keys to the vascular plants of east texas - Botanical Research ...

FERNS AND SIMILAR PLANTS
(PTERIDOPHYTES)
TAXONOMIC TREATMENTS
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AAlthough ferns and similar seedless vascular plants (those reproducing by spores) were formerly
lumped together as Division Pteridophyta, they are here segregated into four separate divisions
(Equisetophyta, Lycopodiophyta, Psilophyta, and Polypodiophyta) to reflect the great
diversity among these ancient plant groups. Thus, the group Pteridophyta is no longer formally
recognized. Together, the four divisions have nearly 10,000 species (Wagner & Smith 1993),
with some authorities indicating as many as 12,000 species (e.g., Hoshizaki & Moran 2001). The
Psilophyta, a very small division of 4–8 species, is recognized as distinct by some authorities,
while included in the Polypodiophyta (ferns) by others (see discussion under Psilophyta). Likewise,
members of the Equisetophyta are sometimes classified as ferns (see discussion under
Equisetophyta). While there are considerable differences between the various groups of ferns
and similar plants, molecular and morphological analyses indicate that all living vascular
plants (ferns and similar plants, gymnosperms, and flowering plants) represent a monophyletic
lineage (Doyle 1998; Pryer et al. 2001). Ferns and similar plants (sometimes called “fern allies”)
dominated the extensive swamps of the Carboniferous Period (360–286 million years ago).
Over geologic time, the compressed ancient remains of the plants from these swamps became
coal (Hoshizaki & Moran 2001). For a Key to Ferns and Similar Plants see page 309.
REFERENCES: Thieret 1980; Tryon & Tryon 1982; Lellinger 1985; Snyder & Bruce 1986; Kramer &
Green 1990; Wagner & Smith 1993; Hasebe et al. 1995; Manhart 1995; Kenrick & Crane 1997;
Doyle 1998; Nauman et al. 2000; Nelson 2000; Hoshizaki & Moran 2001; Pryer et al. 2001;
Moran 2004.
DIVISION PSILOPHYTA
WHISK-FERNS
AThe group is represented by a single very small family of spore-bearing plants. Psilophyta
lack roots (instead they have rhizomes with absorptive rhizoids and mycorrhizal fungi), have
dichotomous branching, and in the East TX species have only small, veinless, scale-like outgrowths
(= enations) on the stems (these are often not considered to be true leaves). This lack of
true roots and leaves makes them morphologically the least complex of all terrestrial vascular
plants. The structure of Psilophyta thus resembles (at least superficially) that of some of the
earliest land plants (Mabberley 1997), and the group has traditionally been linked to the earliest
known vascular plants from the Silurian and Devonian periods—e.g., the fossil genus Rhynia
(Woodland 1997). Alternatively, it has been suggested (e.g., Bierhorst 1977; Judd et al. 2002) that
the simple morphology of Psilophyta may instead be the result of reduction from an ancestral
fern, possibly in association with mycotrophy (= obtaining food from decaying organic material
via a special relationship with a symbiotic fungus). A number of molecular studies (e.g.,
Manhart 1995; Wolf 1997; Vangerow et al. 1999) have linked Psilophyta with the eusporangiate
fern family Ophioglossaceae (Botrychium and Ophioglossum), which is usually considered the
most isolated and basal among the modern plants normally classified as ferns (Wagner 1990).
Since these molecular studies are based on several different data sets, this is a particularly intriguing
connection that needs further study. However, other molecular, chemical, and morphological
data are ambiguous (Cooper-Driver 1977; Wallace & Markham 1978; Gottlieb et al. 1990;
Pryer et al. 1995), and Wolf (1997) noted that “there is no consensus on the relationships of
Psilotaceae to other vascular plants.” Further, based on a cladistic analysis using morphological
characters, Rothwell (1999) concluded that a link between ferns and Psilophyta was not supported.
However, the most recent phylogenetic research (Pryer et al. 2001), based on molecular