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PALEOBOTANY: THE BIOLOGY AND EVOLUTION OF FOSSIL PLANTS
L
VB
P
9.30 Compression of Lepidodendron lycopodites
(Pennsylvanian). Bar 2.5 cm.
Figure
Strobili were borne at the tips of distal branches or in a zone
at the top of the main trunk. The underground portions of the
Lepidodendrales (sometimes called a stigmarian rhizomorph)
consisted of dichotomizing axes that bore helically arranged,
lateral appendages that presumably functioned as roots.
Figure 9.31 Lepidodendron leaf base. L: ligule; P: parichnos;
VB: vascular-bundle scar. (From Taylor and Taylor, 1993.)
Vegetative Features
STEM SURFACE AND LEAF BASES
Some of the most commonly encountered fossils assignable to
the lepidodendrids are compressions of stem surfaces marked
by persistent, somewhat asymmetric, more or less rhomboidal leaf cushions (FIG. 9.31). The leaf cushion actually
represents the expanded leaf base left behind after the leaf
dropped off (FIG. 9.31), since abscission of the leaf did not
occur flush with the stem surface. The top and bottom of the
cushions, which are also called leaf bolsters, generally form
acute angles; the sides are more rounded. The actual scar
left by the abscised leaf is slightly above the midpoint of the
cushion and is generally elliptical or rhombic in outline (FIG.
9.32). On the surface of the leaf scar are three small, pit-like
impressions. The central one represents the single vascular-bundle trace that extended into the leaf. The other two
scars represent the position of channels of loosely arranged
parenchyma tissue, termed parichnos (FIG. 9.31); this tissue originates in the cortex and extends through two grooves
on the abaxial surface of the leaf. On Lepidodendron stem
surfaces, two additional parichnos channels can be identified
at a short distance beneath the leaf scar; these do not occur
in the Diaphorodendraceae, where parichnos is confined to
the foliar scar only. Parichnos is a system of aerating tissues
within the stem. A vertical line extends from the leaf scar
proper to the lower limit of the leaf base. In many specimens,
lateral wrinkles cut across this line. Initially, it was thought
that these wrinkles were of systematic value, but it is now
understood that they are the result of the growth of secondary
tissues in the stem. Just above the leaf scar is a mark that
CHAPTER 9
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Figure 9.32 Several Lepidodendron leaf cushions preserved in
a Mazon Creek nodule (Pennsylvanian). Bar 3.5 mm.
represents the former position of a ligule. Additional markings occur on the leaf base in the form of vertical lines that
result from lateral expansion of the stem. Rarely is preservation so good that all these features can be observed in a single leaf base. In Synchysidendron, the leaf cushion includes a
groove that is formed by folding of the cushion tissue immediately below the leaf scar proper.
Some compression specimens of arborescent lycopsid stems
(FIG. 9.33) also provide information about the epidermis
of these plants. A waxy cuticle covers the stem surface,
including the leaf cushions, but is thought to be absent on
the leaf scar itself (Thomas, 1966). The epidermis is simple
and lacks such specialized cells as hairs and glands. Stomata
are common and sunken in shallow pits (Thomas, 1974).
Another tree-sized lycopsid, Lepidophloios (Q. Wang, 2007)
(originally spelled Lepidofloyos; Sternberg, 1825), occurred
in the Carboniferous coal swamps along with Lepidodendron
(DiMichele, 1979a). Lepidophloios (Lepidodendraceae) was
probably slightly smaller in stature (FIG. 9.34), but in general
its features are quite similar to Lepidodendron. One notable
difference between the two is the arrangement and organization of leaf bases (FIG. 9.35). In Lepidophloios, the leaves are
arranged in a shallow helix, as in other lepidodendrids, but
the leaf bases are flattened and wider than they are tall (FIG.
9.35). They are directed downward on the stem and overlap
the bases below, much like shingles on a roof. When attached,
the leaves bend upward abruptly from the leaf bases; when
a leaf abscised, it left a scar on the bottom third of the base
Figure 9.33 Lepidodendron leaf bases (Pennsylvanian). Bar 1 cm. (Courtesy BSPG.)
(FIG. 9.36). Parichnos and vascular-bundle scars on the leaf
scar are like those of Lepidodendron, but parichnos scars are
not present on the base itself. Lepidophloios was ligulate, with
the ligule attached just above the position of the leaf scar.
Many Lepidophloios stems exhibit large, circular to elliptical scars on the stem surface (FIG. 9.37), some up to several centimeters in diameter. The origin of these scars has
been debated for many years. Some workers regard them
as former sites of vegetative branches that abscised during
the normal growth of the plant, whereas others suggest that
they represent former positions of specialized branches that
bore clusters of strobili. Historically, stems with helically
arranged scars of this type have been given the generic name
Halonia, whereas those with oppositely arranged scars are
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PALEOBOTANY: THE BIOLOGY AND EVOLUTION OF FOSSIL PLANTS
called Ulodendron. Jonker (1976) suggested that Ulodendron
scars on the axes of Lepidodendron, Lepidophloios, and a
related genus, Bothrodendron, represent the former positions of branches that abscised in a manner similar to that
of some existing gymnosperms and angiosperms. Thomas
(1967) regarded Ulodendron as a natural genus, basing this
hypothesis on the persistent leaves, shallow ligule pits, and
rhomboidal leaf bases, whereas DiMichele (1980) suggested
it was congeneric with Paralycopodites.
L
VB
P
Figure 9.36 Diagrammatic representation of Lepidophloios
leaf base showing ligule (L), parichnos (P), and vascular-bundle
scar (VB) (Pennsylvanian). (From Taylor and Taylor, 1993.)
Figure 9.34 Suggested reconstruction of distal region of
crown branches of Lepidophloios hallii (Pennsylvanian). (From
DiMichele, 1979a.)
Figure 9.35 Paradermal section of Lepidophloios leaf bases
Figure 9.37 Three prominent oval branch scars along a
showing four branch scars (Pennsylvanian). Bar 1 cm.
Lepidodendron axis (Pennsylvanian). Bar 4 cm.
CHAPTER 9
as a result of the vascular cambium and phellogen. The
increase in stem diameter results in the sloughing off of the
outer cortical tissues (FIG. 9.45), including the leaf bases,
so that in older parts of the plant (e.g., at the base), the outer
surface of the trunk is protected by periderm. Many of the
older reconstructions of Lepidodendron in museums and
drawings often err in showing leaf bases extending all the
way to the ground on old trunks.
At higher levels in the tree, the branches have smaller
steles and fewer rows of smaller leaves on the surface.
Sections of stems at these levels indicate that less secondary
xylem and periderm are produced. A reduction in stele size
and tissue production continues until the most distal branches,
which contain a tiny protostele with only a few small tracheids, no secondary xylem or periderm, and just a few rows
of leaves. This stage in development, in which the plant literally grows itself out, has been termed apoxogenesis. In other
words, the small, distal twigs of these arborescent lycopsids
do not have the potential of developing into larger branches
with time. This type of growth pattern is called determinate
and contrasts with indeterminate growth, which is typical of
vegetative development in most living woody plants.
Paleobotanists must continually devise new methods of
investigating the biology of the organisms they study. Eggert’s
elegant analysis of growth in the arborescent lycopsids is one
such approach. In another, the focus of the study is the nature of
the unifacial vascular cambium in two Carboniferous lycopsid
morphogenera, Stigmaria and Paralycopodites (Cichan, 1985a).
Cichan (FIG. 9.46) prepared serial tangential sections of the
secondary xylem in order to determine the pattern of production of cambial derivatives and the method of circumferential
increase in the cambium. Cichan’s studies indicate that cambial
activity in these plants was also determinate. Circumferential
increase took place by the enlargement of fusiform initials,
rather than by anticlinal divisions of existing initials, as it does
in seed plants. This type of growth would result in a cambium
that was limited in its capacity for radial expansion. As secondary growth ceased in the plant, fusiform initials ceased to be
meristematic and matured into a cylinder of parenchyma.
LEAVES
The leaves of arborescent lycopsids are linear and some
were up to 1 m long (FIG. 9.47). Chaloner and MeyerBerthaud (1983) demonstrated that stems with the largest
diameters have the longest leaves, a feature they correlate
with the determinate growth of the plants. Many of the different species established for detached leaves were probably
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produced by the same kind of plant and only differed in
size, shape, and anatomy because of their position on the
plant. The generic name Lepidophyllum was initially used
for both structurally preserved and compressed lepidodendrid leaves, but because this name had been used earlier for
a flowering plant, the name Lepidophylloides was proposed
in its place (Snigirevskaya, 1958). A single vascular bundle, flanked by two shallow grooves on the abaxial surface,
extends the entire length of the lamina in Lepidophylloides
(FIG. 9.48). Stomata occur on the abaxial surface aligned
in rows that parallel the grooves and sunken in shallow pits.
A well-developed hypodermal zone of fibers surrounds the
mesophyll parenchyma and vascular bundle of the leaf; no
palisade parenchyma has been reported. In L. sclereticum
from Permian coal balls, the vascular bundle is convex in
transverse section and surrounded by transfusion tracheids
(S. J. Wang et al., 2002).
UNDERGROUND ORGANS
Underground axes of the Lepidodendrales are given the
name Stigmaria. These dichotomizing structures represent
one of the most common lycopsid fossils and constitute the
Figure 9.46 Michael A. Cichan.
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PALEOBOTANY: THE BIOLOGY AND EVOLUTION OF FOSSIL PLANTS
Figure 9.48 Cross section of Lepidodendron leaf. Note
two abaxial furrows (arrows) where stomata are located (Pennsylvanian). Bar 1.5 mm. (Courtesy BSPG.)
Figure 9.47 Lepidophylloides in Mazon Creek nodule
(Pennsylvanian). Bar 2 cm.
principal organ found in the clay layer or underclay immediately beneath most Carboniferous coal deposits. The underclay represents the soil layer (paleosol) in which these plants
were rooted in the coal swamps (Mosseichik et al., 2003).
Extensive specimens of Stigmaria have been uncovered in
growth position, some with rootlike structures, commonly
called stigmarian appendages, still attached (FIG. 9.49).
Although there are several species of Stigmaria, our knowledge of the anatomy of these underground systems is based
principally on the species Stigmaria ficoides (FIG. 9.50)
(Williamson, 1887b). The stigmarian system arises from the
base of the trunk as four primary axes, each of which extends
out horizontally, so that the rooting system is relatively shallow. Helically arranged lateral appendages were attached
to each axis. These appendages abscised during the growth
of the plant, leaving characteristic circular scars (FIG.
9.51) on the main axis, and these can be seen on a variety
of casts, compressions, and impressions of Stigmaria. The
lateral appendages are sometimes called stigmarian rootlets
(FIG. 9.52), although their helical arrangement (i.e., phyllotaxy) and abscission are characteristic of leaves rather than
lateral roots (see Chapter 7). The primary axes in Stigmaria
dichotomize repeatedly to form an extensive subterranean
system that may have radiated up to 15 m from the trunk.
Primary axes of Stigmaria have a parenchymatous pith
that may include scattered tracheids at more distal levels.
Primary xylem is endarch and arranged in a series of dissected bands, which are, in turn, surrounded by a vascular
cambium. What is interpreted as secondary xylem is distinctive because the wide vascular rays give the wood a
segmented appearance (FIG. 9.53). Secondary xylem tracheids are aligned in radial files and possess scalariform wall