marsilea, looks like a four-leaf clover,
but circinate vernation gives it away as a fern. It is rhizomatous
from which leaves emerge at the nodes. The the base of some of the
leaves, a sporocarp (a hardened folded leaf with sporangia inside)
develops (Top a&b). The sporocarp develops as a gelatinous
ring which allows the sori to emerge into the water (Bottom
a&b). These are heterosporous. Microspores develop into
multiflagellate sperm and the megaspores develop into a megagametophyte
which does not exceed the bounds of the spore wall. The daughter
sporophyte grows from the zygote in the archegonium in the megaspore and
appears almost like a germinating seed.
Image from Ditmer (1964)
The other types of water ferns are the floating ferns. Azolla,
the mosquito fern (Figure 29), floats on the surface of the water. It
resembles small sprigs of red cedar on the water. When the sunlight is
most intense, the plants protect themselves with a red pigments that turns small
ponds in the southern US red in the middle of the day. Although they float
on the water surface, they have a noticeable layer of wax on their upper
surface. This serves to reduce desiccation and to help them remain afloat
by being caught in the surface tension. Very often Azolla has a
symbiotic Nostoc associated with the plant, presumably providing the
plant with usable nitrogen compounds. I have seen them grow in such
densities that they effectively seal off the water surface from
mosquitoes. However, when they are that abundant, they prevent the
penetration of light and the pond becomes anoxic.
Cyathea (Figure 30), a common tree fern with a fossil
history which goes back to the Jurassic, can be a dominant
plant in some tropical forests, particularly the mountain forests. One
such dominant can be seen in El Yunque, the montane rainforest of Puerto
Rico. Cyathea arborea is a robust member of the forest understory
and even forms the canopy on steep areas of the mountain. The trunk is an
upright rhizome that can grow 12 o more meters high with a tuft of large leaves
at its growing tip. Thus, from a distance, they resemble palms.
However, the large fiddleheads emerging from the crown label them for what they
are. The sori
are rounded on veins and are sheathed by a globose indusium. Dehiscence of
the sporangia occurs by a
transverse slit, and the gametophyte is thalloid with a midrib.
Dennstaedtia (Figure 31), the hay-scented fern, is
common on the edges of woods in the northeastern US. They grow from vigorous
rhizomes that can dive many cm deep into the soil and shoot quickly into
clearings. They share these characteristics with their relatives, Pteridium,
the bracken ferns. They also produce allelopathic compounds that tend to
discourage the growth of seed plants. Thus, they can, when established,
have a major impact on the regrowth of a forest. Some of them are of some
economic importance because they are poisonous to sheep and cattle.
Adiantum (Figure 32), the maiden hair fern, is one of
the most beautiful ferns. They grow from a creeping rhizome with
distinctive thrice cut compound leaves. Fertile pinnae are narrower than
the sterile ones because the marginal sori are surrounded by a false
indusium formed by the margins of the leaves curling over the sori.
Dryopteris (Figure 33), the wood fern, is one of the
most conspicuous fern genera in the Eastern Deciduous Forest. They grow
from a rhizome that remains upright and does not creep. Thus, the leaves
tend to emerge in one vase-like cluster. Each sorus is associated with a
bean-shaped indusium. The species in this genus readily hybridize making
the identification of some individuals quite a challenge.
Polypodium (Figure 34) is a common small evergreen
plant in the US woodlands. The common polypody grows as an understory
plant in the Eastern Deciduous Forest. The southern polypody, however,
grows as an epiphyte on the branches of large trees like the Live Oak of the
southern coastal forests of the US. Polypodium has sori that are
naked. That is, they are not associated with an indusium, either true
According to Bold et al. (1987) and Lelinger (1985), the
ferns have been a problem in phylogenetics for some time. The classical relationships of the groups of ferns can be seen in Pearson
(1995) and Rothwell (1999), both of which
are similar to the view of Bold et al. (1987). Pryer et
al. (2001), however, through molecular phylogenetic analysis indicate that the
ferns, if considered as a monophyletic group, must include the psilophytes and
horsetails. After that, Scheuttpelz et al. (2006) and Schuettpelz and
Pryer (2007 and 2008) confirmed the relationship. Smith et al. (2006)
created a revised Linnaean taxonomy of extant ferns using the recently confirmed
relationships. Schneider et al. (2009) analyzed fern phylogeny by a
cladistic analysis using morphological characters and found a similar patterns
that included Psilotum and Equisetum within the ferns. We used Kenrick and
Crane (1997b) for support in the inclusion of the extinct taxa, particularly the
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